Climate change in the

Past

Palaeoclimate

Data collection and presentation by Carl

Denef January 2014

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Past climate is the key to preview future climate and helps to explain present climate change.

Understanding present climate change and projecting climate change and impacts into the future can be greatly helped by knowledge of climate changes in the past.

The next slides will show that most of the Earth’s geological history was characterized by a warm climate, with average global surface temperatures 9-12 °C warmer than now, and with atmospheric CO2 levels 3-5 times higher than in the pre-industrial Era. The warm climate sometimes turned into major glaciation periods (Ice Ages) that lasted 30-300 million years. At present we live again in a glaciation period interupted by cycles of warming every 100,000 years. During glacial periods CO2 levels dropped significantly, as did sea level.

The major glaciations (blue areas) during the Earth's entire existence.[Ref]

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How can we assess climate of the past?

Past climate can not directly be assessed but can be reconstructed on the basis of what is called « proxies ». These are present physical parameters, that have signatures of certain climate parameters in the past.

Temperature reconstruction proxies

1) Oxygen and Hydrogen isotope ratios in ice cores and in sediments in sea, land and lake floors : By drilling in polar ice sheets of Greenland and Antarctica and in mountain glaciers, cylindric specimens can be sampled and the relative quantity of the stable oxygen18 (18O) and deuterium (D) isotopes be determined. Water molecules containing the heavier 18O or D evaporate at a higher temperature than water molecules containing the normal 16O and hydrogen, due to the higher atomic weight of the former. Similarly, when water vapor condenses, heavier water molecules holding 18O atoms tend to precipitate first. Changes in 18O /16O ratio over time (δ18O) is therefore indicative of temperature change and can be followed in air trapped within fallen snow that compacts to ice or in the ice itself. δ18O in ice layers is indicative for the temperature at the time the ice was formed. The deeper the drilling the older the cores. The deepest drills are >4 km and contain proxies of > 800,000 years old.

Sediments are often analyzed for δ18O in foraminifera (or forams) and diatoms. These are shelled microorganisms found in aquatic and marine environments. Forams are either planktonic (floating in the water column) or benthic (bottom dwelling). Foram shells are made up of calcium carbonate (CaCO3) while diatom shells are composed of SiO2. Relatively more δ 18O in shells is found in carbonate when ocean waters are cold and ice.

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covers the Earth, because at lower temperature the proportion of H218O that

evaporates becomes lower, leaving more H218O in the residual water for shell

formation.

2) Alkenones and Mg/Ca ratio of calcite in foraminifera and diatoms in sediments may yield information about their temperature at formation.

3) Remnants of vegetation, animals, plankton, corals or pollen in land, lakes and ocean floor sediments may be characteristic of certain climatic zones

4) Direct temperature measurements in rocks: Rock has a very low thermal conductivity. It can take centuries for rocks underground to become aware of changes in surface temperatures. By taking very careful measurements of the temperature of rock in boreholes tens and hundreds of meters underground, it is possible to detect shifts in the long-term mean surface temperature at that location. As thermal diffusion is a very slow process, short term changes are averaged out. This technique only provides information about changes in the average temperature at the century resolution.

5) The type of living species in fossils can be typical for a temperature range. For example, plankton live in narrow temperature ranges.

6) Tree rings are indicative for warmth, although also for humidity and nutrient conditions

The map on the left shows the locations of the 951 boreholes in the University of Michigan global database of boreholes.

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Reconstruction of CO2 levels:

1) Direct analysis in air trapped in ice core layers up to 800,000 years ago.

2) Reconstruction from carbon isotopic ratio (δ13C) in carbonate of fossilized soils (paleosols) or of phytoplankton (foramenifera) shells, that remained intact over millions of years. δ13C is the ratio of the stable carbon isotopes 13C/12C.[1]

Carbon in inorganic carbonates not derived from living organisms shows the natural isotope ratio signature of 1/99 without preferential choice for 13C or 12C, while carbon in materials originated from photosynthesis is depleted of 13C, because plants prefer 12C over 13C in photosynthesis. Carbon in fossil shells is therefore also depleted in 13C. δ13C depends on the levels of CO2 in the atmosphere and on the amounts of CO2 being respired by organic matter in the soil itself.

