Global warmingThis page is about the current warming of the Earthsclimatesystem. "Climatechange"canalsorefer toclimate trends at any point in Earths history. For otheruses see Global warming (disambiguation)Global LandOcean Temperature Index1880 1900 1920 1940 1960 1980 2000Temperature Anomaly (C)0.40.200.20.40.6Annual Mean5year Running MeanGlobal mean surface temperature change from 1880 to2014, relative to the 19511980 mean. The black line isthe annual mean and the red line is the 5-year runningmean. The greenbars showuncertaintyestimates.Source: NASA GISS.World map showing surface temperature trends (C perdecade) between1950and2014. Source: NASAGISS.[1]IEA CO2 Emissions per Year vs. IPCC ScenariosBillions of Tonnes of Fossil Fuel CO2 Emitted322224262830Year1990 1995 2000 2005 2010IEA dataB2A1A1TA2A1F1Fossil fuel related carbon dioxide (CO2) emissions com-pared to ve of the IPCCs "SRES" emissions scenarios.The dips are related to global recessions.Image source:Skeptical Science.Globalwarming and climate change are terms for theobserved century-scale rise in the average temperature ofthe Earths climate system and its related eects.Multiple lines of scientic evidence show that the cli-mate systemis warming.[2][3]Althoughthe increaseof near-surfaceatmospherictemperatureis themea-sure of global warming often reported in the popularpress,most of the additional energy stored in the cli-mate system since 1970 has gone into ocean warming.The remainder has melted ice, and warmed the con-tinents and atmosphere.[4][lower-alpha 1]Many of the ob-served changes since the 1950s are unprecedented overdecades to millennia.[5]Scientic understanding of global warming is increasing.In its 2014 report the Intergovernmental Panel on Cli-mate Change (IPCC) reported that scientists were morethan 95% certain that most of global warming is causedbyincreasingconcentrationsofgreenhousegasesandother human (anthropogenic) activities.[6][7][8]Climatemodelprojections summarized in the report indicatedthat during the 21st century the global surface temper-ature is likely to rise a further 0.3 to 1.7 C (0.5 to 3.1F) for their lowest emissions scenario using stringentmitigation and 2.6 to 4.8 C (4.7 to 8.6 F) for theirhighest.[9] These ndings have been recognized by thenational science academies of the major industrializednations.[10][lower-alpha 2]Future climate change and associated impacts will dif-fer from region to region around the globe.[12][13] An-12 1 OBSERVED TEMPERATURE CHANGESticipatedeects includewarmingglobal temperature,rising sea levels, changing precipitation, and expansionof deserts in the subtropics.[14] Warming is expected tobe greatest in the Arctic, with the continuing retreat ofglaciers, permafrost and sea ice. Other likely changesinclude more frequent extreme weather events includ-ingheat waves, droughts, heavyrainfall, andheavysnowfall;[15] ocean acidication; and species extinctionsdue to shifting temperature regimes. Eects signicantto humans include the threat to food security from de-creasing crop yields and the abandonment of populatedareas due to ooding.[16][17]Possiblesocietal responsestoglobal warmingincludemitigation by emissions reduction, adaptation to its ef-fects, building systems resilient to its eects, and possiblefuture climate engineering. Most countries are parties tothe United Nations Framework Convention on ClimateChange (UNFCCC),[18] whose ultimate objective is toprevent dangerous anthropogenic climate change.[19] TheUNFCCC have adopted a range of policies designed toreduce greenhouse gas emissions[20][21][22][23] and to as-sist in adaptation to global warming.[20][23][24][25] Partiesto the UNFCCC have agreed that deep cuts in emissionsare required,[26] and that future global warming shouldbe limited to below 2.0 C (3.6 F) relative to the pre-industrial level.[26][lower-alpha 3]1 Observed temperature changesMain article: Instrumental temperature recordThe global average (land and ocean) surface tempera-Energy change inventory, 1971-2010Oceans93%Ice3%Continents3%Atmosphere1%Earth has been in radiative imbalance since at least the 1970s,where less energy leaves the atmosphere than enters it. Most ofthis extra energy has been absorbed by the oceans.[28]It is verylikely that human activities substantially contributed to this in-crease in ocean heat content.[29]ture shows a warming of 0.85 [0.65 to 1.06] C in theperiod 1880 to 2012, based on multiple independentlyproduced datasets.[30] Earths average surface tempera-ture rose by 0.740.18 C over the period 19062005.The rate of warming almost doubled for the last half ofthat period (0.130.03 C per decade, versus 0.070.02Two millennia of mean surface temperatures according to dif-ferent reconstructions from climate proxies, each smoothed on adecadal scale, with the instrumental temperature record overlaidin black.NOAA graph of Global Annual Temperature Anomalies 19502012, showing the El Nio Southern OscillationC per decade).[31]The average temperature of the lower troposphere has in-creased between 0.13 and 0.22 C (0.23 and 0.40 F)per decade since 1979, according to satellite tempera-ture measurements. Climate proxies show the temper-ature to have been relatively stable over the one or twothousand years before 1850, with regionally varying uc-tuations such as the Medieval Warm Period and the LittleIce Age.