Module 2 The climate science behind climate changeclimate change. To this end, this module draws on...

Module 2The climate science behind

climate change

The climate science behind climate change

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21 Introduction

Module 1 showed that human actions can influ-ence the climate because the economy and the environment are interdependent. This module focuses specifically on the climate system and the climate science behind climate change. By reviewing key points, the module enables read-ers to understand how the climate system works, why climate change occurs, and how humans induce climate change. The module highlights recent empirical evidence pointing towards the existence of human-induced climate change, and discusses climate-change-related impacts that can already be observed today. To conclude, the module outlines different scenarios for the

future and discusses the anticipated impacts of climate change.

To this end, this module draws on the work of the Intergovernmental Panel on Climate Change (IPCC) Working Group I, which deals with the physical science basis of climate change. As mentioned in Module 1, the IPCC is the leading body collecting, reviewing, and assessing recent state-of-the-art scientific work related to climate change. The five IPCC assessment reports and the different IPCC special reports19 together form the most comprehensive and reliable source of scien-tific work today on climate change. The module therefore uses some of the terminology intro-duced by the IPCC (see Box 9).

TheIPCC’sterminologytoreportfindingstothepublicBox 9

Relationshipbetweenagreement,evidenceandconfidencelevels

The Intergovernmental Panel on Climate Change (IPCC) developed specific terminology to report its findings to the public. In the fifth assessment report (IPCC, 2013a), a finding is assessed in terms of the underlying evi-dence and the agreement among scientists regarding that finding. In addition, the IPCC often assigns a level of confidence to the different findings that is based on evidence and agreement (see Figure 11).

Figure 11

19 All these reports are available on the IPCC home-page at: http://www.ipcc.ch/publications_and_data/pu-blications_and_data_reports.shtml#1.

High agreementLimited evidence

High agreementMedium evidence

High agreementRobust evidence

Medium agreementLimited evidence

Medium agreementMedium evidence

Medium agreementRobust evidence

Low agreementLimited evidence

Low agreementMedium evidence

Low agreementRobust evidence

Source: Mastrandera et al. (2010: 3).

Source: Author's elaboration based on Mastrandera et al. (2010).

Confidence scale

Agre

emen

t

Evidence (type, amount, quality, consistency)

To rate the degree of agreement among scientists regarding the evidence, it uses the terms “low,” “medium,” and “high.” Confidence levels are expressed using the terms “very low,” “low,” “medium,” “high,” and “very high.” If the likelihood of an outcome or a result has been assessed using statistical techniques, the IPCC reports probability values using the following terminology: “virtually certain” (99–100 per cent probability); “very likely” (90–100 per cent probability); “likely” (66–100 per cent probability); “about as likely as not” (33–66 per cent probability); “unlikely” (0–33 per cent probability); “very unlikely” (0–10 per cent probability); and “exceptionally unlikely” (0–1 per cent probability). Sometimes it also uses the terms “extremely likely” (95–100 per cent probability); “more likely than not” (>50–100 per cent probability); “more unlikely than likely” (0–<50 per cent probability); and “extremely unlikely” (0–5 per cent probability). For more details, see Mastrandrea et al. (2010).

Section 2 introduces the theoretical basis of the climate system and climate change. Section 2.1 familiarizes the reader with the concepts of weather, climate, and climate change, and intro-duces the five components of the climate system (i.e. atmosphere, hydrosphere, cryosphere, land surface, and biosphere). Section 2.2 focuses on the planet’s energy balance, which influences all five components of the climate system and is

therefore of crucial importance for the climate. The section stresses that the natural greenhouse effect plays an integral role in the planet’s en-ergy balance and is generally responsible for the relatively warm average temperature on the sur-face. With the module having outlined how the climate system works, Section 2.3 then explains that the climate not only changes in response to factors within the climate system but also in re-


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20 This section draws on the work of the IPCC, in particu-lar Chapter 1 of the contri-bution of Working Group I to the third IPCC assessment report (Baede et al., 2001), as well as several chapters of the fifth assessment report (especially Boucher et al., 2013, Cubasch et al., 2013, Masson-Delmotte et al., 2013, and Myhre et al., 2013).

sponse to some factors that are external to the climate system. In fact, some external factors (e.g. human activities) affect climate change drivers (e.g. greenhouse gases), which in turn affect the energy balance of the planet. Because the earth’s energy balance affects all five components of the climate system, external factors can thus affect the climate. Section 2.4 introduces two impor-tant concepts that allow for assessing how the planet’s energy balance changes as a result of externally induced variations in climate change drivers: radiative forcing and effective radiative forcing. These two important concepts allow for quantifying the extent to which different exter-nal factors influence the climate. Finally, Section 2.5 specifically focuses on the different ways in which humans affect the climate, altering at-mospheric concentrations of greenhouse gases and aerosols, and influencing properties of the land surface. It concludes by assessing the rela-tive strength of these different human-induced perturbations of the climate system, and high-lights the importance of feedback effects.

Section 3 then provides an overview of the main changes that have occurred in the climate system and highlights to what extent human activities have contributed to these observed changes. To this end, the section discusses observed changes in the means of temperature, precipitation, ice and snow cover, and sea levels. It then shows that not only the mean state of climate variables has changed, but that the frequency of extreme cli-mate events has also been affected. The section concludes with a short discussion on the impact of these observed changes on human and natu-ral systems.

To conclude Module 2, Section 4 looks at the fu-ture of the planet’s climate system. To this end, it introduces different scenarios describing pos-sible pathways of climate change drivers that are used by the IPCC’s climate models to project fu-ture changes in the climate system. After discuss-ing these scenarios, Section 4 highlights the most important predicted changes in the main climate variables (temperature, precipitation, snow and ice cover, and sea levels) that are likely to occur until 2100. The section ends by providing selected examples of anticipated future risks for humans and natural systems that might result from the simulated changes in the climate system.

At the end of this module, readers should be able to:

• Distinguish the five components of the cli-mate system;

• Understand the radiative balance of the planet;

• Explain how the natural greenhouse effect operates;

• Define and understand the concept of radia-tive forcing;

• Explain how economic activities can alter the climate;

• List major observed human-induced changes of the climate system;

• Discuss anticipated future impacts of climate change.

To support the learning process, readers will find several exercises and discussion questions in Sec-tion 5 covering the issues introduced in Module 2. Additional reading material can be found in Annex 2.

2 Thetheoreticalbasisoftheclimate systemandclimatechange20

When speaking about climate change, it is im-portant to clearly distinguish three distinct but related concepts: weather, climate, and climate change. Weather can be defined as the changing state of the atmosphere, which is characterized by temperature, precipitation, wind, clouds, etc. (Baede et al. 2001). Weather fluctuates frequently as a result of fast-changing weather systems. Weather systems, and hence the weather, can only be predicted with some degree of reliabil-ity for a very short period of time (one or two weeks). They are unpredictable over longer time horizons. Climate, on the other hand, is, loosely speaking, “long-term average weather.” IPCC (2001a: 788) defines climate as “the statistical description in terms of mean and variability of relevant quantities [such as temperature, precip-itation, and wind] over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization.” Climate not only varies from location to location (depending on a variety of factors such as distance to the sea, latitude and longitude, the presence of moun-tains, etc.) but also over time (e.g. from season to season, year to year, century to century, etc.). Based on this definition, IPCC (2001a: 788) defines climate change as “a statistically significant vari-ation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer).”

Understanding the interactions of the variety of factors that influence the climate is complicated. Climate, climate change, and the role of human activities in changing the climate can only be understood if one has an understanding of the whole climate system. The following section


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2looks briefly at the components of the planet’s climate system and shows how they interact.

2.1 Thefivecomponentsoftheclimatesystem

The climate system, schematically displayed in Figure 12, is part of the environmental system

discussed in Module 1. It consists of five main components (Baede et al., 2001): the atmosphere, hydrosphere, cryosphere, land surface, and bio-sphere (in bold in Figure 12). These five compo-nents interact with each other (thin arrows in Fig-ure 12) and are all affected by the planet’s energy balance, which will be discussed in Section 2.2.

TheclimatesystemFigure 12

Source: Baede et al. (2001: 88).

The most variable and rapidly changing part of the climate system is the atmosphere (Baede et al., 2001), which marks the boundary of our envi-ronmental system as defined in Module 1. The en-tire atmosphere lies within 500 km from the sur-face of the planet,21 with 99 per cent of its total mass within 50 km from the surface (Common and Stagl, 2004). The atmosphere can be subdi-vided (or stratified) into the five layers displayed in Figure 13. The troposphere is the lowest layer of the atmosphere and extends up to approxi-mately 11 km from the planet’s surface. It con-tains most of the atmosphere’s mass and plays a key role in determining the planet’s climate.

Mean temperature in the troposphere decreases with distance from the surface. In the text that follows, we will often refer to the so-called sur-face-troposphere system, which encompasses the planet’s surface and the troposphere. The next layer is called the stratosphere and extends to roughly 50 km above the surface. Most of the incoming ultraviolet radiation is absorbed by ozone that is concentrated in the stratosphere. The boundary between the troposphere and the stratosphere is called tropopause. The strato-sphere is followed by the mesosphere (50–90 km above the surface) and the thermosphere (90–500 km above the surface).22

21 We exclude here the exos-phere and consider that the atmosphere stops at the top of the thermosphere.

22 Many consider the ther-mosphere as the boundary of the atmosphere. But strictly speaking the thermosphere is followed by the exosphere (500–10,000 km above surface), which is considered by some to mark the actual boundary of the atmosphere and thus the environmen-tal system (discussed in Module 1) with the rest of the universe.

Atmosphere

Atmosphere-iceinteraction

Land-atmosphereinteraction

Precipitation-evaporation

Atmosphere-biosphereinteraction

Heatexchange

Changes insolar inputs

Soil-biosphereinteraction

Biosphere

Land surfaceSea ice

Changes in the ocean:

Ice-ocean coupling

Windstress Human influences

Terrestrial

Volcanic activity

radiation

Hydrosphere:rivers & lakes

Cryosphere:sea ice, ice sheets, glaciers

Clouds

Changes in thehydrological cycle

Hydrosphere:ocean

text

Changes in the atmosphere:composition, circulation

circulation, sea level, biogeochemistryChanges in/on the land surface:

orography, land use, vegetation, ecosystems

N2, O2, Ar,H2O, CO2,CH4, N2O, O3, etc.Aerosols

Glacier Ice sheet


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leLayersoftheatmosphereFigure 13

Source: National Aeronautics and Space Administration (NASA), Climate Science Investigations, available at: http://www.ces.fau.edu/nasa/module-2/atmosphere/earth.php.Note: The average temperature varies with altitude and is indicated by the red line.