3) Determination of stomata in fossil plants: Stomata are pores to breathe in the CO2 that plant leaves need for photosynthesis. When CO2 is abundant, plants down regulate the number of stomata in their leaves. Stomata density in fossil plants is therefore an important proxy for atmospheric CO2 concentrations.

Reconstruction of ocean currents. Ocean sediment made up of microfossils and mineral grains delivered to the sea from continent erosion can tell about ocean currents in the past. Diatoms particularly take advantage of upwelling ocean water that is richer in nutrients.

Reconstruction of ocean pH: Ocean pH can be reconstructed up to >20 million years ago from the ratio of stable boron isotopes 11B/10B in ocean sediments of foraminifera.

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Reconstruction of Wind directions. Volcanic dust, sea salt, black

carbon and desert dust in the air are deposited on glaciers and ice sheets and accumulated with snow in the ice. When the dust shows up in ocean cores, its chemistry can be used to determine where it came from. By mapping the distribution of the dust, wind direction and strength can be inferred. The dust also may reveal how dry and dusty the climate may have been at a particular time. Read more

Climate models. It has become possible to represent the different physical processes associated with the climate system as differential equations that can be numerically resolved by computers. These physical processes include atmospheric and ocean circulation, the reciprocal relationships of the latter, ice formation and melting dynamics, the distribution of δ18O in the oceans, ecological parameters of carbonate forming organisms and others. If the various forcing parameters are known over time they can be entered in the model that then computes the likely climate evolution over time. The calculated climate is then compared to the climate evolution constructed from proxies. The closer the modeled changes match those observed in the sediments or ice cores, the greater the confidence in the realism of the models. Multiple regression analysis can dissect out the different forcing factors involved in the past climate change.

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Ocean sediment cores (source)

Foranimifera samples (Wikipedia)

Ice Cores (source)

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Palaeoclimate drivers At time scales of 1-10 million years tectonic activity is the major driver of

climate change. Increased tectonic activity causes continental drift and increases CO2 release by volcanism and sea-floor spreading, promoting global warming by the increased greenhouse effect. Continental drift, in turn , determines the position of the continents and affects the thermohaline circulation. The latter transfers heat between the equatorial regions and the poles and in this way affects climate. When landmasses are concentrated near the poles, there is an increased chance for

snow and ice to accumulate as there is more cooling over polar regions. Small changes in the Earth’s obliquity, eccentricity and precession change the

amount of solar radiation reaching the Earth (Milankovitch cycles) which can tip the balance between summers in which the packed winter snow completely melts and summers in which the winter snow persists until the following winter. If snow remains accumulating it strongly increases albedo, resulting in cooling. Moreover, accumulation of snow and ice on land decreases sea level.

Source

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Geological time scales (source)

Look here or he re how the Earth land and sea surface evolved over those times ! Source

Numbers are million years

Period Epoch Date

Quaternary

Holocene 0–0.0117

Pleistocene

0.0117–0.126

0.126–0.781

0.781–1.806

1.806–2.588

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Eocene (50 Ma)

Mid-Jurassic (170 Ma)

Cretaceous-Paleogene boundary (65 Ma)

Pleistocene (0.5 Ma)

Gondwana

Mid-Ordovician (470 Ma)

Ice

Continents during geological history

Present

Ma = million years ago Source

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Climate changes in the Phanerozoic - from 500 million years ago

Temperature: There have been periods when global average temperature was 9-12 °C higher than present (15 °C), as derived from δ18O in fossils. [1] The Earth was a hothouse without any ice caps on the poles. Tropical waters are assumed to have been around 45°C.

Sporadically there were long periods of glaciation (indicated by the blue bars in the Figure) over the Antarctic area. During warm periods, masses of water evaporated from the oceans that, upon cooling over the huge supercontinent known as ‘Gondwana’, that was located over the Southern hemisphere and Antarctic region at that era, precipitated as snow. Packed snow became ice, giving cooling of the Earth (increased ice-albedo).