[32]The warming that is evident in the instrumental temper-ature record is consistent with a wide range of obser-vations, as documented by many independent scienticgroups.[33] Examples include sea level rise,[34] widespreadmelting of snow and land ice,[35]increased heat con-tent of the oceans,[33]increased humidity,[33]and theearlier timing of spring events,[36] e.g., the owering ofplants.[37] The probability that these changes could haveoccurred by chance is virtually zero.[33]31.1 TrendsTemperature changes vary over the globe. Since 1979,land temperatures have increased about twice as fast asocean temperatures (0.25 C per decade against 0.13C per decade).[38] Ocean temperatures increase moreslowly than land temperatures because of the larger ef-fective heat capacity of the oceans and because the oceanloses more heat by evaporation.[39] The northern hemi-sphere is also naturally warmer than the southern hemi-sphere mainly because of meridional heat transport in theoceans, which has a dierential of about 0.9 petawattsnorthwards,[40] with an additional contribution from thealbedo dierences between the polar regions. Since thebeginning of industrialisation the temperature dierencebetween the hemispheres has increased due to melting ofsea ice and snow in the North.[41] Average arctic temper-atures have been increasing at almost twice the rate ofthe rest of the world in the past 100 years; however arctictemperatures are also highly variable.[42] Although moregreenhouse gases are emitted in the Northern than South-ern Hemisphere this does not contribute to the dierencein warming because the major greenhouse gases persistlong enough to mix between hemispheres.[43]The thermal inertia of the oceans and slow responses ofother indirect eects mean that climate can take centuriesor longer to adjust to changes in forcing. Climate com-mitment studies indicate that even if greenhouse gaseswere stabilized at year 2000 levels, a further warming ofabout 0.5 C (0.9 F) would still occur.[44]Global temperature is subject to short-term uctuationsthat overlay long-term trends and can temporarily maskthem.The relative stability in surface temperature from2002 to 2009, which has been dubbed the global warminghiatus by the media and some scientists,[45] is consistentwith such an episode.[46][47] Recent updates to account fordiering methods of measuring ocean surface tempera-ture measurements show a signicant positive trend overthe recent decade.[48][49]1.2 Warmest yearsNine of the 10 warmest years in the instrumental recordoccurred since 2000, with 2014 being the warmest yearon record.[50] 2014 was also the 38th consecutive yearwith above-average temperatures.[51] Before 2014, 2005and 2010 had tied for the planets warmest year, exceed-ing 1998 by a few hundredths of a degree.[52][53][54] Sur-face temperatures in 1998 were unusually warm becauseglobal temperatures are aected by the El Nio SouthernOscillation (ENSO), and the strongest El Nio in the pastcentury occurred during that year.[55]2 Initial causes of temperaturechanges (external forcings)Main article: Attribution of recent climate changeEarth's land and ocean surface warmed to an average of 14Heat and energyin the atmosphereGreenhouse gasabsorption: 350Solar Radiationabsorbed by Earth235 W/mThermal radiationinto space: 195Directly radiatedfrom surface: 4016832449267TheGreenhouseEect 492452Greenhouse eect schematic showing energy owsbetweenspace, theatmosphere, andEarths surface.Energyexchanges areexpressedinwatts per squaremeter (W/m2).Thisgraph, knownastheKeelingCurve, documentsthe increase of atmospheric carbon dioxide (CO2)concentrations from 19582015. Monthly CO2mea-surements displayseasonal oscillations inanupwardtrend; each years maximum occurs during the NorthernHemisphere's late spring, and declines during its growingseason as plants remove some atmospheric CO2.Theclimatesystemcanrespondtochangesinexter-nal forcings.[56][57] External forcings can push the cli-mateinthedirectionofwarmingorcooling.[58]Ex-amplesofexternal forcingsincludechangesinatmo-spheric composition (e.g., increased concentrations ofgreenhouse gases), solar luminosity, volcanic eruptions,and variations in Earths orbit around the Sun.[59] Orbitalcycles vary slowly over tens of thousands of years, and atpresent are in a cooling trend.[60] The variations in orbitalcycles may produce a glacial period about 50,000 yearsfrom now.[61]2.1 Greenhouse gasesMain articles: Greenhouse gas, Greenhouse eect,Radiative forcing, Carbon dioxide in Earths atmosphereand Earths energy budget4 2 INITIAL CAUSES OF TEMPERATURE CHANGES (EXTERNAL FORCINGS)See also: List of countries by carbon dioxide emissionsand History of climate change scienceThe greenhouse eect is the process by which absorptionand emission of infrared radiation by gases in a planetsatmosphere warm its lower atmosphere and surface. Itwas proposed by Joseph Fourier in 1824, discovered in1860 by John Tyndall,[62] was rst investigated quantita-tively by Svante Arrhenius in 1896,[63] and was developedin the 1930s through 1960s by Guy Stewart Callendar.[64]Annual worldgreenhousegasemissions, in2010, bysector.Percentageshareof global cumulativeenergy-relatedCO2 emissions between 1751 and 2012 across dierentregions.On Earth, naturally occurring amounts of greenhousegases have a mean warming eect of about 33 C (59F).[65][lower-alpha 4] Without the Earths atmosphere, theEarths average temperature would be wellbelow thefreezing temperature of water.