The atmosphere is mainly a mixture of different gases but also contains some solid and liquid matter, namely aerosols and clouds. The com-position of the atmosphere has been changing throughout the history of the planet. Today the main bulk of the volume of the earth’s atmos-phere is composed of approximately 78.1 per cent nitrogen (N2), 20.9 per cent oxygen (O2), and 0.93 per cent argon (Ar). Additionally, the atmosphere contains several trace gases such as carbon di-oxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). These trace gases, which consti-tute less than 0.1 per cent of the atmosphere’s volume, are called greenhouse gases (GHG). Wa-ter vapour (H2O) is also a greenhouse gas whose volume in the atmosphere is volatile depending on the hydrosphere’s hydrological cycle. Despite their small share in the total volume of the at-mosphere, greenhouse gases are of crucial im-portance for the earth’s climate. While the three main gases (N2, O2, and Ar) do not absorb or re-flect the infrared radiation emitted by the planet, greenhouse gases are different – they absorb in-frared radiation coming from the planet’s surface and emit this radiation towards space and back towards the earth’s surface, thereby increasing the temperature at the surface (see Section 2.2 for a detailed discussion). Consequently, green-house gases are crucial for our planet’s climate (Baede et al., 2001).

The hydrosphere consists of all forms of liquid water and includes oceans, rivers, and lakes. Ap-proximately 70 per cent of the total surface of the planet is covered by water. Oceans alone store roughly 97 per cent of all forms of water (liquid, solid, and gas) available on earth, while rivers and lakes store roughly 0.009 per cent (Common and Stagl, 2004). The basic process that takes place in the hydrosphere is called the hydrological cycle. The cycle starts with the water evaporating from the oceans, lakes, and rivers and being released into the atmosphere, which leads to an exchange of heat between the hydrosphere and the atmos-phere. The water then returns from the atmos-phere to the surface in the form of precipitation, either directly into the oceans or indirectly on the land from where it reaches the oceans through riv-ers. Water that returns from land to oceans then in turn influences the composition and circulation of oceans. Through this cycle, oceans not only ex-change water but also heat energy, carbon dioxide, and aerosols with the atmosphere. As oceans are able to gradually store and release large quantities of heat, carbon dioxide, and aerosols over a long time period, they act as the planet’s climate regu-lator and thus are an important source of long-term natural climate variability (Baede et al., 2001).

The cryosphere, consisting of solid water, includes continental glaciers, snow fields, sea ice, perma-

2o1o

3o4o5o6o7o8o9o

1101oo

120130140150160170

490500510520

Exosphere

Thermosphere

Mesosphere

Stratosphere

Troposphere Altit

ude

(km

)100 150 200 500/150050-50 0-100

Temperature °C


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2frost, and the large ice sheets of Antarctica and Greenland. The cryosphere is relevant for the plan-et’s climate because of its capacity to store heat, its low capacity to transfer heat (in other words, its low thermal conductivity), its high reflectivity of incoming solar radiation, and its influence on ocean circulation and sea levels (Baede et al., 2001).

The land surface, made up of soils and vegetation, encompasses all parts of the planet that are not covered by oceans. Land surface matters for the climate because it determines how energy from solar radiation is returned to the atmosphere. Part of the energy from the sun is directly returned to the atmosphere as longwave infrared radiation and part of the energy is used to evaporate wa-ter, which then returns as water vapour to the at-mosphere. The texture of the land surface, which depends on the type of soil and/or vegetation cov-ering the surface, also influences the atmosphere indirectly, as different textures influence winds in different ways (Baede et al., 2001).

The biosphere is composed of all living organ-isms (also called biota). While it only represents a very thin layer of the planet (roughly 0.4 per cent of the planet’s radius (Common and Stagl, 2004), biota plays a key role in influencing the composition of the atmosphere and thus the climate of the planet. Plants perform photosyn-thesis, a process by which they use energy from solar radiation and specific enzymes to trans-form carbon dioxide from the atmosphere and water from the hydrosphere into glucose (a car-bohydrate) and oxygen (Figure 14). During this process, they extract large amounts of carbon from carbon dioxide present in the atmosphere and store it in the form of glucose, and release oxygen, a by-product of photosynthesis, into the atmosphere. The biosphere also has an impact on atmospheric concentrations of other greenhouse gases like methane or nitrous oxide (Baede et al., 2001). Some digestive processes of animal species release for instance methane as a by-product.

Photosynthesis–thechemicalreactionFigure 14

6 CO2 + 6 H2O C6H12 O6 + 6 O2

Carbon dioxide Water Glucose Oxygen

Enzymes

Solar radiation

Source: Author.

The five components of the climate system inter-act in numerous ways, as schematically illustrat-ed by the thin arrows in Figure 12. For instance, water vapour is exchanged between the atmos-phere and the hydrosphere, carbon dioxide is con-stantly extracted from the atmosphere by plants from the biosphere, and ice sheets from the cryo-sphere influence the hydrosphere’s ocean circu-lations and levels. These are just a few examples of the physical, chemical, and biological interac-tions that make the climate system extremely complex. Some of these processes are still only partly known, and there may be processes that are still completely unknown (Baede et al., 2001).

All five components of the system and all pro-cesses happening within the system use energy. The balance between energy flowing into the sys-tem and energy leaving the system is called the energy balance. Changes in the energy balance have a profound impact on all the components and processes of the system. Given the crucial importance of the energy balance, Section 2.2 fa-

miliarizes readers with this term and discusses the natural greenhouse effect that is an integral part of the balance.

2.2Theearth’senergybalanceandthenatural greenhouseeffect

The planet’s energy balance influences all five components of the climate system that were in-troduced in Section 2.1.23 Understanding the en-ergy balance and how it can be altered is there-fore of crucial importance for understanding the mechanisms behind climate change.

Solar radiation is the energy source that powers the entire climate system. Roughly 50 per cent of solar radiation consists of visible light, with the rest consisting mostly of infrared and ultraviolet light (Baede et al., 2001). New satellite-based data allow for accurately quantifying the exchange of radiative energy between the sun, the earth, and space. However, it is more difficult to quantify energy flows within the climate system because

23 In this material, we also use the term “radiative balance” as a synonym for energy balance.


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TheglobalmeanenergybalanceoftheearthFigure 15

Source: Wild et al. (2013: 3108).Note: TOA: top of the atmosphere. Wm-2: watts per square meter.

those flows cannot be directly measured. Conse-quently, it is not surprising that estimates of the global energy balance differ considerably (Wild, 2012). IPCC (2013a) updated its energy balance diagram in the fifth assessment report build-ing on the newly available estimates of Wild et al. (2013),24 which use satellite and ground-based radiation network data combined with models from the fifth IPCC assessment report.

Figure 15 provides a schematic illustration of the planet’s energy balance as outlined by Wild

et al. (2013) and used in the fifth IPCC report. Ar-rows represent radiation flows, numbers indicate best estimates of the magnitude of these flows, and numbers in parentheses provide the uncer-tainty range of these magnitudes, representing present-day climate conditions at the beginning of the 21st century. Accounting for day and night as well as for different yearly seasons, an aver-age amount of energy equivalent to 340 watts per square meter (Wm-2) enters the earth’s at-mosphere, and hence the environmental system, each second.

24 In the third and fourth assessment reports, the IPCC used the energy balance dia-gram of Kiehl and Trenberth (1997).

Of these 340 Wm-2, roughly 76 Wm-2 are directly reflected back to space by clouds, atmospheric gases, and aerosols. Another 24 Wm-2 reach the earth’s surface and are directly reflected back to space by the surface (due to surface reflec-tivity, technically referred to as surface albedo). As white light-coloured surfaces reflect more light compared to dark-coloured surfaces, most of these 24 Wm-2 are reflected back to space by snow fields, glaciers, ice sheets, and deserts. Of the remaining 240 Wm-2, roughly 79 Wm-2 are ab-sorbed by the atmosphere. This leaves 161 Wm-2 that warm the planet’s land surface and oceans (see the lower left-hand side of Figure 15). The planet’s land surface and oceans subsequently return this energy towards the atmosphere and space as sensible heat, water vapour, and long-wave infrared radiation.

In order to have a stable climate, there needs to be a balance between incoming solar radiation and out-going radiation emitted by the earth. Thus, the 240 Wm-2 of incoming radiation absorbed by the plan-et’s surface and atmosphere should be returned back to space. If this does not happen, the radiative balance of the planet is not in equilibrium. If signifi-cantly more radiation were to enter the planet than leave, the planet would become too hot for life; if significantly more radiation were to leave than en-ter, the planet would become too cold for life. Note that our planet’s energy balance is currently not in a complete equilibrium: Wild et al. (2013) as well as other recent studies (Hansen et al., 2011; Murphy et al., 2009; Trenberth et al., 2009) find a small posi-tive imbalance of the earth’s radiative balance. In fact, instead of 240 Wm-2, only 239 Wm-2 leave the planet (see the upper right-hand side of Figure 15).


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2The earth returns the incoming solar radiation as longwave infrared radiation,25 which is the heat energy you can feel, for instance, emanating out from a fire. The quantity and wavelengths of en-ergy radiated by physical objects are specific to the temperature of the object. Hotter objects ra-diate more longwave infrared radiation (in terms of magnitude and energy) than colder objects. In order to radiate 239 Wm-2 in longwave infrared radiation, the radiating surface should have an average temperature of roughly -19°C (Baede et al., 2001). However, the average temperature of the earth’s surface is not -19°C but 14°C. At this temperature, the surface alone radiates on aver-age 397 Wm-2 (Figure 15), which is a considerable higher amount than 239 Wm-2. So how is the planet able to “only” radiate the 239 Wm-2 re-quired to (almost) maintain its radiative balance and have at the same time a relatively high aver-age temperature on the surface?