Source (adapted)11

CO2: Overall, temperature changes correlated with atmospheric CO2 levels,

lowest levels being reached during periods of glaciation. However, absolute values were very divergent according to the proxy used (See Figure). Boron, phytoplankton and fossil plant stomata proxies generally give lower CO2 levels. A recent paper in PNAS showed previous δ13C determinationsin in soil carbonates overestimated atmospheric CO2 levels; the paper calculated with a new method that CO2 levels during the Phanerozoic was maximum 1500 ppm and that the fluctuations were best in line with those reconstructed from the stomata proxies.

Source: IPCC AR4 Chapter 612

Sea level: Over most of geologic history, long-term average sea level has been significantly higher than today. Sea level rose upon warming and lowered during appearance or expansion of land ice (ice sheets and glaciers), due to the retention of rain once frozen on land.

The Figure shows two sea level reconstructions during the Phanerozoic. The scale of change during the last glacial/interglacial transition is indicated with a black bar. (From Wikipedia)13

The Paleocene–Eocene Thermal Maximum (PETM) – a high CO2 and high temperature World

The Paleocene Epoch followed the mass extinction event of the dinosaurs at the end of the Cretaceous, ~65 million years ago. The paleocene was cooler and dryer than the preceding Cretaceous. It was followed by the Eocene during which the Earth became a “hothouse”.

PETM is a short period of warming, with average global temperature 4–7°C higher than pre-industrial, from ~55.5 to 55.3 million years ago. Sea surface temperatures in the Tropics was ~35 °C.[16] There was little or no ice on the poles. Due to the reduced albedo, temperature anomaly was greatest at the poles (polar amplification). During the summer temperature in the Arctic was probably over 20 °C. Fossils of tropical plants and animals were found at high latitudes (for example, giant turtles and alligators were living north of the Arctic circle), consistent with the unusual warming in the northern hemisphere.[Ref]

There was a a prominent global drop in carbon isotope ratio (δ13C), consistent with a massive carbon release (CO2 and/or CH4) into the atmosphere and ocean (4500–6800 gigatonnes Carbon equivalents), leading to global ocean acidification and dissolution of carbonate deposited on ocean basins (see IPCC AR5 WG1, chapt. 5, section 5.3.1). This occurred in 5,000–20,000 years. Notice that this roughly corresponds to an average rate of ~0.5–1.0 gigatonnes Carbon per year which is ~10-20 times less than the present yearly anthropogenic emission.

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Various reconstructions found atmospheric CO2 values of 2000-3500 ppm[6] [7], although IPCC concluded in IPCC AR5 WG1 chapter 5 that values remained below 1125 ppm. Strong continental drift, that made continents move toward their present positions, may have been the initial driving force. This caused excessive volcanic activity injecting large amounts of CO2 (13C-depleted) in the atmosphere. Warming by the increased greenhouse effect of CO2 may have been further enhanced by temperature-induced destabilization of methane clathrate (stable methane-water cristals in the sea floor). The PETM was accompanied by a mass extinction of 35-50% of benthic foraminifera over the course of ~1,000 years, probably due to to rapid temperature increases in ocean bottom water and a concomitant reduction in dissolved oxygen. However, planktonic foraminifera diversified, and dinoflagellates bloomed. There is no evidence of increased extinction rate among the most terrestrial biota[Ref] On the contrary, despite the “hothouse” nature of the Earth, several major mammalian orders, including the Artiodactyla, horses, and primates, abruptly evolved from archaic mammals and spread across the globe over a few hundred or thousand generations [Ref].

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The Early Eocene Climatic Optimum EECO is a period between 54 and 48 million years ago following the PETM.

Average global surface temperature further rose up to 8–14°C above preindustrial, established in about 30,000 years. High latitude sea surface temperature was 14 - 16 °C higher and over land 9-24 °C, CO2 concentrations were 1000-2000 ppm and continental ice sheets were absent. Carbon release into the atmosphere was somwhat moxer than during the PETM.