[66] The major greenhousegases are water vapor, which causes about 3670% of thegreenhouse eect; carbon dioxide (CO2), which causes926%; methane (CH4), which causes 49%; and ozone(O3), which causes 37%.[67][68][69] Clouds also aect theradiation balance through cloud forcings similar to green-house gases.Human activity since the Industrial Revolution has in-creased the amount of greenhouse gases in the atmo-sphere, leading to increased radiative forcing from CO2,methane, tropospheric ozone, CFCs and nitrous oxide.According to work published in 2007, the concentrationsof CO2 and methane have increased by 36% and 148%respectively since 1750.[70] These levels are much higherthan at any time during the last 800,000 years, the pe-riod for which reliable data has been extracted from icecores.[71][72][73][74] Less direct geological evidence indi-cates that CO2values higher than this were last seenabout 20 million years ago.[75] Fossil fuel burning has pro-duced about three-quarters of the increase in CO2 fromhuman activity over the past 20 years. The rest of thisincrease is caused mostly by changes in land-use, par-ticularly deforestation.[76] Estimates of global CO2 emis-sions in 2011 from fossil fuel combustion, including ce-ment production and gas aring, was 34.8 billion tonnes(9.5 0.5 PgC), an increase of 54% above emissions in1990. Coal burning was responsible for 43% of the to-tal emissions, oil 34%, gas 18%, cement 4.9% and gasaring 0.7%[77] In May 2013, it was reported that read-ings for CO2 taken at the worlds primary benchmark sitein Mauna Loa surpassed 400 ppm. According to profes-sor Brian Hoskins, this is likely the rst time CO2 lev-els have been this high for about 4.5 million years.[78][79]Monthly global CO2 concentrations exceeded 400 p.p.m.in March 2015, probably for the rst time in several mil-lion years.[80]Over the last three decades of the twentieth century,gross domestic product per capita and population growthwere the main drivers of increases in greenhouse gasemissions.[81] CO2 emissions are continuing to rise due tothe burning of fossil fuels and land-use change.[82][83]:71Emissions can be attributed to dierent regions. Attribu-tion of emissions due to land-use change is a controversialissue.[84][85]:289Emissions scenarios, estimates of changes in future emis-sionlevelsofgreenhousegases, havebeenprojectedthat depend upon uncertain economic, sociological,technological, and natural developments.[86] In most sce-narios, emissions continue to rise over the century, whilein a few, emissions are reduced.[87][88]Fossil fuel re-serves are abundant, and will not limit carbon emissionsin the 21st century.[89]Emission scenarios, combinedwith modelling of the carbon cycle, have been used toproduce estimates of how atmospheric concentrations ofgreenhouse gases might change in the future. Using thesix IPCC SRES marker scenarios, models suggest thatby the year 2100, the atmospheric concentration of CO2could range between 541 and 970 ppm.[90] This is 90250% above the concentration in the year 1750.The popular media and the public often confuseglobal warming with ozone depletion, i.e., the destruc-tionofstratosphericozone(e.g., theozonelayer)bychlorouorocarbons.[91][92]Although there are a fewareas of linkage, the relationship between the two is notstrong. Reduced stratospheric ozone has had a slight cool-ing inuence on surface temperatures, while increasedtropospheric ozone has had a somewhat larger warmingeect.[93]2.2 Particulates and sootGlobal dimming, a gradual reduction in the amount ofglobal direct irradiance at the Earths surface, was ob-2.3 Solar activity 5Atmospheric CO2 concentration from 650,000 years ago to nearpresent, using ice core proxy data and direct measurements.Ship tracks can be seen as lines in these clouds over the AtlanticOcean on the east coast of the United States. The climatic im-pacts from particulate forcing could have a large eect on cli-mate through the indirect eect.served from 1961 until at least 1990.[94] The main causeof this dimming is particulates produced by volcanoesand human made pollutants, which exerts a cooling ef-fect by increasing the reection of incoming sunlight. Theeects of the products of fossil fuel combustion CO2and aerosols have partially oset one another in re-cent decades, so that net warming has been due to the in-crease in non-CO2 greenhouse gases such as methane.[95]Radiative forcing due to particulates is temporally lim-ited due to wet deposition, which causes them to have anatmospheric lifetime of one week. Carbon dioxide hasa lifetime of a century or more, and as such, changes inparticulate concentrations will only delay climate changesdue to carbon dioxide.[96] Black carbon is second only tocarbon dioxide for its contribution to global warming.[97]In addition to their direct eect by scattering and absorb-ing solar radiation, particulates have indirect eects onthe Earths radiation budget. Sulfates act as cloud con-densation nuclei and thus lead to clouds that have moreand smaller cloud droplets. These clouds reect solar ra-diation more eciently than clouds with fewer and largerdroplets, phenomenon known as the Twomey eect.[98]This eect also causes droplets to be of more uniformsize, which reduces growth of raindrops and makes thecloud more reective to incoming sunlight, known as theAlbrecht eect.