As discussed in Section 2.1, the atmosphere con-tains several trace gases such as water vapour (H2O), carbon dioxide (CO2), methane (CH4), ni-trous oxide (N2O), and ozone (O3). These gases are able to absorb longwave infrared radiation from the surface of the planet and from the atmos-phere itself.26 Greenhouse gases then emit in-frared radiation in all directions. This means that they emit radiation towards space but also back towards the surface (see the right-hand side of Figure 15). The downward-directed flux, currently estimated at 342 Wm-2, heats up the lower lay-ers of the atmosphere and the surface, and thus maintains the relatively high surface tempera-ture of 14°C. Hence, greenhouse gases act like a “blanket” that traps heat in the lower layer of the atmosphere.27 This effect, known as the natural greenhouse effect, results in a net transfer of in-frared radiation from warm areas near the sur-face to higher levels of the atmosphere (Baede et al., 2001). The main part 28 of the 239 Wm-2 of out-going longwave radiation that are needed to bal-ance the incoming solar radiation is subsequent-ly radiated back towards space from relatively high altitudes and not directly from the surface. These areas in the higher level of the troposphere are approximately five km above the surface at mid-latitudes and have an average temperature of roughly -19°C (see the upper right-hand part of Figure 15). Thus, the natural greenhouse effect is an integral part of the planet’s energy balance system that is responsible for the relatively warm average temperature on the planet’s surface.

Having covered the components of the climate system as well as the radiative balance that in-fluences all these components, we can now turn

our attention towards climate change. Section 2.3 provides an overview and a classification of factors that can alter the climate system.

2.3 Internallyandexternallyinduced climatechange

Sections 2.1 and 2.2 showed that the earth’s cli-mate is shaped by factors that are internal to the climate system (i.e. processes within and be-tween components of the climate system, such as interactions between the atmosphere and oceans). It is important to understand that in addition to these internal factors, some external factors are also able to shape the climate. Why is this so?

Section 2.2 showed that the climate system is in its equilibrium if the net incoming solar radia-tion is balanced by the outgoing longwave radia-tion. Such an equilibrium is marked by a stable climate (e.g. stable mean temperature, mean precipitation, etc.). If the radiative balance of the planet changes, the climate is likely to change as well because through various interactions and feedback mechanisms a change in the radiative balance affects virtually all components of the climate system. For example, a change of the ra-diative balance can affect means or variances of climate variables but also other statistics such as the occurrence of extreme events (Baede et al., 2001; see Box 10 for a definition of extreme events). Hence, factors that are external to the climate system but somehow influence the ra-diative balance of the planet can also shape the climate. External factors can be further subdi-vided into natural external factors and human-induced (i.e. anthropogenic) external factors. The most obvious example of a natural external fac-tor is solar activity whose variations result in a changing amount of incoming solar radiation. Volcanic activity is another example: volcanic eruptions emit aerosol particles into the atmos-phere, which can influence the amount of incom-ing solar radiation reflected back towards space. Among the human-induced external factors, hu-man industrial activity influences greenhouse gas concentrations in the atmosphere and there-by affects the amount of longwave radiation that is being radiated from earth back towards space. In short, external factors (e.g. solar activ-ity, volcanic activities, or human activities) have an influence on so-called climate change drivers (e.g. solar radiation, aerosol particles, greenhouse gases),29 which in turn affect the radiative bal-ance and thereby shape the climate (see Figure 18 for a schematic illustration of externally induced climate changes).

25 In the text that follows we use the terms “longwave radiation,” “infrared radia-tion,” and “longwave infrared radiation” synonymously.

26 In other words, these greenhouse gases make the atmosphere opaque (i.e. impenetrable) to a lot of the longwave radiation emitted from the planet’s surface, but not to the incoming shortwave radiation, which explains why much of this incoming radiation can directly reach the surface.

27 Note that clouds also act as such a “blanket.” At the same time, due to their brightness, they reflect incoming solar radiation. As a net impact, clouds tend to have a slight cooling effect on the climate system (Baede et al., 2001).

28 With the exception of a small share of infrared radia-tion that is directly radiated from the surface through the so-called atmospheric window towards space.

29 Drivers of climate change are substances and processes that alter the planet’s energy balance.


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Extremeevents-schematicpresentationFigure 16

ExtremeeventsBox 10

30 Internally induced climate change means that factors internal to the climate sys-tem affect the mean or the variability of the climate, while externally induced climate change means that factors external to the climate system affect the mean or the variability of the climate.

31 See the IPCC reports listed in Annex 2 for additional rea-dings on internally induced climate variability.

32 Note that these concepts can a priori also be used to measure the influence of most internal factors on climate change.

Climate change does not only affect the means of climate variables such as temperature or precipitation, but can also affect the likelihood of the occurrence of extreme weather and climate events (Cubasch et al., 2013). Examples of such extreme events are droughts, cyclones, or heat waves. The Intergovernmental Panel on Climate Change (IPCC) defines an extreme weather event as an event that is “rare at a particular place and/or time of year. Definitions of ‘rare’ vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations” (Cubasch et al., 2013: 134). Extreme climate events can be defined as extreme weather events that persist for some time (Cubasch et al., 2013).

Cubasch et al. (2013) show that statistical reasoning can illustrate that increases or decreases of the frequency of extreme weather events (e.g. an increase of extremely hot days) can result from small changes in the dis-tribution of climate variables (e.g. an increase in the mean temperature). For example, Figure 16 displays the probability density function of temperature. Note that temperature is almost normally distributed (other climate variables such as precipitation are not normally distributed but have skewed distributions). Now suppose that climate change increases the mean temperature. As a result, the probability density function of temperature shifts to the right (i.e. average temperature increases) as illustrated by the solid curve in Figure 16. This shift of the average temperature affects the frequency of extreme events. On the one hand, one ob-serves more hot extremes; on the other, one observes fewer cold extremes. Changes in the variance, skewness, or shape of distributions can also affect the frequency of extreme events (see Cubasch et al., 2013: 134–35, for a more detailed discussion).

HotAverage

Temperature

Cold

Fewer cold extremes

(a) Increase in mean

More hot extremes

Source: Author's elaboration based on Cubasch et al. (2013:134).

The climate of our planet is thus shaped by fac-tors that are internal to the climate system and by factors that are external to the climate sys-tem (Baede et al., 2001). This implies that climate change, which is defined as a persistent variation in either the mean state of the climate or in its variability (see the introduction to Section 2), can be induced internally or externally.30 The El Niño-Southern Oscillation (ENSO), described in Box 11, is an example of an internally induced climate vari-ability (Baede et al., 2001). The ENSO is the result of an interaction between the atmosphere and

the Pacific ocean, and affects different climate variables such as precipitation and temperature in many parts of the world. As this teaching ma-terial focuses on human-induced (i.e. externally induced) climate change, we will not address internally induced climate variability further,31 and limit ourselves to a discussion of externally induced climate change. To do so, Section 2.4 will introduce two concepts – radiative forcing and ef-fective radiative forcing – that allow for measur-ing the influence of natural and human-induced external factors on climate change.32


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2

Correlationsofsurfacetemperature,precipitationandmeansealevelpressurewiththeSouthernOscillationIndex

Figure 17

TheElNiño-SouthernOscillation–anexampleofaninternalinteractionamongcomponentsoftheclimatesystemaffectingthemeansandvariabilityofdifferentclimatevariables

Box 11

El Niño-Southern Oscillation (ENSO) events are naturally occurring phenomena that result from an interaction be-tween the atmosphere and the hydrosphere. According to the Intergovernmental Panel on Climate Change (IPCC), “El Niño involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America, weakening the usually strong sea surface temperature (SST) gradient across the equato-rial Pacific, with associated changes in ocean circulation. Its closely linked atmospheric counterpart, the Southern Oscillation (SO), involves changes in trade winds, tropical circulation and precipitation” (Trenberth et al., 2007: 287). Historically, the ENSO alternates between two states: El Niño and La Niña, each of which has specific regional impacts on climate variables such as temperature or precipitation (Figure 17). For example, surface temperature is above average during El Niño events and below average during La Niña events in the eastern tropical Pacific region. El Niño events occur every 3 to 7 years and alternate with their counterpart La Niña (Trenberth et al., 2007).1

Source: Trenberth et al. (2007). Note: Correlations with the Southern Oscillation Index (SOI), based on standardized Tahiti minus Darwin sea level air pres-sure, for annual (May to April) means of sea level air pressure (top left), surface temperature (top right) for 1958 to 2004, and precipitation for 1979 to 2003 (bottom left). In the SOI graph (bottom right), red (blue) values indicate El Niño (La Niña) condi-tions. The graph shows the long-term periodic fluctuation between these conditions since 1850.

Source: Author's elaboration based on Trenberth et al. (2007).1 An intuitive explanation of the El Niño-Southern Oscillation can be found in geoscientist Keith Meldahl’s video (available at https://www.youtube.com/watch?v=GTgz6ie2eSY). A more extensive explanation of the phenomenon is provided by a Yale University open course given by Ronald Smith, a professor of geoscience, geophysics and mechanical engineering, available at https://www.youtube.com/watch?v=bK-n0CeFWtk.

The ENSO influences regional climate patterns in several parts of the globe and has a global impact on climate variables such as surface temperature and precipitation. Figure 17 illustrates these effects based on annual mean correlations between the climate variables and the Southern Oscillation Index (SOI). The SOI (bottom right panel) uses observed atmospheric pressure at sea level to infer the presence of El Niño and La Niña events. It is calculated as the standardized air pressure in Tahiti (Eastern Pacific) minus the standardized air pressure in Darwin, Australia (Western Pacific). Positive values (higher pressure in Tahiti) indicate a La Niña event; negative values (higher pressure in Darwin) indicate an El Niño event. The bottom left panel of Figure 17 shows the correlation between SOI and precipitation: one observes, for example, a strong positive correlation among the variables over the Western Pacific, indicating that this region experiences above (below) average precipitation during La Niña (El Niño) events. The upper right panel displays the correlation between SOI and surface temperature. One observes, for example, that there is a strong negative correlation between the two variables over the eastern tropical Pacific region, indicating that in this region, surface temperature is above (below) average during El Niño (La Niña) events. Thus, as shown in Figure 17, internal interactions such as the El Niño-Southern Oscillation can have significant effects on the climate.

192018901860 1950 1980 2010

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2.4Measuringtheimportanceoffactors drivingclimatechange:radiativeforcing andeffectiveradiativeforcing

In theory, there are many ways to assess how strongly different external factors change the climate. One could, for instance, try to directly measure the effect of a change in a single cli-mate change driver (e.g. greenhouse gases) that has been induced by an external factor (e.g. hu-man industrial activity) on different climate variables (e.g. temperature and precipitation). However, identifying and isolating such effects is extremely difficult. Climate scientists there-fore rely on intermediate measures that quan-tify the influence of external factors on climate change indirectly. The basic idea behind these measures is rather intuitive. First, climate scien-tists measure how strongly the radiative balance is affected by an externally induced variation in a climate change driver. Then they estimate how this change of the radiative balance affects the climate (Figure 18). To do so, they use a measure called “radiative forcing.”