The PETM and EECO are considered “case studies” for global warming and massive carbon input into the atmosphere under present anthropogenic carbon emission scenarios. However, IPCC attributes low to medium confidence to the available proxy data and extrapolations need to be done with caution (see IPCC AR5 WG1, chapt. 5, section 5.3.1).

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The Azolla event

The Azolla event occurred in the middle Eocene epoch,[1] ~49 million years ago and lasted 800,000 years. Enormous Blooms of the freshwater fern Azolla developed in the Arctic Ocean over a surface of 4,000,000 km². The continental configuration at that epoch was such that the Arctic sea was almost entirely cut off from the wider oceans (see next slide). Reconstructed average year temperature was 13 °C over the arctic sea at that epoch and caused large evaporation making the sea water saltier and hence more dense. Heavy rainfall[7] over land caused high discharge of fresh water from rivers, which caused accumulation of a layer of fresh water over the surface of the dense sea water [8]. At the average temperature of 13 °C Azolla could easily grow on that layer, especially during summer when days are long, as it is a very fast growing species and converts Nitrogen of the air to nutrients. Massive amounts of dead Azolla sank to the sea floor where it did not rotten since the dense sea layer underneath was probably anoxic because it did not mix with ocean flows nor with the less dense layer above. Dead vegetation was incorporated into sediments and buried. Azolla photosynthesis removed massive amounts of CO2 from the atmosphere and its incorporation into sediment sequestered the carbon. This occurred for 800,000 years over a surface of 4,000,000 km², largely enough to have lowered atmospheric CO2 and initiate cooling. 17

Figure source Continental configuration during the Eocene as seen above the Arctic (Figure source)

the freshwater fern Azolla (Wikipedia)

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The transformation from a "greenhouse Earth" state to an “icehouse Earth”. After the Azolla event the Earth continued to cool, up to the Industrial

Era. Antartic glaciation was present at ~35 million years ago. In the Arctic a significant ice sheet was present since ~3 million years ago. The Figures below show the temperature decline over different oceans and the Antarctic, as reconstructed from ice cores and ocean sediment cores.

A major contribution to the expansion of the Antarctic ice sheet was the creation of the Antarctic circumpolar current,[26] that presumably isolated the cold water around the Antarctic and reduced heat transport from the (sub)tropical ocean to the Antarctic.

From Nature 486, 97–100 (2012)

Subtropical East Pacific

Northeast Pacific

Northwest Pacific

SS

T °C

SS

T °

C

15

20

25

5

10

1

5

20

2

5

30

15 12 9 6 3 0

From Nature 486, 97–100 (07 June 2012)

Million years ago

From Wikipedia

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Proxy-based reconstructions show the steady decrease of atmospheric CO2

levels in parallel to the temperature decrease between 49 and 23 million years ago. Reconstruction with a new boron/calcium (B/Ca) ratio proxy found CO2 concentrations of 350 - 450 ppm between 20 and 10 million years ago and a recent alkenone-based proxy study showed a further CO2 decrease from 5 to 2 million years ago(Ref).

Millions of years ago

Figure from IPCC AR5 WG1

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Climate change during the Pliocene In the mid-pliocene (3.3 – 3 million years ago) the global average temperature

was again 3–10 °C higher than today,[1] [PNAS ], 3–4° C warmer at low latitudes, and up to 10° C warmer nearer the poles. Evidence from Lake El'gygytgyn, in northeast Arctic Russia, shows that 3.6 to 3.4 million years ago, summer temperatures were ~8°C warmer than today.There was near complete deglaciation of the Greenland and West Antarctic Ice Sheet. Average sea surface temperature increased with 2-6 °C, particularly between Greenland and W-Europe (see Figure). [Ref] (Science. 340:1421-7, 2013).

Recent studies showed that only a relatively small rise (~35% higher) in atmospheric CO2 levels was associated with this substantial global warming, and that CO2 levels at peak temperatures were between about 365-415 ppm, [Ref] [Ref] [Ref] [Ref]

Global sea level was 7-20 m higher [2].