[99] Indirect eects are most noticeable inmarine stratiformclouds, and have very little radiative ef-fect on convective clouds. Indirect eects of particulatesrepresent the largest uncertainty in radiative forcing.[100]Soot may either cool or warm Earths climate system,depending on whether it is airborne or deposited. At-mospheric soot directly absorbs solar radiation,whichheats the atmosphere and cools the surface. In isolatedareas with high soot production, such as rural India, asmuch as 50%of surface warming due to greenhouse gasesmay be masked by atmospheric brown clouds.[101] Whendeposited, especially on glaciers or on ice in arctic re-gions, the lower surface albedo can also directly heatthe surface.[102] The inuences of particulates, includingblack carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the eects of green-house gases are dominant in the extratropics and southernhemisphere.[103]ChangesinTotalSolarIrradiance(TSI)andmonthlysunspotnumbers since the mid-1970s.2.3 Solar activityMain articles: Solar variation and Solar windSince 1978, solar irradiance has beenmeasuredbysatellites.[106] These measurements indicate that the Sunsradiative output has not increased since 1978, so thewarming during the past 30 years cannot be attributedto an increase in solar energy reaching the Earth.Climate models have been used to examine the role of theSun in recent climate change.[107] Models are unable toreproduce the rapid warming observed in recent decadeswhen they only take into account variations in solar outputand volcanic activity. Models are, however, able to sim-ulate the observed 20th century changes in temperaturewhen they include all of the most important external forc-ings, including human inuences and natural forcings.Another line of evidence against the Sun having caused6 4 CLIMATE MODELS-1.5-1-0.500.511.522.5-1.5-1-0.500.511.522.5-1.5-1-0.500.511.522.5Radiative Forcing ComponentsRadiative forcing (W/m2)CO2CH4HalocarbonsTroposphericN20StratosphericStratospheric water vapourLand useBlack carbonon snowDirect effectCloud albedoeffectLinear contrailsSolar irradianceNet AnthropogenicComponentOzone AlbedoAerosolsGreenhouseGasesContribution of natural factors and human activities to radiativeforcing of climate change.[104]Radiative forcing values are forthe year 2005, relative to the pre-industrial era (1750).[104]Thecontribution of solar irradiance to radiative forcing is 5% thevalue of the combined radiative forcing due to increases in theatmospheric concentrations of carbon dioxide, methane and ni-trous oxide.[105]recent climate change comes from looking at how tem-peratures at dierent levels in the Earths atmospherehave changed.[108]Models and observations show thatgreenhousewarmingresultsinwarmingoftheloweratmosphere (called the troposphere) but cooling ofthe upper atmosphere (called the stratosphere).[109][110]Depletion of the ozone layer by chemical refrigerants hasalso resulted in a strong cooling eect in the stratosphere.If the Sun were responsible for observed warming, warm-ing of both the troposphere and stratosphere would beexpected.[111]3 FeedbackMain articles: Climate change feedback and Climate sen-sitivityThe climate system includes a range of feedbacks, whichSeaice, shownhere inNunavut, innorthernCanada, reectsmoresunshine, whileopenoceanabsorbsmore, acceleratingmelting.alter the response of the system to changes in externalforcings. Positive feedbacks increase the response of theclimate system to an initial forcing, while negative feed-backs reduce the response of the climate system to aninitial forcing.[112]There are a range of feedbacks in the climate system,including water vapor, changes in ice-albedo (snow andice cover aect how much the Earths surface absorbsor reects incoming sunlight), clouds, and changes intheEarthscarboncycle(e.g., thereleaseofcarbonfrom soil).[113]The main negative feedback is the en-ergy the Earths surface radiates into space as infraredradiation.[114]According to the Stefan-Boltzmann law,if the absolute temperature (as measuredinkelvin)doubles,[lower-alpha 5] radiated energy increases by a factorof 16 (2 to the 4th power).[115]Feedbacks are an important factor in determining thesensitivityof theclimatesystemtoincreasedatmo-spheric greenhouse gas concentrations. Other factors be-ing equal, a higherclimatesensitivity means that morewarming will occur for a given increase in greenhousegas forcing.[116] Uncertainty over the eect of feedbacksis a major reason why dierent climate models projectdierent magnitudes of warmingfor agivenforcingscenario. More research is needed to understand therole of clouds[112] and carbon cycle feedbacks in climateprojections.[117]TheIPCCprojectionspreviouslymentionedspanthelikely range (greater than 66%probability, based on ex-pert judgement)[6] for the selected emissions scenarios.However, the IPCCs projections do not reect the fullrange of uncertainty.[118] The lower end of the likelyrange appears to be better constrained than the upper endof the likely range.[118]4 Climate modelsMain article: Global climate modelCalculations of global warming prepared in or before2001 from a range of climate models under the SRESA2 emissions scenario, which assumes no action is takentoreduceemissionsandregionallydividedeconomicdevelopment.7Projected change in annual mean surface air temperaturefrom the late 20th century to the middle 21st century,based on a medium emissions scenario (SRES A1B).[119]This scenario assumes that no future policies are adoptedto limit greenhouse gas emissions. Image credit: NOAAGFDL.