Radiative forcing is the most commonly used in-dicator that allows for capturing how externally induced changes in climate change drivers affect the radiative balance and subsequently change the climate (Myhre et al. 2013). The term “forcing” indicates that the radiative balance of the earth is forced away from its equilibrium state by an externally induced variation in a climate change driver (Perman et al., 2011).33 Intuitively, radiative forcing measures the radiative imbalance that occurs from an externally induced change in a climate change driver. IPCC (2001a: 795) defines radiative forcing as “the change in the net vertical irradiance (expressed in Watts per square metre: Wm-2) at the tropopause34 due to… a change in the external forcing of the climate system, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun.”

While the radiative forcing concept is the most widely used measure to assess and compare the size of the radiative imbalance created by externally induced variations in climate change drivers, it has some weaknesses. The main one is that the concept keeps all surface and tropospheric properties fixed and does not allow them to respond to the changes induced by the variations in climate change driv-ers. In the fifth assessment report, the IPCC there-fore introduced a new, complementary concept called “effective radiative forcing.” Radiative forc-ing and effective radiative forcing are very similar, with the exception that effective radiative forcing allows some surface and tropospheric properties to respond to perturbations in the short term. Ef-fective radiative forcing is defined as “the change in [the] net top of atmosphere downward radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust, but with sur-face temperature or a portion of surface conditions unchanged… Hence effective radiative forcing in-cludes both the effects of the forcing agent itself and the rapid adjustments to that agent (as does radiative forcing, though stratospheric tempera-ture is the only adjustment for the latter)” (Myhre et al., 2013: 665). Due to the inclusion of short-term adjustments of some surface and tropospheric properties, the effective radiative forcing concept is believed to be a better indicator of potential tem-perature responses (Myhre et al., 2013).

Radiative forcing (and effective radiative forc-ing) can be negative or positive. Positive radiative forcing implies that incoming radiation is larger than outgoing radiation, leading to an energy increase in the environmental system (i.e. a posi-tive energy imbalance). To rebalance the system, temperatures in the surface-troposphere system have to increase. Negative radiative forcing im-plies that incoming radiation is smaller than out-going radiation (i.e. a negative imbalance), lead-ing to an energy decrease in the environmental system. To rebalance the system, temperatures in the surface-troposphere system have to decrease.

33 Climate change drivers are subsequently also called “for-cing agents,” while externally induced variations in climate change drivers are also called “external forcings” or some-times simply “forcings.” For the sake of simplicity, we do not adopt this terminology in this teaching material and continue to use the terms “climate change driver” and “externally induced variations in climate change drivers.” However, the terms “external forcings,” “forcings,” and “forcing agents” do appear in direct citations from the IPCC.

34 The tropopause is the boundary between the troposphere and the stratosphere (see Figure 13). For practical reasons, the tropopause is defined as the top of the atmosphere (see Ramaswamy et al., 2001, for a detailed discussion).

Source: Author's elaboration based on Forster et al. (2007: 134). Note: Non-initial radiative forcing effects have been omitted from the figure as they are not addressed by this teaching material. Feedback effects are discussed in Section 2.5.

Natural external factors

(e.g. solar activity, volcanic activity)

Human-inducedexternal factors(e.g. industrial activity)

Radiativeforcing

Feedbackeffects

Direct and indirectchanges in climate change drivers

(e.g. greenhouse gases, aerosols, solar irradiance)

Climate perturbationsand responses

(e.g. temperature, precipitation, extreme weather events)

ExternallyinducedclimatechangesFigure 18


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2Before we focus entirely on human influences on the climate in Section 2.5, let us briefly il-lustrate the radiative forcing concept by listing some examples of radiative forcing induced by certain natural external factors. First, let us con-sider solar activity. Solar activity, and hence solar output, is not constant but fluctuates over time at various time scales, including centennial and millennial scales (Myhre et al., 2013). Solar output has increased gradually during the industrial era (from 1750 up until today), which has led to an increase in incoming solar radiation and thereby caused a small amount of positive radiative forc-ing. This has had a small warming effect on the surface-troposphere system (Forster et al., 2007). Another natural external factor, the astronomical alignment of the sun and the earth, also varies and induces cyclical changes in radiative forcing. However, these changes are only substantial over very long time horizons, and partially explain, for instance, different climatic periods such as ice ages (Myhre et al., 2013). Volcanic eruptions are another natural external factor that can, over a short period of time lasting from several months up to a year, increase the concentration of sul-phate aerosol particles in the stratosphere that block parts of incoming solar radiation, induc-ing short-term negative radiative forcing, which tends to have a cooling effect on the surface-troposphere system (Forster et al., 2007).

While such natural external factors are important, they have played a relatively small role during the industrial era. In its fifth assessment report, the IPCC states that “there is a very high confidence that industrial-era natural forcing is a small frac-tion of the anthropogenic forcing except for brief periods following large volcanic eruptions. In particular, robust evidence from satellite obser-vations of the solar irradiance and volcanic aero-sols demonstrates a near-zero (-0.1 to +0.1) Wm-2 change in the natural forcing compared to the anthropogenic effective radiative forcing increase of 1.0 (0.7 to 1.3) Wm-2 from 1980 to 2011. The natu-ral forcing over the last 15 years has likely offset a substantial fraction (at least 30 per cent) of the anthropogenic forcing” (Myhre et al., 2013: 662).

2.5 Human-inducedclimatechange

After outlining basic mechanisms that influence the climate system and introducing concepts that allow for measuring climate change, we are now equipped with the necessary tools to ana-lyse human-induced climate change.

Human activities cause changes in the amounts of greenhouse gases, aerosols, and clouds in the earth’s atmosphere. These human-induced

changes in climate change drivers influence the planet’s radiative balance and hence the climate system. Human activities also change the land surface of the planet, which can effect, for in-stance, surface reflectivity (albedo), influencing the radiative balance and thus also the climate system (IPCC 2001b, 2007, 2013a). The sections that follow first discuss these human-induced changes in climate change drivers separately, then assess their respective impact on the radia-tive balance, and finally take a look at feedback effects that can amplify or reduce the impacts of the different climate change drivers.

2.5.1 Human-induced greenhouse gas emissions and the enhanced greenhouse effect

The main greenhouse gases emitted by human activities are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halocarbons (Forster et al., 2007). Carbon dioxide, methane, nitrous oxide, and many halocarbons are called “well-mixed greenhouse gases” because they mix sufficiently in the troposphere for reliable concentration measurements to be made from only a few re-mote observations (Myhre et al., 2013).

Anthropogenic carbon dioxide emissions mainly result from human use of fossil fuels in sectors such as transportation, energy, and ce-ment production. Anthropogenic methane is mostly emitted during agricultural activities and natural gas distribution, and from landfills. Anthropogenic nitrous oxide emissions stem mostly from burning of fossil fuels and the use of fertilizers in agricultural soil. Anthropogenic halocarbons, which include chlorofluorocarbons (see also Box 14 in Module 3), are emitted by di-verse industrial activities and have in the past also been released by refrigeration processes. In addition to the four main greenhouse gases, human activities also emit other pollutants such as carbon monoxide (CO), volatile organic compounds, nitrogen oxides (NOx), and sulphur dioxide (SO2). While these gases are negligible greenhouse gases, they indirectly influence con-centrations of other greenhouse gases such as methane or ozone through chemical reactions (Cubasch et al., 2013).

So how strongly have humans influenced atmos-pheric concentrations of greenhouse gases?

Instrumental measurements provide accurate atmospheric GHG concentrations back to 1950, while indirect measures are used for dates prior to 1950: ice core data allow for analysing air bub-bles enclosed in ice and thus provide an indirect record of past atmospheric concentrations of


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Source: Forster et al. (2007: 135).Note: Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample.

well-mixed greenhouse gases (Masson-Delmotte et al., 2013). Figure 19 combines data based on in-strumental measurements and ice core data to show that while atmospheric carbon dioxide,

methane, and nitrous oxide concentrations were fairly stable for more than a thousand years be-fore the industrial revolution (starting around 1750), they have increased rapidly since then.

Atmosphericcarbondioxide,methaneandnitrousoxideconcentrationsfromyear0to2005Figure 19

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Ice core data also allow us to look further back in history and track atmospheric GHG concentra-tions dating several hundreds of thousands of years. The fifth IPCC assessment report provides information covering the past 800,000 years (IPCC, 2013a). Data indicate that pre-industrial ice core GHG concentrations stayed within natural limits. For carbon dioxide, maximum concentra-tions of 300 parts per million (ppm) and mini-mum concentrations of 180 ppm have been found. For methane, data indicate maximum concentra-tions of 800 parts per billion (ppb) and minimum concentrations of 350 ppb. And for nitrous oxide, ice core data show maximum concentrations of 300 ppb and minimum concentrations of 200 ppb (Masson-Delmotte et al., 2013). The fifth IPCC assessment report reaches the conclusion that “it is a fact that present-day (2011) concentrations of CO2 (390.5 ppm), CH4 (1803 ppb) and N2O (324 ppm) exceed the range of concentrations recorded in the ice core records during the past 800 ka.35 With very high confidence, the rate of change of the observed anthropogenic WMGHG [well-mixed greenhouse gases] rise and its RF [radiative forc-ing] is unprecedented with respect to the highest resolution ice core record back to 22 ka for CO2, CH4 and N2O, accounting for the smoothing due to ice core enclosure processes. There is medium confi-dence that the rate of change of the observed an-thropogenic WMGHG rise is also unprecedented with respect to the lower resolution records of the past 800 ka” (Masson-Delmotte et al., 2013: 391).

In short, given the data and the methods to de-termine the origin of GHG emissions,36 the sci-

entific community has reached a consensus: hu-mans have substantially altered the composition of the atmosphere and continue to do so. Since the industrial revolution, anthropogenic GHG emissions have substantially increased GHG con-centrations in the atmosphere (IPCC 2001b, 2007, 2013a). The rate of this increase is unprecedented over the last 800,000 years, and the concentra-tions are currently higher than all concentra-tions recorded in ice cores over those years (IPCC, 2013a). So how does this human-induced change of the atmosphere affect the climate?