-10 -6 -2 2 6 10 °C

Februari August

Source21

Both temperature and CO2 levels continued to decrease subsequent to

the Pliocene

Millions of years ago 5 4 3 2 1 0

From Nature. 2013 Apr 4;496(7443):43-9

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The pleistocene glaciation and the Ice Age cycles The post-Pliocene temperature decrease resulted in the onset of the Northern

Hemisphere glaciation.[9] [3] Greenland ice sheet started to grow significantly ~3 million years ago. But on top of the downward temperature trend, rapid cycles of warming and cooling started to develop , marking a new epoch, the Pleistocene. These cycles (now 52 in total) are known as glacial-interglacial cycles. Between 2.5 and 1 million years ago an average cycle was ~41,000 years. During the last million years cycle period became ~100,000 years. Warm interglacial periods have an abrupt onset and last some 20,000 years after which there is stepwise cooling at a slower rate and glaciation (Ice Age).

During the interglacial warmer periods, surface temperature was 10-14 °C higher than during the coldest period (glaciation maximum). Glaciation periods were drier and dustier.

CO2 changes followed the same pattern. Levels dropped as low as ~190 ppm during glaciation and increased up to ~280 ppm during interglacial periods. Several studies have found that changes in CO2 levels lag 400-800 years behind the changes in temperature (Read more). However a paper in Science in 2013 shows synchrony between temperature and CO2 during the last interglacial warming period, based on N15 isotope data in trapped air in Antarctic ice cores.

The glacial-interglacial cycles are now explained on the basis of the Milankovitch cycles in solar radiation input. The initial trigger for warming are particular values of the Earth’s obliquity, eccentricity and precession that increase the amount of incoming solar energy. However, these variations alone cannot account for the large differences between glacial and interglacial temperatures. The warming from solar input is believed to be enhanced by several internal feedback systems between the climate, the ice sheets and the warming-induced release of CO2 from the oceans (read more in Nature 500, 190–193 , 2013).

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The right Figure shows Antarctic temperature, insolation (energy input from sun in W/m2) and atmospheric CO2 over the last 800,000 years reconstructed from Antarctic ice cores and Milankovitch cycle calculations. Variations of CO2 level are highly correlated with temperature variations. Notice that the CO2 levels integrated over time (area under the CO2 curve) were higher during the MIS-11 and the Eemian interglacials and that this was also the case with temperature. The Figure also shows that temperature rose in parallel with increased insolation, although this was less pronounced during the MIS-11 interglacial. However, during MIS-11 CO2 forcing integrated over time was larger. Also notice the coordinated fluctuations in atmospheric CH4.

MIS-11 Eemian

Su

mm

er s

ols

tic

e i

ns

ola

tio

n a

t 65

° N

ort

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W/m

2 )

Source

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It is during the Pleistocene epoch that the genus Homo (Homo habilis) has evolved. Homo sapiens evolved during the last two Ice Ages

The plot underneath shows the linear regression line between temperature and CO2 data over the last 850,000 years. A 0.89 correlation exists. The point encircled and shaded in red is the present temperature/CO2 coordinates. It is located 9 standard deviations away from the mean, consistent with a non-natural forcing.

Source

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The Marine Isotopic Stage 11 (MIS 11) MIS-11 is the interglacial period between 424,000 and 374,000 years

ago.[1] Its duration was considerably longer than that of other interglacials. It was 1.5–2.0 °C warmer than preindustrial at high latitudes [see PNAS article].

CO2 concentration was similar to that of the pre-industrial period, but integrated over time CO2 forcing was larger (see previous slides).

Beach deposits in Alaska, Bermuda and the Bahamas, as well as uplifted reef terraces in Indonesia, suggest that global sea level reached as much as 6 -20 m above the present.[5][6][7] [see also PNAS], consistent with the near absence of Arctic continental ice sheets

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The Eemian Interglacial The Eemian interglacial is the last interglacial before the present

interglacial (Holocene) and dates from 130,000 to 115,000 years ago. Global mean surface temperature was ∼1–2 °C warmer than preindustrial,

although a recent paper in Nature, 24 Jan 2013, using new Greenland ice cores, concludes it was 8 ± 4 °C warmer than the last thousand years.