[120]A climate model is a computerized representation of thephysical, chemical and biological processes that aect theclimate system.[121] Such models are based on scienticdisciplines such as uid dynamics and thermodynamics aswell as physical processes such as radiative transfer. Themodels predict a range of variables such as local air move-ment, temperature, clouds, and other atmospheric prop-erties; ocean temperature, salt content, and circulation;ice cover on land and sea; the transfer of heat and mois-ture from soiland vegetation to the atmosphere; andchemical and biological processes, among others.Although researchers attempt to include as many pro-cesses as possible, simplications of the actual climatesystem are inevitable because of the constraints of avail-able computer power and limitations in knowledge of theclimate system. Results from models can also vary dueto dierent greenhouse gas inputs and the models cli-mate sensitivity. For example, the uncertainty in IPCCs2007 projections is caused by (1) the use of multiplemodels[118] with diering sensitivity to greenhouse gasconcentrations,[122] (2) the use of diering estimates ofhumanitys future greenhouse gas emissions,[118] (3) anyadditional emissions from climate feedbacks that werenot included in the models IPCC used to prepare its re-port, i.e., greenhouse gas releases from permafrost.[123]The models do not assume the climate will warm due toincreasing levels of greenhouse gases. Instead the modelspredict how greenhouse gases will interact with radiativetransfer and other physical processes. Warming or cool-ing is thus a result, not an assumption, of the models.[124]Clouds and their eects are especially dicult to pre-dict. Improving the models representation of clouds istherefore an important topic in current research.[125] An-other prominent research topic is expanding and improv-ing representations of the carbon cycle.[126][127][128]Models are also used to help investigate the causes of re-cent climate change by comparing the observed changesto those that the models project from various natural andhuman causes. Although these models do not unambigu-ously attribute the warming that occurred from approxi-mately 1910 to 1945 to either natural variation or humaneects, they do indicate that the warming since 1970 isdominated by man-made greenhouse gas emissions.[59]The physical realismof models is testedby exam-iningtheir abilitytosimulate contemporaryor pastclimates.[129] Climate models produce a good match toobservations of global temperature changes over the lastcentury, but do not simulate all aspects of climate.[130]Not all eects of global warming are accurately predictedby the climate models used by the IPCC. Observed Arcticshrinkage has been faster than that predicted.[131] Precip-itation increased proportionally to atmospheric humidity,and hence signicantly faster than global climate modelspredict.[132][133] Since 1990, sea level has also risen con-siderably faster than models predicted it would.[134]5 Observedandexpectedenviron-mental eectsMain article: Eects of global warmingAnthropogenic forcing has likely contributed to some ofProjections of global mean sea level rise by Parris and others.[135]Probabilities havenot beenassignedtotheseprojections.[136]Therefore, none of these projections should be interpreted as abest estimate of future sea level rise. Image credit: NOAA.the observed changes, including sea level rise, changes inclimate extremes (such as the number of warm and colddays), declines in Arctic sea ice extent, glacier retreat, andgreening of the Sahara.[137][138]During the 21st century, glaciers[139] and snow cover[140]are projected to continue their widespread retreat. Pro-jections of declines in Arctic sea ice vary.[141][142] Recentprojections suggest that Arctic summers could be ice-free(dened as ice extent less than 1 million square km) asearly as 2025-2030.[143]Detection is the process of demonstrating that climatehas changed in some dened statistical sense, withoutproviding a reason for that change. Detection does notimply attribution of the detected change to a particularcause. Attribution of causes of climate change is theprocess of establishing the most likely causes for the de-tected change with some dened level of condence.[144]Detection and attribution may also be applied to observedchanges in physical, ecological and social systems.[145]8 5 OBSERVED AND EXPECTED ENVIRONMENTAL EFFECTS5.1 Extreme weatherMain articles: Extreme weather and Physical impacts ofclimate change Extreme eventsChanges inregional climateareexpectedtoincludegreater warming over land, with most warming at highnorthern latitudes, and least warming over the SouthernOcean and parts of the North Atlantic Ocean.[146]Future changes in precipitation are expected to followex-isting trends, with reduced precipitation over subtropicalland areas, and increased precipitation at subpolar lati-tudes and some equatorial regions.[147] Projections sug-gest a probable increase in the frequency and severity ofsome extreme weather events, such as heat waves.