The answer is relatively straightforward: the anthropogenic increase in GHG concentrations amplifies the natural greenhouse effect (see Sec-tion 2.2). Increased GHG concentrations lead to an increased rate of absorption and subsequent emissions of infrared radiation coming from the surface. In other words, increased GHG concen-trations increase the atmosphere’s opacity to longwave radiation, but more so at lower alti-tudes where air density and GHG concentrations are higher than at higher altitudes where they are both lower. Thus the share of upward flux longwave radiation leaving the atmosphere from higher, relative to lower altitudes, increases. As a result of the increase in GHG concentrations, the altitudes from where earth’s radiation is emit-ted towards space thus become higher. At these higher altitudes, the troposphere is colder (Figure 13) and therefore, less energy is emitted towards space, causing a positive radiative forcing (Baede et al., 2001). This effect is called the enhanced greenhouse effect.

35 ka is a unit of time indica-ting a thousand years. 36 For instance, one can analyse the changing isotopic composition of atmospheric CO2. By doing so, it can be shown that the observed increase in the atmospheric CO2 concen-tration is of anthropogenic origin, because the changing isotopic composition of atmospheric CO2 betrays the fossil origin of the increase (Baede et al., 2001).


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22.5.2 Human-induced changes in aerosols

In addition to increasing concentrations of greenhouse gases, human activities also increase the amount of aerosols in the atmosphere (IPCC 2001b, 2007, and 2013a). Atmospheric aerosols are generated either by emissions of primary particulate matter or by formation of second-ary particulate matter from gaseous precursors. These consist mainly of inorganic compounds (e.g. sulphate, nitrate, ammonium, sea salt), or-

ganic matter, black carbon, mineral compounds (e.g. desert dust) and primary biological aerosol particles (Boucher et al., 2013). Humans emit aer-osols through diverse industrial, energy-related, and land-use activities (Baede et al., 2001). Unlike greenhouse gases, aerosols remain in the atmos-phere only for a short time as they are washed out by rain. Figure 20 provides an overview of key properties of the main aerosols in the tropo-sphere, including their main sources and their most important climate-relevant properties.

KeypropertiesofmainaerosolsinthetroposphereFigure 20

Aerosol species Size distribution Main sources Main sinks Tropospheric

lifetimeKey climate-

relevant properties

Sulphate Primary: Aitken, accumulation and coarse modesSecondary: Nucleation, Aitken, and accumula-tion modes

Primary: marine and volcanic emissionsSecaondary: oxidation of SO2 and other S gases from natural and anthro-pogenic sources

Wet depositionDry deposition

~ 1 week Light scattering. Very hygroscopic. Enhances absorp-tion when depos-ited as a coating on black carbon. Cloud condensation nudei (CCN) active.

Nitrate Accumulation and coarse modes

Oxidation of NO2 Wet depositionDry deposition

~ 1 week Light scattering. Hygroscopic. CCN active

Black carbon Freshly emitted: <100 nmAged: accumulation mode

Combustion of fossil fuels, biofuels and biomass


1 week to 10 days

Large mass absorp-tion efficiency in the shortwave. CCN active when coated. May be ice nudei (IN) active.

Organic aerosol

POA: Aitken and accumulation modes. SOA: nucleation, Aitken and mostly accumula-tion modes. Ages OA: accumulstion mode

Combustion of fossil fuel, biofuel and biomass. Continetal and marine ecosystems. Some an-thropogenic and biogenic non-combustion sources


~ 1 week Light scattering. Enhances absoption when desposited as a coatind on black carbon: CCN active (depending on ag-ing time and size).

… of which brown carbon

Freshly emitted: 100–400 nmAged: accumulation mode

Combustion of biofuels and biomass. Natural humic-like substances from the biosphere


~ 1 week Medium mass absorption efficiency in the UV and visible. Light scattering.

… of which terrestrial PBAP

Mostly coarse mode Terrestrial ecosystems SedimentationWet depositionDry deposition

1 day to 1 week depending on size

May be IN active. May from giant CCN

Mineral dust Coarse and super-coarse ,modes, with a small accumulation mode

Wind erosion, soil resus-pension. Some agricultur-al practices and induytrial activities (cement)

SedimentationWet depositionDry deposition


IN active: Light scattering and absorption. Grennhouse effect.

Sea Spray Coarse and accumulation modes

Breaking of air bubbles induces e.g., by wave breaking. Wind erosion.



Light scattering. Very hygroscopic. CCN active. Can include primary organiccompounds in smaller size range

… of whichmarine POA

Preferentially Aitken and accumulation modes

Emitted with sea spray in biologically active oceanic regions


~ 1 week CCN active.

Source: Boucher et al. (2013: 597).Note: CNN: cloud condensation nuclei; IN: ice nuclei; OA: organic aerosols; POA: primary organic aerosols; PBAP: primary biological aerosol particles; SOA: secondary organic aerosols; UV: ultraviolet.


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The effects of increased amounts of aerosols on ra-diative forcing and hence the climate are still not fully known (Baede et al., 2001). As the fifth IPCC assessment report states, aerosols and clouds are still contributing the largest uncertainty to estimates of the changing energy balance of the planet (Boucher et al., 2013). Two main channels of effect have been identified. First, some aerosols (Figure 20) absorb and scatter incoming solar ra-diation, and to some extent also longwave radia-tion from the surface. They thus directly affect the radiative balance of the planet by sending parts of the incoming radiation back into space (hence one speaks of the “direct effect”). This direct ef-fect causes a negative radiative forcing (Baede et al., 2001; Boucher et al., 2013). Second, aerosols interact with clouds. According to Boucher et al. (2013), some aerosols (Figure 20) act as cloud con-densation nuclei (CCN) and ice nuclei (IN), around which cloud droplets and ice crystals form. Aero-sols thus play an important role in cloud forma-tion and are often described as “cloud seeds.” By interacting with clouds, aerosols indirectly influ-ence cloud albedo and the lifetimes of clouds (this effect is often referred to as the “indirect effect”). This has an impact on the reflection of incoming shortwave radiation as well as the absorption of outgoing longwave radiation by the atmosphere and thus on the planet’s energy balance.

2.5.3 Human-induced land-use change

Humans not only affect the climate by emit-ting GHG emissions and aerosols, but also by changing the surface characteristics of land. This so-called “land-use change” can result from various human activities including agriculture, irrigation, deforestation, urbanization, and traf-fic, and it influences physical and/or biologi-cal properties of the land surface (Baede et al., 2001). Hurtt et al. (2006) estimate that between 42 and 68 per cent of the total land surface was affected by human activities over the 1700–2000 period.

By changing the land surface, humans directly and indirectly alter the planet’s energy balance, water cycles, carbon cycles, and heat fluxes (Cu-basch et al., 2013; Myhre et al., 2013). Changes in physical properties of the land surface can influ-ence the reflectivity of the surface (land albedo) and hence affect the amount of incoming solar radiation reflected towards space, thereby direct-ly influencing the energy balance of the planet (Baede et al., 2001). Irrigation and other water-intensive activities can affect the water cycle and thus indirectly affect the energy balance. Chang-

es in biological properties can also have impor-tant consequences, especially in terms of GHG emissions. If humans convert forests into culti-vable land, they cut or burn the existing forests. They thereby destroy carbon sinks, which reduces carbon storage, releases carbon dioxide into the atmosphere, and changes surface albedo (Cu-basch et al., 2013). The combined effect of these physical and biological changes is complex and difficult to assess.

2.5.4 Human-induced radiative forcing

Sections 2.5.1 to 2.5.3 explained different ways in which humans influence climate change drivers, and thereby affect the radiative balance of the earth and thus change the climate. This leads to the question of the importance of the different ef-fects as well as the overall effect humans have on the climate. The previously introduced radiative forcing and effective radiative forcing concepts are very useful to this end. IPCC (1996, 2001, 2007, 2013a) estimated mean radiative forcing of each of the climate change drivers that are affected by humans. As knowledge about the climate system and climate change is constantly growing, these numbers have been revised several times. Figure A1 in Annex 1 provides an overview of the esti-mates of the different assessment reports.

Figure 21 displays the IPCC (2013a) estimates of radiative forcing (hatched) and effective radia-tive forcing (solid) of different climate change drivers over the 1750–2011 period. Uncertainties are displayed by dotted and solid lines that in-dicate 5 to 95 per cent confidence intervals. They vary widely depending on the climate change driver under consideration. The largest positive radiative forcings are due to carbon dioxide and other well-mixed greenhouse gases. Together, their human-induced concentration increases are responsible for an estimated radiative forc-ing of 2.83 Wm-2. Human-induced increases of tropospheric ozone (where ozone acts as a green-house gas) caused an effective radiative forcing of 0.4 Wm-2. Ozone increases in the stratosphere (where they block parts of incoming solar radia-tion) caused a negative radiative forcing of -0.1 Wm-2. Changes in surface albedo caused by land-use change are estimated to cause negative ef-fective radiative forcing (-0.15 Wm-2), while black carbon particles on snow and ice cause positive effective radiative forcing (0.04 Wm-2). The great-est uncertainty is associated with aerosols. While both aerosol-radiation and aerosol-cloud inter-actions are estimated to cause negative radiative forcing, confidence intervals are very large.


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2Radiativeforcingoftheclimatebetween1750and2011

Figure 21

Anth

ropo

geni

c

Forcing agent

Well-mixed greenhouse gases

Ozone

Surface albedo

Contrails

Aerosol-radiation interaction

Aerosol-cloud interaction

Total anthropogenic

Solar irradiance

Stratospheric water vapour from CH4

Nat

ural

Radiative forcing (Wm-2)-1 0 1 2 3

Contrail-induced cirrus

Black carbon on snowLand use

Stratospheric

Other WMGHGHalocarbons

CH4 N2O

CO2

Tropospheric

Source: Myhre et al. (2013: 697).Note: This figure is a bar chart for radiative forcing (hatched) and effective radiative forcing (solid) for the period 1750–2011. Uncertainties (5 to 95 per cent confidence range) are given for radiative forcing (dotted lines) and effective radiative forcing (solid lines). WMGHG: well-mixed greenhouse gases.