CO2 concentration was similar to preindustrial (280 ppm) but over a longer time (see previous slide).

There was a stronger solar forcing than during the Holocene, due to the high orbital eccentricity in phase with a high precession index, resulting in large positive solar radiative forcing during boreal summer in the Northern Hemisphere and austral spring in the Southern Hemisphere.

Mean sea level was 5.5-9 m higher than present and Ice-sheets were smaller. The thickness of the N.W. Greenland ice sheet decreased by 400 m (Nature, 24 Jan 2013)

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The Last Glacial Maximum (LGM) During the coldest part of the last Ice Age, about 22,000 years ago, the

northern part of the North America, Asia and Europe were covered with a giant ice sheet. Central Greenland was 17-25 °C cooler and north-Atlantic sea surface temperature at mid-latitude was 10 °C cooler (see more data in IPCC AR5 Table 5.2). Atmospheric CO2 was 190 ppm. Due to so much water being stored as ice at the poles, sea level was about 120 m below the current level.

The Figure[Ref] gives an overview of the cooling in different locations. The circle size represents the difference in temperature between the coldest glacial and the peak interglacial temperature. Notice that the cooling is highest in the Arctic and Antarctic, due to the ice-albedo cooling phenomenon (polar amplification).

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Sudden warming events during the last Ice Age Climate during the last glaciation period

was very chaotic. The so called Heinrich and Dansgaard-Oeschger (D-O) events, occurred repeatedly throughout most of this time (25 times). Each D-O event is characterized by an abrupt warming of 4-6 °C to near-interglacial conditions that occurred within decades, - a faster rise than during present anthropogenic warming - and is followed by a gradual cooling. However, in the Antarctic warming was gradual.

Less frequent events were the Heinrich events (lower panel) also with sudden warmings followed by a gradual cooling. Even though Heinrich and D-O events seem to have been initiated in the North Atlantic, they had a global footprint.

Atmospheric CO2 rose with ~20 ppm (from ~190-200 to ~200-220 ppm) several thousand years before the onset of the D-O event[Ref].

Holocene

Data from NOAA.

These cycles ended at the onset of the Holocene, which experienced a much more stable climate.

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Sudden cooling events during transition to the Holocene The Younger Dryas episode is a period of rapid cooling that was named

after a flower Dryas octopetala that grows in the cold. It occurred after initial warming at the end of the last Glacial Maximum about 14,500 years ago, in the Northern Hemisphere. It lasted ~1300 years. Temperature in different areas of the Northern Hemisphere fell to near-glacial conditions within a decade or, according some proxies, in just a few years. The higher the latitude, the greater was cooling (8 °C in Greenland; see next slide). It has been hypothesized that massive amounts of ice sheet meltwater reduced the salinity and density of the surface ocean in the North Atlantic, causing a slowdown in the ocean's thermohaline circulation. This reduced the flow of warm water from the Tropics into the Atlantic ocean resulting in cooling. Atmospheric CO2 was rising during the deglaciation but remained on a plateau (240 ppm) during the cooling phase.[Ref]

The end of the Younger Dryas, ~11,500 years ago, was also particularly abrupt. In Greenland, temperatures rose 10° C in a decade, which is a faster change than during present anthropogenic warming. Data from NOAA and here

The 8.2 ka (kiloyear) cooling event High-resolution analyses of a Greenland ice core indicate that temperature around 8200 years ago cooled ~3.3°C in Greenland within two decades. The entire event lasted about 150 years and then temperatures returned to previous values. Lake and ocean sediments show that European climate was also affected, with temperatures dropping about 2°C. Global CO2 dropped with 25 ppm. [Ref] It is thought that the event was caused by sudden collapse of an ice sheet dam south of the Hudson Bay, that held huge amounts of melt water in a large lake. Fresh water release

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into the Labrador sea may have resulted in a slowdown of the ocean thermohaline circulation and hence cooling. Data from NOAA.

Temperature changes during the Younger Dryas. Circles denote the size of the temperature change. Blue is cooling, red warming (Shakun and Carlson, 2010).