[148]A 2015 study published in Nature, states: About 18% ofthe moderate daily precipitation extremes over land are at-tributable to the observed temperature increase since pre-industrial times, which in turn primarily results from hu-man inuence. For 2 C of warming the fraction of pre-cipitation extremes attributable to human inuence rises toabout 40%. Likewise, today about 75% of the moderatedaily hot extremes over land are attributable to warming.It is the most rare and extreme events for which the largestfraction is anthropogenic, and that contribution increasesnonlinearly with further warming.[149][150]5.2 Sea level riseMain articles: Sea level rise and DeglaciationSea level rise, has been estimated to be on average +2.6Sparse records indicate that glaciers have been retreating sincethe early 1800s. In the 1950s measurements began that allowthe monitoring of glacial mass balance,reported to the WorldGlacier Monitoring Service (WGMS) and the National Snow andIce Data Center (NSIDC).mm and +2.9 mm per year 0.4 mm since 1993. Addi-tionally, sea level rise has accelerated in recent years.[151]Over the 21st century, the IPCC projects for a high emis-sions scenario, that global mean sea level could rise by5298 cm.[152] The IPCCs projections are conservative,and may underestimate future sea level rise.[153] Other es-timates suggest for the same period that global mean sealevel could rise by 0.2 to 2.0 m (0.76.6 ft), relative tomean sea level in 1992.[135]Widespread coastal ooding would be expected if sev-eral degrees of warming is sustained for millennia.[154]For example, sustained global warming of more than 2C(relative to pre-industrial levels) could lead to eventualsea level rise of around 1 to 4 m due to thermal expan-sion of sea water and the melting of glaciers and smallice caps.[154] Melting of the Greenland ice sheet couldcontribute an additional 4 to 7.5 m over many thousandsof years.[154] It has been estimated that we are alreadycommitted to a sea-level rise of approximately 2.3 me-ters for each degree of temperature rise within the next2,000 years.[155]5.3 Ecological systemsMain article: Climate change and ecosystemsInterrestrial ecosystems, theearliertimingofspringevents, and poleward and upward shifts in plant and ani-mal ranges, have been linked with high condence to re-cent warming.[156] Future climate change is expected toparticularly aect certain ecosystems, including tundra,mangroves, and coral reefs.[146] It is expected that mostecosystems will be aected by higher atmospheric CO2levels, combinedwithhigherglobal temperatures.[157]Overall, it is expected that climate change will result inthe extinction of many species and reduced diversity ofecosystems.[158]Increases in atmospheric CO2concentrations have ledto an increase in ocean acidity.[159] Dissolved CO2 in-creases ocean acidity, which is measured by lower pHvalues.[159]Between1750to2000, surface-oceanpHhas decreased by 0.1, from 8.2 to 8.1.[160] Surface-ocean pH has probably not been below 8.1 during thepast 2 million years.[160] Projections suggest that surface-ocean pHcould decrease by an additional 0.30.4 units by2100.[161] Future ocean acidication could threaten coralreefs, sheries, protected species, and other natural re-sources of value to society.[159][162]Ocean deoxygenation is projected to increase hypoxia by10%, and triple suboxic waters (oxygen concentrations98% less than the mean surface concentrations), for each1 C of upper Ocean warming.[163]5.4 Long-term eectsMain article: Long-term eects of global warmingOn the timescale of centuries to millennia, the magni-tude of global warming will be determined primarily by6.1 Habitat inundation 9anthropogenic CO2 emissions.[164] This is due to carbondioxides very long lifetime in the atmosphere.[164]Stabilizing global average temperature would require re-ductions in anthropogenic CO2emissions.[164]Reduc-tions in emissions of non-CO2 anthropogenic greenhousegases (GHGs) (e.g., methane and nitrous oxide) wouldalso be necessary.[164][165] For CO2, anthropogenic emis-sions would need to be reduced by more than 80%relativeto their peak level.[164] Even if this were achieved, globalaverage temperatures would remain close to their highestlevel for many centuries.[164]5.5 Large-scale and abrupt impactsMain article: Abrupt climate changeClimate change could result in global, large-scale changesinnatural andsocial systems.[166]Twoexamplesareocean acidication caused by increased atmospheric con-centrations of carbon dioxide, and the long-term meltingof ice sheets, which contributes to sea level rise.[167]Some large-scale changes could occur abruptly, i.e., overa short time period, and might also be irreversible. Anexample of abrupt climate change is the rapid releaseof methane and carbon dioxide from permafrost, whichwould lead to amplied global warming.[168][169] Scien-tic understanding of abrupt climate change is generallypoor.[170] The probability of abrupt change for some cli-mate related feedbacks may be low.[168][171] Factors thatmay increase the probability of abrupt climate change in-clude higher magnitudes of global warming, warming thatoccurs more rapidly, and warming that is sustained overlonger time periods.[171]6 Observed and expected eects onsocial systemsFurther information: Eects of global warming Socialsystems and Regional eects of global warming Regional impactsThe eects of climate change on human systems, mostlydue to warming or shifts in precipitation patterns, orboth, havebeendetectedworldwide. Productionofwheat and maize globally has been impacted by climatechange. While crop production has increased in somemid-latitude regions such as the UKand Northeast China,economic losses due to extreme weather events have in-creased globally. There has been a shift from cold- toheat-related mortality in some regions as a result of warm-ing. Livelihoods of indigenous peoples of the Arctichave been altered by climate change, and there is emerg-ing evidence of climate change impacts on livelihoods ofindigenous peoples in other regions. Regional impacts ofclimate change are now observable at more locations thanbefore, on all continents and across ocean regions.