According to IPCC findings, total anthropogenic radiative forcing is virtually certain to be positive and is estimated to be 2.3 Wm-2. The probability of negative anthropogenic radiative forcing is estimated to be smaller than 0.1 per cent and thus exceptionally unlikely (Myhre et al., 2013). This confirms that human activities contribute to increase the level of solar energy stored in the climate system, which results in the warming of the lower atmosphere and the earth’s surface. Compared to the human influence, natural fac-tors had only a low influence on the planet’s en-ergy balance and thus on climate change (Figure 21). The IPCC (Myhre et al., 2013: 661) therefore concludes that “it is unequivocal that anthropo-genic increases in the well-mixed greenhouse gases [WMGHG] have substantially enhanced the greenhouse effect, and the resulting forcing continues to increase. Aerosols partially offset the forcing of the WMGHGs and dominate the uncertainty associated with the total anthropo-genic driving of climate change.”

2.5.5 Feedback effects and non-linearity

While we have seen that anthropogenic radia-tive forcing has been positive and has increased during the industrial era, hence warming the climate system, we have not yet discussed what specific impacts this radiative forcing has had on the climate system (see Section 3 for a dis-cussion of observed impacts, and Section 4 for a discussion of anticipated impacts). A priori we know that positive (negative) radiative forcing increases (decreases) the temperature of the cli-mate system.

However, this relationship is complicated because of the existence of so-called feedback effects that can either amplify or diminish the effect that specific radiative forcings have on variables such as temperature or precipitation (IPCC 2001, 2007, 2013a). Feedback effects that amplify the effect of driver-induced radiative forcing are called “posi-tive feedbacks,” while those that reduce the ef-fect of driver-induced radiative forcing are called “negative feedbacks” (Le Treut et al., 2007). Due to these internal feedback mechanisms, climate variables like temperature will in general not re-act in a linear way to changes in climate change drivers (IPCC 2001, 2007, 2013a).37 While a detailed discussion of all known feedback mechanisms is out of the scope of this teaching material, Figure 22 provides some examples that have been listed in the first chapter of the fifth IPCC report (Cu-basch et al., 2013).

Figure 22 schematically displays some key feed-back mechanisms related to increases in carbon dioxide concentrations that, as we have seen, cause positive radiative forcing and hence tend to warm the surface-troposphere system, all else being equal. Some of these feedback mecha-nisms are positive (i.e. they additionally warm the system), some are negative (i.e. they cool the system), and some can be positive or negative. For example, the so-called water-vapour feed-back is a positive feedback: as the planet gets warmer due to the stronger greenhouse effect, more water evaporates. This leads to an increase of water vapour in the atmosphere, where it acts as a strong greenhouse gas, further enhancing the greenhouse effect. Snow and ice albedo are

37 The example in the next paragraph illustrates this non-linearity.


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PositiveandnegativefeedbackmechanismsFigure 22

Source: Cubasch et al. (2013:128).Note: Climate feedbacks related to increasing CO2 and rising temperature include negative feedbacks (-), positive feedbacks (+) and positive or negative feedbacks (±). The smaller box highlights the large difference in timescales for the various feedbacks. GHG: greenhouse gas.

at the root of another positive feedback mecha-nism: increasing temperatures cause additional snow and ice to melt, which changes the size of the land surface. White reflecting surfaces are transformed into darker, absorbing surfaces. This lowers the planet’s reflectivity of incoming solar radiation and thus increases the amount of energy the planet’s surface absorbs, leading

to additional warming. If the average tempera-ture of the planet increases, the planet starts to radiate more infrared radiation, sending more energy back towards the atmosphere and ulti-mately space. This infrared radiation mechanism is an example of a negative feedback mechanism, which is however almost negligible for a small temperature change of the order of 1-4°C.

Shortsummary

Section 2 reviewed basic elements of climate science. It showed that the climate system has five components: atmosphere, hydrosphere, cryosphere, land surface, and biosphere. These components interact in complex ways. All of them depend on the planet’s energy balance (or radiative balance), i.e. the difference between the energy that flows into the climate system and the energy that flows out of the climate system. For a stable climate, incoming energy should equal outgoing energy. Section 2 explained that climate can change due to variations in factors that are either internal or external to the climate system. External factors such as hu-man activities can affect the climate by influencing climate change drivers such as atmospheric greenhouse gas concentrations. Externally induced changes in climate change drivers then change the energy balance of the planet and thus affect the climate. The radiative forcing concept introduced in Section 2 measures how strongly external factors influence the radiative balance. Positive radiative forcing indicates that more energy is flowing into the system than out of the system, leading to higher temperatures. Negative radiative forcing indicates that less energy is flowing into the system than out of the system, leading to lower tempera-tures. Section 2 then showed that human activities influence several climate change drivers (atmospheric greenhouse gases, aerosol concentrations, reflectivity of the planet’s surface) and have thereby caused overall positive radiative forcing since the start of the industrial era, which is heating up the planet. Section 2 con-cluded by highlighting the importance of feedback mechanisms that can amplify or diminish the effects that positive or negative radiative forcing has on different climate variables such as temperature or precipitation.


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23 Observedchangesin theclimatesystem

Section 2 introduced the climate system and ex-plained how it can be affected by human activi-ties. This section provides a short summary of hu-man-induced changes of the climate system that have been observed and assessed by the scientific community. To do so, we focus on observed chang-es as reported in the fifth IPCC assessment report.

IPCC reports observed changes in the climate based on research undertaken using a variety of different instrumental measurements. These measurements are either on-site measurements, which measure variables such as temperature or precipitation di-rectly on site, or off-site measurements. Off-site measurements (also called “remote sensing”) gath-er data from so-called remote sensing platforms,38 which measure climate variables from a certain distance (i.e. “off-site”). Instrumental data are com-plemented with paleoclimate reconstructions (e.g. the ice core data used to analyse carbon dioxide concentrations mentioned in Section 2.5), which al-low for extending the covered period considerably.

Based on an extensive review of available re-search, IPCC (2013b: 4) finds that the “warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprec-edented over decades to millennia. The atmos-phere and ocean have warmed, the amounts of snow and ice have diminished, sea level has ris-en, and the concentrations of greenhouse gases have increased.” In the text that follows, we take a closer look at the most important climate pertur-bations and responses (see the scheme in Figure

18) – namely, observed changes in mean tempera-ture, precipitation, ice cover, sea levels, and occur-rence of extreme events. The section concludes with a short overview of the impacts of these ob-served changes on human and natural systems.

3.1 Observedchangesintemperature

Climate change has resulted in an increase in the annual average global surface temperature. Data from several independent datasets concur that the combined mean land and ocean surface tem-peratures increased by 0.85°C over the 1880–2012 period. The magnitude of this increase lies with 90 per cent probability within the range of 0.65°C to 1.06°C. Over the 1951–2012 period, this increase is estimated to have been 0.72°C, lying with 90 per cent probability within the range of 0.49°C to 0.89°C (Hartmann et al., 2013). The longest avail-able dataset with data sufficiently detailed to al-low for calculating regional trends shows that al-most the entire planet experienced an increase of average surface temperature over the 1901–2012 period, as illustrated by Figure 23. It is important to note that while the long-term warming trend is highly robust, short-term temperature trends vary due to natural variability and are thus sensi-tive to the start and end year of the period under observation. This leads to a high inter-annual and decadal variation in short-term warming trends (Hartmann et al., 2013). As an example, the IPCC states that the rate of warming from 1998 to 2012 was slower than the overall rate from 1951 to 2012. The IPCC explains this lower rate by the fact that the 1998–2012 period started with a strong El Niño event, which led to a relatively high mean temper-ature in the first year of the measurement period.

38 Satellites are examples of remote sensing platforms.

Observedsurfacetemperaturechangesfrom1901to2012Figure 23

Source: IPCC (2013b: 6) based on Hartmann et al. (2013).


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Mean temperature increases can be observed not only at the planet’s surface, but also in oceans and in the troposphere. The IPCC states that it is virtually certain that the average tem-perature in the upper ocean (0 to 700 meters below the surface) increased over the 1971–2012 period, and it is likely that the temperature also increased during the 1870–1971 period (Rhein et al., 2013). Ocean temperature also likely in-creased in lower ocean segments during the 1975–2009 period (700 to 2,000 meters below the surface) and the 1992–2005 period (3,000 meters below the surface to the bottom of the ocean). Rhein et al. (2013) estimate that oceans have absorbed by far the most of the climate system’s energy increase (i.e. 93 per cent of the energy accumulated during 1971–2010). It is also virtually certain that the troposphere has warmed since the mid-20th century (Hartmann et al., 2013).

The IPCC states that it is extremely likely that human-induced changes in the climate sys-tem have been the dominant cause of these observed temperature increases. More spe-cifically, it states that it is extremely likely that more than 50 per cent of the increase in mean surface temperature is attributable to human-induced increases in GHG concentrations. Hu-man-induced variations in atmospheric GHG concentrations alone likely increased average surface temperature by 0.5°C to 1.3°C between 1951 and 2010. The joint effect of other human-induced variations in climate change drivers (e.g. aerosols) on mean surface temperature is likely to be within -0.6°C to 0.1°C, while naturally

induced changes only account for -0.1°C to 0.1°C. Human activities have also very likely substan-tially contributed to the observed warming of upper ocean levels and the troposphere (Bindoff et al., 2013).

3.2 Observedchangesinprecipitation

Climate change also affects precipitation lev-els. While the fourth IPCC assessment report concluded that global precipitation increased north of 30°N between 1900 and 2005 and has decreased in the tropics since 1971, the fifth as-sessment report relativizes these findings (Hart-mann et al., 2013). It states that even if all availa-ble long-term datasets point towards an increase in global mean precipitation over the 1901–2008 period, the order of magnitude of this increase varies widely depending on the data source. The IPCC thus attributes only a low confidence level to evidence indicating global precipitation in-creases over land surfaces prior to 1950 and a medium confidence level to evidence indicating precipitation increases after 1950.

Changes in precipitation seem to differ widely by region. Figure 24 displays precipitation increases in middle and higher latitudes in the northern and southern hemisphere. Hartmann et al. (2013) have medium confidence in the evidence sug-gesting that precipitation increased in mid-lati-tudes of the northern hemisphere for the period before 1950, and high confidence for the period after 1950. For all other regions, however, confi-dence in the evidence of precipitation increases is low due to data quality.

ObservedchangeinannualprecipitationoverthelandsurfaceFigure 24

Source: IPCC (2013b: 8) based on Hartmann et al. (2013).Note: mm yr-1: millimeters per year.