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Climate change during the Holocene The Holocene epoch started at the end of the last Ice Age is characterized by a

relatively stable climate, when looked at it at the global level, with global average temperature fluctuations of only +/- 0.5 °C. However, around 8,000-5,000 years ago there was a period of significant warming in the Northern Hemisphere at high and polar altitudes (the Holocene climate optimum), but no change at low and mid latitudes. Tropical reefs tended to show temperature increases of less than 1 °C, while the southern hemisphere was cooler. There is evidence that the world’s northernmost ice cap melted away and was rebuilt when the climate got colder again [Ref] . The Holocene climatic optimum is also seen in Antarctic ice cores.

The climate may have been particularly forced by increased solar irradiation (see Milankovitch cycles), as the Earth’s axial tilt was 24° and the nearest approach to the Sun (perihelion) was during boreal summer. The calculated forcing would have provided 8% more solar radiation (+40 W/m2) to the Northern Hemisphere in the summer, tending to cause greater heating at that time.

Mid 20th century average temperature set at zero. Source

Thousands of years ago32

Sea level rose ~120 m compared to the glacial maximum of the last Ice Age.

According to IPCC AR5 WG1 the initial rate of sea level rise at the beginning of the Holocene (13,000 -14,600 years ago) was at a very high rate - about 20 m in less than 500 years,[1] perhaps just 200 years.[2] [3] The present sea level rise rate is more than 10 times lower.

Importantly, sea level remained fairly constant during the last 3000 years but started to rise from the beginning of the Industrial Era.

Two natural climate anomalies were seen during the last millennium: the Medieval Climate Anomaly (MCA) or Medieval Warm Period and the Little Ice Age (LIA). MCA was a warm period (as warm as the late 20th century in some regions) in the North Atlantic region lasting from about AD 950 to 1250.[8] LIA is a cold period, particularly in Europe and North America, between 1550 and 1850.[7]

Years before present

From IPCC AR5 Figure 13.3

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Arctic temperature over the last 2000 years Temperature reconstruction based on lake sediments, ice and tree ring

proxies (17 different records) show a cooling during the last 1800 years, with a steep warming during the last 200 years. The millennial-scale cooling is –0.22° ± 0.06°C per 1000 years. The cooling correlates with the reduction (about 6 W/m2 at 65°N) in summer solar irradiance, primarily driven by the orbital precession of the Earth and enhanced by ice albedo.

Values are 10-year means standardized relative to the reference period of 980 to 1800.

From Science, 325:1236-1239, 2009

Sta

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ize

d

tem

pe

ratu

re c

ha

ng

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Temperature in the Tropics A coral-based reconstruction of sea surface temperatures (SST) over the

last 250 years for the whole of the Tropics (30°N-30°S) was recently reported. It was developed from 14 disparate coral records located in the Indian and Pacific oceans. The Figure shows a small decreasing trend in SST between 1600 and1800, after which there was a steady increase of 0.5 °C until present.

Source35

How did life on Earth react in response to climate change? Fossil data have shown that climate change can profoundly affect life on

Earth. There have been several mass extiction events in the past, that were associated with dramatic changes in temperature and atmospheric and ocean CO2 and oxygen levels or with the appearance of toxic gas in the atmosphere. However, there are examples also that certain phyla prospered during the same period. For example during the PETM mammals expanded rapidly.

On the basis of paleoclimatic data the end of the Acadian empire (Read more) and of the Maya culture (Read more) are thought to be caused or facilitated by abrupt climate change (prolonged drought). The Acadian empire in Mesopotamia flourished for about 100 years until, at 4170 +/- 150 years before present, it suddenly collapsed. The Maya culture collapsed around 800-900 AD with many cities being abandoned.

In an article in Science 27 April 2001: Vol. 292 no. 5517 pp. 667-673, entitled “Cultural Responses to Climate Change During the Late Holocene”, 4 case studies drawn from New and Old World civilizations documented societal responses to prolonged drought, including population dislocations, urban abandonment, and state collapse and concluded that further study of past cultural adaptations to persistent climate change may provide valuable perspective on possible responses of modern societies to future climate change.

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View more slide shows on palaeoclimate

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