[172]Thefuturesocial impactsofclimatechangewill beuneven.[173]Many risks are expected to increase withhigher magnitudes of global warming.[174]All regionsare at risk of experiencing negative impacts.[175] Low-latitude, less developed areas face the greatest risk.[176] Astudy from 2015 concluded that economic growth (Grossdomestic product) of poorer countries is much more im-paired with projected future climate warming, than pre-viously thought.[177]Ameta analysis of 56 studies concluded in 2014 that eachdegree of temperature rise will increase violence by up to20%, which includes st ghts, violent crimes, civil unrestor wars.[178]Examples of impacts include:Food:Crop production will probably be negativelyaected in low latitude countries, while eects atnorthern latitudes may be positive or negative.[179]Global warming of around 4.6 C relative to pre-industrial levels could pose a large risk to global andregional food security.[180]Health: Generally impacts will be more negativethan positive.[181] Impacts include: the eects of ex-treme weather, leading to injury and loss of life;[182]and indirect eects, such as undernutrition broughton by crop failures.[183]6.1 Habitat inundationMap showing where natural disasters caused/aggravatedbyglobal warming may occur.Further information: Eects of climate change onhumans Displacement/migrationSee also: Climate refugeeIn small islands and mega deltas, inundation as a result ofsea level rise is expected to threaten vital infrastructureand human settlements.[184][185] This could lead to issuesof homelessness in countries with low lying areas suchas Bangladesh, as well as statelessness for populations incountries such as the Maldives and Tuvalu.[186]10 8 DISCOURSE ABOUT GLOBAL WARMING7 Possible responses to globalwarming7.1 MitigationMain article: Climate change mitigationMitigationof climatechangeareactions toreduceThe graph on the right shows three pathways to meet the UN-FCCCs 2 C target, labelled global technology, decentralisedsolutions, and consumption change. Each pathway shows howvariousmeasures(e.g., improvedenergyeciency, increaseduse of renewable energy) could contribute to emissions reduc-tions. Image credit: PBL Netherlands Environmental AssessmentAgency.[187]greenhouse gas (GHG) emissions, or enhance the ca-pacity of carbon sinks to absorb GHGs fromtheatmosphere.[188] There is a large potential for future re-ductionsinemissionsbyacombinationofactivities,including: energyconservationandincreasedenergyeciency; the use of low-carbon energy technolo-gies, suchas renewableenergy, nuclear energy, andcarbon capture and storage;[189][190] and enhancing car-bon sinks through, for example, reforestation and pre-venting deforestation.[189][190]Near- and long-term trends in the global energy systemare inconsistent with limiting global warming at below1.5or 2 C, relative to pre-industrial levels.[191][192] Pledgesmade as part of the Cancn agreements are broadly con-sistent with having a likely chance (66 to 100% proba-bility) of limiting global warming (in the 21st century) atbelow 3 C, relative to pre-industrial levels.[192]In limiting warming at below 2 C, more stringent emis-sion reductions in the near-term would allow for lessrapid reductions after 2030.[193] Many integrated mod-els are unable to meet the 2 C target if pessimistic as-sumptions are made about the availability of mitigationtechnologies.[194]7.2 AdaptationMain article: Adaptation to global warmingOther policyresponses includeadaptationtoclimatechange. Adaptation to climate change may be planned,either in reaction to or anticipation of climate change, orspontaneous, i.e., without government intervention.[195]Plannedadaptationisalreadyoccurringonalimitedbasis.[189] The barriers, limits, and costs of future adap-tation are not fully understood.[189]Aconcept relatedtoadaptationis adaptive capacity,which is the ability of a system (human, natural or man-aged) toadjust toclimatechange(includingclimatevariabilityandextremes) tomoderatepotential dam-ages, to take advantage of opportunities, or to cope withconsequences.[196] Unmitigated climate change (i.e., fu-ture climate change without eorts to limit greenhousegas emissions) would, in the long term, be likely to exceedthe capacity of natural, managed and human systems toadapt.[197]Environmental organizations and public gures have em-phasized changes in the climate and the risks they entail,while promoting adaptation to changes in infrastructuralneeds and emissions reductions.[198]7.3 Climate engineeringMain article: Climate engineeringClimate engineering (sometimes called by the more ex-pansive term 'geoengineering'), is the deliberate modi-cation of the climate. It has been investigated as a pos-sible response to global warming, e.g. by NASA[199]and the Royal Society.