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Selectedobservedchangesinsnowcover,iceextentandsealevelFigure 25

Despite this uncertainty, the IPCC has medium confidence that human actions have contributed to these observed changes in mean precipitation since 1950. The IPCC also has medium confidence that since 1973, human activities have affected atmospheric humidity, which is another impor-tant variable of the hydrological cycle (Bindoff et al., 2013).

3.3 Observedchangesiniceandsnowcover

Ice and snow cover are also affected by climate change. The IPCC reports a very likely decrease of 3.5 to 4.1 per cent per decade in the extent of annual Arctic sea ice over the 1979–2012 period (in fact, the extent decreased in every season and in every decade after 1979 – see panel b in Figure 25). There is also high confidence that per-ennial and multi-year Arctic sea ice shrank over the same period and that the average winter sea ice thickness decreased between 1980 and 2012. Both the Greenland and Antarctic ice sheets have also been losing mass at an accelerated speed (Vaughan et al., 2013). Finally, there is very high confidence that the average snow cover in the northern hemisphere has decreased since the 1950s (see panel a in Figure 25).

These observed changes in the cryosphere are also at least partly human-driven. The IPCC esti-mates that it is (a) very likely that human actions have contributed to the sea ice loss in the Arc-tic since 1979; (b) likely that human actions have contributed to the ice surface melting of Green-land since 1993; (c) likely that human actions have contributed to the retreat of glaciers since the 1960s; and (d) likely that human actions have contributed to the snow cover reductions in the northern hemisphere (Bindoff et al., 2013).

3.4Observedchangesinsealevels

Global sea levels also respond to variation in climate change drivers. The IPCC has high confidence in find-ings suggesting that the rate of sea level rise since the 1850s is higher than the mean increase over the last 2,000 years. As panel c of Figure 25 shows, global sea levels increased by almost 0.2 meters over the 1901 to 2010 period. Most of these increases since the 1970s are attributable to the aforementioned re-ductions in glaciers and to the thermal expansion of oceans, which are due to the mean temperature in-creases in oceans (Rhein et al., 2013). Hence, the IPCC concludes that it is very likely that humans have also substantially contributed to the observed increase of sea levels since the 1970s (Bindoff et al., 2013).

Source: IPCC (2013b: 10).

1940 1960 1980 20001900 1920Year

1940 1960 1980 20001900 1920Year

(mill

ion

km2 )

(mill

ion

km2 )

(mm

)

30

35

40

8

6

4

10

12

14

1940 1960 1980 20001900 1920Year

8

6

4

10

12

14

45Northern hemisphere spring snow cover(a) (b) Artic summer sea ice extent

(c) Global average sea level change


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3.5 Observedchangesinextremeevents

Besides affecting mean states of variables such as temperature, precipitation, snow cover, ice cover, and sea levels, variations in climate change drivers have also altered the probability of occurrence of extreme events (see Box 10 for a

definition of extreme events), such as droughts, floods, heat waves, cyclones, or wildfires. Table 4 summarizes the IPCC’s evidence of changes in the occurrence of these types of extreme events, and provides an overview of its assessment of the role of humans in driving these observed changes.

ChangesintheprobabilityofoccurrenceofextremeeventsTable 4

Extreme event and direction of trendAssessment of whether changes

occurred (typically since 1950 unless otherwise indicated)

Assessment of a human contribution to observed changes

Warmer and/or fewer cold days and nights over most land areas. Very likely Very likely

Warmer and/or more frequent hot days and nights over most land areas Very likely Very likely

Warm spells/heat waves. Frequency and/or duration of increases over most land areas.

Medium confidence on a global scale.Likely in large parts of Europe, Asia, and Australia.

Likely

Heavy precipitation events. Increase in the frequency, intensity, and/or amount of heavy precipitation

Likely more land areas with increases than decreases. Medium confidence

Increases in intensity and/or duration of drought.

Low confidence on a global scale. Likely changes in some regions. Low confidence

Increases in intense tropical cyclone activity.

Low confidence in long-term (centen-nial) changes.Virtually certain in North Atlantic since 1970

Low confidence

Increased incidence and/or magnitude of extreme high sea level Likely (since 1970) Likely

Source: Author's elaboration based on IPCC (2013b: 7)

3.6Impactsonnaturalandhumansystems

Sections 3.1–3.5 showed that human-induced variations in climate change drivers such as at-mospheric greenhouse gas or aerosol concentra-tions have already affected the climate system to a considerable extent. These observed changes in temperature, precipitation, ice and snow, sea levels, and occurrence of extreme events have in turn had several impacts on natural and hu-man systems on a global scale (see Figure 26 for a schematic overview).

The IPCC’s evidence is the strongest for climate change impacts on natural systems (Field et al.,

2014). The IPCC has medium confidence that the observed changes in precipitation and snow and ice cover have affected hydrological systems and the quantity and quality of water resources. There is high confidence that climate change has af-fected various terrestrial, freshwater, and marine species. Some of these species have changed their geographical range, seasonal activities, and mi-gration routes in response to a changing climate (Field et al., 2014). The population sizes of species have also been affected by climate change: the IPCC reports, for instance, that climate change has increased tree mortality in some regions and con-tributed to the extinction of some animal species in Central America (Field et al., 2014).


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Source: Field et al. (2014: 42).

ObservedimpactsattributedtoclimatechangeFigure 26

Shortsummary

Section 3 reviewed changes in the climate system that have been observed and reported by the IPCC. It high-lighted that, in response to human activities, mean temperature increased, precipitation patterns changed, ice and snow cover diminished, and sea levels rose. The section also pointed out that the frequency of some extreme events such as the number of hot days increased as a result of human activities. The section con-cluded by reviewing the key impacts of these observed changes on human and natural systems.

While evidence is the strongest for natural sys-tems, some impacts on humans that are attrib-utable to a changing climate have also been reported. Overall, crop yields have been nega-tively affected. The IPCC has high confidence in evidence suggesting that negative impacts on crop yields have been more frequent than posi-tive impacts on crop yields (Field et al., 2014). In particular, wheat and maize yields seem to have been reduced as a result of a changing climate (medium confidence). While economic losses due to extreme weather events have increased on a global scale, the IPCC has only low confi-dence in evidence suggesting that these ob-served economic losses are directly attributable to climate change (Field et al., 2014). Evidence of negative health impacts attributable to climate change is spare but growing. The IPCC has me-

dium confidence in findings suggesting that heat-related mortality has increased and cold-related mortality has decreased regionally as a result of mean temperature increases. Medium confidence is also attributed to findings indicat-ing that the distribution of some waterborne illnesses has changed due to climate change (Field et al., 2014).

Evidence suggests that climate change has al-ready negatively affected human and natural systems. Furthermore, the IPCC expects that “the likelihood of severe, pervasive and irreversible im-pacts on people and ecosystems” will increase if humans continue to emit large quantities of GHG emissions and thereby change the climate system (Field et al., 2014: 62). Section 4 briefly discusses these possible future impacts of climate change.


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4 Anticipatedchangesintheclimate systemandpotentialimpactsof climatechange

Section 3 outlined the changes in the climate sys-tem that have been observed over past decades and explained that the main part of these chang-es is attributable to human actions. To inform policymakers about possible future changes in the climate system and to assess potential future impacts on humankind and natural systems, the IPCC relies on model predictions. This section in-troduces readers briefly to the different scenar-ios used by climate scientists to predict future changes in the climate system, outlines the most important anticipated changes, and lists key ex-pected impacts on human and natural systems.

In the fifth assessment report, IPCC reviews and assesses the results of numerous studies that use climate models to simulate future changes within the climate system. Climate models are mathematical models that describe key aspects and processes of the climate system. To simulate changes in the climate system, these models need information on how climate change drivers such as atmospheric greenhouse gas and aerosol con-centrations will evolve over the coming decades. As this information cannot yet be observed, re-searchers have to rely on plausible scenarios of the future. This means that they have to rely on infor-mation generated using plausible assumptions on how human activities will evolve and what consequences they will have on climate change drivers. To facilitate worldwide climate research, the IPCC provides such scenarios, which are called Representative Concentration Pathways (RCPs).

Each RCP is based on different assumptions about future economic activities, population growth,

energy sources, and other factors, and contains corresponding values of estimated future green-house gas and aerosol emission trajectories and concentrations until 2100. The IPCC provides four main scenarios, which are called RCP2.6, RCP4.5, RCP6.0, and RCP8.5. The numbers indicate the projected radiative forcing by the end of the 21st century (van Vuuren et al., 2011), e.g. RCP8.5 pro-jects radiative forcing of 8.5Wm-2 by 2100.

While a detailed discussion of all the assump-tions behind the RCPs is out of the scope of this teaching material, we will provide a short over-view of the main characteristics of each scenario. RCP2.6 assumes that drastic climate policy inter-ventions manage to reduce GHG emissions such that they peak before 2020, leading in 2100 to a slight reduction in today’s emission levels and atmospheric GHG concentrations that result in radiative forcing of 2.6 Wm-2 (Wayne, 2013). RCP2.6 can thus be viewed as the IPCC’s best-case scenario. Directly opposed to RCP2.6 is RCP8.5, which can be viewed as the IPCC’s worst-case scenario. RCP8.5 assumes a world with no addi-tional climate change policies and high popula-tion growth (Wayne, 2013). In this scenario, green-house gas emissions continue to grow over the entire century and result in radiative forcing of 8.5 Wm-2. RCP4.5 and RCP6.0 are located between these two “extreme” scenarios. In both of them, GHG emissions would peak during the century (however, considerably later than in RCP2.6), leading to radiative forcing of 4.5 Wm-2 and 6.0 Wm-2 by 2100, respectively. The solid lines start-ing from 2012 in Figure 27 illustrate the projected pathways of carbon dioxide emissions for all four scenarios. Note that IPCC (2014) estimates that if humans were to continue to live as they are today, they would end up in a scenario between RCP6.0 and RCP8.5.

CarbondioxideemissiontrajectoriesaccordingtothefourrepresentativeconcentrationpathwaysFigure 27

1950 2000 2050 2100Year

200

100

-100

0

Annu

al e

mis

sion

s (Gt

CO2/y

r)

Annual anthropogenic CO2 emissions

Historical emissions

RCP scenarios:

RCP8.5

RCP6.0

RCP4.5

RCP2.6

Source: IPCC (2014: 9).Note: GTCO2/yr: Gigatonnes of carbon dioxide per year; RCP: representative concentration pathways.


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PredictedincreasesinmeansurfacetemperatureFigure 28

Source: IPCC (2014: 63). Note: Dots indicate decadal averages, with selected decades labelled. The two overlapping dark-orange and grey surfaces indicate the spread of total human-induced as well as carbon-dioxide-induced warming obtained by different models and scenarios. GTCO2: Gigatonnes of carbon dioxide; GtC: Gigatonnes of carbon.

Numerous studies have used these four RCP sce-narios in their climate models to predict future changes in the climate system. Based on these studies, the IPCC anticipates major changes within the climate system that we summarize below separately for different climate variables (temperature, precipitation, snow and ice cover, and sea level).

Simulations based on all four scenarios predict an increase of mean temperature over the 21st century, as indicated by Figure 28. While the mag-nitudes of the rise in global mean surface tem-perature are comparable for all four scenarios over the 2016–2035 period, mean temperature estimates differ widely for the 2035–2100 period,

depending on the scenario (IPCC, 2014). Compared to the mean of pre-industrial temperature levels (1861–1880), RCP8.5 is estimated to lead to an in-crease of more than 4°C, RCP6.0 to an increase of roughly 3°C, RCP4.5 to an increase of roughly 2.5°C, and RCP2.6 to an increase below 2°C.40 As explained in Modules 1 and 4 of this teaching material, a mean temperature increase below 2°C is the main target of the Paris Agreement. This implies that “limiting total human-induced warming (accounting for both CO2 and other hu-man influences on climate) to less than 2°C rela-tive to the period 1861–1880 with a probability of >66 per cent would require total CO2 emissions from all anthropogenic sources since 1870 to be limited to about 2900 Gt” (IPCC, 2014: 63).

40 For the sake of clarity, the spread of these estimates has been omitted. However, the total spread (using all models and scenarios) of total human-induced war-ming and carbon dioxide-in-duced warming is displayed in Figure 28.

0

1

2

3

4

51000 2000 3000 4000 5000 6000 7000 8000

2090s

2090s

2090s

2090s

2000s

1990s

1970s

5000 1000 1500 2000 2500

Cumulative total anthropogenic CO2 emissions from 1870 (GtC)

Cumulative total anthropogenic CO2 emissions from 1870 (GtCO2)

Total human-induced warming

CO2 -induced warming

Tem

pera

ture

chan

ge re

lativ

e to

1861

– 18

80 (˚

C)

1940s

1880s

RCP8.5

RCP6.0

RCP4.5

RCP2.6

Unlike temperature, no clear global trend is iden-tifiable for precipitation. Some scenarios (RCP8.5) project an increase in annual mean precipitation for high latitude regions, for the equatorial Pacif-ic region, and for some mid-latitude regions, but a decrease in other mid-latitude and subtropical regions (IPCC, 2014). The IPCC also expects an in-crease in extreme precipitation events in some regions of the globe.

All simulations predict decreases in Arctic sea ice (RCP8.5-based simulations even predict a nearly ice-free Arctic ocean in the summer season be-fore the 2050s), near-surface permafrost (with expected decreases of 37 per cent in RCP2.6 up to 81 per cent in RCP8.5), and decreasing glacier

volume (with expected decreases of 15 to 55 per cent in RCP2.6 up to 35 to 85 per cent in RCP8.5, excluding glaciers in the Arctic and Greenland, and Antarctic ice sheets) (IPCC, 2014).

Finally, all RCP scenario-based simulations pre-dict ongoing increases in sea level over the entire 21st century. IPCC (2014) estimates that by 2081–2100, sea levels will have risen from 0.26 to 0.55 meters (RCP2.6) up to 0.45 to 0.82 meters (RCP8.5) compared to their 1986–2005 levels.

All these anticipated changes in the climate sys-tem will have impacts on human and natural systems. Generally speaking, IPCC (2014: 64) ex-pects that climate change “will amplify existing


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Source: IPCC (2014: 14).

risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of devel-opment. Increasing magnitudes of warming increase the likelihood of severe, pervasive, and irreversible impacts for people, species, and eco-systems. Continued high emissions would lead to mostly negative impacts for biodiversity, eco-system services, and economic development, and would amplify risks for livelihoods and for food and human security.”

Some of these risks are global, including (a) in-creased risk of ecosystem and biodiversity losses; (b) increased risk of food and water insecurity;

(c) increased risk of loss of rural livelihoods and income, especially affecting poor segments of the population; (d) increased risk to human health and of disrupting livelihoods due to ex-treme events and sea level rise; and (e) increased systemic risk as the anticipated increase in the frequency of extreme weather events could lead to breakdowns of infrastructure networks (IPCC, 2014). Other anticipated risks vary locally, as shown in Figure 29. IPCC (2014: 73) emphasizes that many “aspects of climate change and its as-sociated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warm-ing increases.”

KeyanticipatedrisksperregionFigure 29

Shortsummary

Section 4 introduced readers to different scenarios used by the IPCC to predict future changes in the climate system. The section highlighted that it is likely that future changes in the climate system (such as increases in temperature) will occur as a result of human activities. It then concluded by showing that these anticipated changes in the climate system will have important negative impacts on human and natural systems.


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21 Whatisthedifferencebetweenweatherandclimate?

2 Name and define the five components of the climate system. Discuss two interactions among selectedcomponents.

3 DiscusstheglobalenergybalanceoftheplanetbyusingFigure15asabasisforyourdiscussion.Whyistheglobalenergybalanceofcrucialimportancefortheplanet’sclimatesystem?

4 Whyistheaveragesurfacetemperature14°Candnot-19°C?Definethenaturalgreenhouseeffect.

5 Listtwointernalfactorsthatcanaffecttheclimate.

6 Listfourexternalfactorsthatcanchangetheclimateandexplainhowtheycandoso.

7 Defineradiativeforcing.Whyisthisconceptausefultooltomeasuretheinfluenceofexternalfactorsontheplanet’sclimate?

8 Discussthedifferentwaysinwhichhumanactivitiescanaffecttheclimateoftheplanet.

9 Whathuman-inducedchangesinclimatevariablescanalreadybeobservedtoday?Discusschangesthataffectyourcountry.

10 WhatistheroleofscenariosintheIPCC’ssimulationoffutureclimatechangeimpacts?

11 Listfiveanticipatedimpactsofclimatechange.Discusswhichchangeswillbemostimportantforyourcountry.

5 Exercisesandquestionsfordiscussion


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ANNEX 1

EstimatesofglobalmeanradiativeforcingprovidedbythedifferentIPCCassessmentreports

IPCCassessmentreports

Figure A1

Global mean radiative forcing (Wm-2) ERF (Wm-2)

SAR(1750–1993)

TAR(1750–1998)

AR4(1750–2005)

AR5(1750–2011) Comment AR5

Well-mixed greehouse gases(CO2, CH4, N2O, and halocarbons)

2.45 (2.08 to 2.82)

2.43 (2.19 to 2.67)

2.63 (2.37 to 2.89)

2.83 (2.54 to 3.12)

Change due to increase in concentrations

2.83 (2.26 to 3.40)

Tropospheric ozone

+0.40 (0.20 to 0.60)

+0.35 (0.20 to 0.50)

+0.35 (0.25 to 0.65)

+0.40 (0.20 to 0.60)

Slightly modified estimate

Stratosphericozone

-0.1 (-0.2 to -0.05)

-0.15 (-0.25 to -0.05)

-0.05 (-0.15 to +0.05)

-0.05 (-0.15 to +0.05)

Estimate unchanged

Stratospheric water vapour from CH4

Not estimated +0.01 to +0.03 +0.07 (+0.02 to +0.12)

+0.07 (+0.02 to +0.12)

Estimate unchanged

Aerosol-radiation interactions

Not estimated Not estimated -0.50 (-0.90 to +0.10)

-0.35 (-0.85 to +0.15)

Re-evaluated to be smaller in magnitude

-0.45 (-0.95 to + 0.05)

Aerosol-cloudinteractions

0 to -1.5(sulphate only)

0 to -2.0(all aerosols)

-0.70 (-1.80 to -o.30)(all aerosols)

Not estimated

Replaced by ERF and re-evaluated to be smaller in magnitude

-0.45 (-1.2 to 0.0)

Surface albedo (land use) Not estimated -0.20

(-0.40 to 0.0)-0.20

(-0.40 to 0.0)-0.15

(-0.25 to -0.05)

Re-evaluated to be slightly smaller in magnitude

Surface albedo (black carbon aerosol on snow and ice)

Not estimated Not estimated +0.10 (0.0 to +0.20)

+0.04 (+0.02 to

+0.09)

Re-evaluated to be weaker

Contrails Not estimated +0.02 (+0.006 to +0.07)

+0.01 (+0.003 to +0.03)

+0.01 (+0.005 to +0.03) No major change

Combined contrails and contrail-induced cirrus

Not estimated 0 to +0.04 Not estimated Not estimated 0.05 (0.02 to 0.15)

Total anthropologic Not estimated Not estimated 1.6 (0.6 to 2.4) Not estimated

Stronger positive due to changes in various forc-ing agents

2.3 (1.1 to 3.3)

Solar irradiance +0.30 (+0.10 to +0.50)

+0.30 (+0.10 to +0.50)

+0.12 (+0.06 to +0.30)

+0.05 (0.0 to +0.10)

Re-evaluated to be weaker

Source: Myhre et al. (2013: 696).Note: ERF: Effective radiative forcing; SAR: Second IPCC Assessment Report; TAR: Third IPCC Assessment Report; AR4: Fourth IPCC Assessment Report; AR5: Fifth IPCC Assessment Report.

URL

Fifth Assessment Report (AR5) https://www.ipcc.ch/report/ar5/

Fourth Assessment Report (AR4) https://www.ipcc.ch/report/ar4/

Third Assessment Report (TAR) https://www.ipcc.ch/ipccreports/tar/

Second Assessment Report (SAR) https://www.ipcc.ch/pdf/climate-changes-1995/ipcc-2nd-assessment/2nd-assessment-en.pdf

First Assessment Report (FAR) https://www.ipcc.ch/publications_and_data/publications_ipcc_first_assessment_1990_wg1.shtmlhttps://www.ipcc.ch/publications_and_data/publications_ipcc_first_assessment_1990_wg2.shtmlhttps://www.ipcc.ch/publications_and_data/publications_ipcc_first_assessment_1990_wg3.shtml

ANNEX 2


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