[200]Techniques under researchfall generally into the categories solar radiation manage-ment and carbon dioxide removal, although various otherschemes have been suggested. A study from 2014 inves-tigated the most common climate engineering methodsand concluded they are either ineective or have poten-tially severe side eects and cannot be stopped withoutcausing rapid climate change.[201]8 Discourse about global warming8.1 Political discussionMain article: Politics of global warmingFurther information: 2011 United Nations ClimateChange Conference, 2012 United Nations Climate8.2 Scientic discussion 11Change Conference and 2013 United Nations ClimateChange ConferenceMost countries are Parties to the United Nations Frame-Article 2 of the UN Framework Convention refers explicitly tostabilization of greenhouse gas concentrations.[202]To stabilizethe atmospheric concentration of CO2, emissions worldwide would need to be dramatically reducedfrom their present level.[203]work Convention on Climate Change (UNFCCC).[204]The ultimate objective of the Convention is to preventdangerous human interference of the climate system.[205]As is stated in the Convention, this requires that GHGconcentrations are stabilized in the atmosphere at a levelwhere ecosystems can adapt naturally to climate change,food production is not threatened, and economic devel-opment can proceed in a sustainable fashion.[206]TheFramework Convention was agreed in 1992, but sincethen, global emissions have risen.[207]During negotia-tions, the G77 (a lobbying group in the United Nationsrepresenting133developingnations)[208]:4pushedfora mandate requiring developed countries to "[take] thelead in reducing their emissions.[209] This was justiedon the basis that: the developed worlds emissions hadcontributed most to the stock of GHGs in the atmo-sphere; per-capita emissions (i.e., emissions per head ofpopulation) were still relatively low in developing coun-tries;and the emissions of developing countries wouldgrow to meet their development needs.[85]:290 This man-date was sustained in the Kyoto Protocol to the Frame-work Convention,[85]:290 which entered into legal eectin 2005.[210]In ratifying the Kyoto Protocol, most developed coun-tries accepted legally binding commitments to limit theiremissions. These rst-round commitments expired in2012.[210] United States President George W. Bush re-jected the treaty on the basis that it exempts 80% ofthe world, including major population centers such asChina and India, from compliance, and would cause se-rious harm to the US economy.[208]:5At the 15th UNFCCC Conference of the Parties, held in2009 at Copenhagen, several UNFCCC Parties producedthe Copenhagen Accord.[211][212] Parties associated withthe Accord (140 countries, as of November 2010)[213]:9aim to limit the future increase in global mean tempera-ture to below 2 C.[214] The 16th Conference of the Par-ties (COP16) was held at Cancn in 2010. It producedan agreement, not a binding treaty, that the Parties shouldtake urgent action to reduce greenhouse gas emissions tomeet a goal of limiting global warming to 2 Cabove pre-industrial temperatures. It also recognized the need toconsider strengthening the goal to a global average rise of1.5 C.[215]8.2 Scientic discussionSee also: Scientic opiniononclimate change andSurveys of scientists views on climate changeMost scientists agree that humans are contributing to ob-served climate change.[82][216] A meta study of academicpapers concerning global warming, published between1991 and 2011 and accessible from Web of Knowledge,found that among those whose abstracts expressed a po-sition on the cause of global warming, 97.2% supportedthe consensus view that it is man made.[217] In an Octo-ber 2011 paper published in the International Journal ofPublic Opinion Research, researchers from George Ma-son University analyzed the results of a survey of 489American scientists working in academia, government,and industry. Of those surveyed, 97% agreed that thatglobal temperatures have risen over the past century and84% agreed that human-induced greenhouse warmingis now occurring, only 5% disagreeing that human activ-ity is a signicant cause of global warming.[218][219] Na-tional science academies have called on world leaders forpolicies to cut global emissions.[220]In the scientic literature, there is a strong consensusthat global surface temperatures have increased in recentdecades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scienticbody of national or international standing disagrees withthis view.[221][222]12 9 ETYMOLOGY8.3 Discussion by the public and in popu-lar mediaMain articles: Climate change denial, Global warmingcontroversy and Media coverage of climate changeThe global warming controversy refers to a variety ofdisputes, substantially more pronounced in the popularmedia than in the scientic literature,[223][224] regardingthe nature, causes, and consequences of global warming.The disputed issues include the causes of increased globalaverage air temperature,especially since the mid-20thcentury,whether this warming trend is unprecedentedor within normal climatic variations, whether humankindhas contributed signicantly to it, and whether the in-crease is wholly or partially an artifact of poor measure-ments. Additional disputes concern estimates of climatesensitivity, predictions of additional warming, and whatthe consequences of global warming will be.From19901997 in the United States, conservative thinktanks mobilized to challenge the legitimacy of globalwarming as a social problem. They challenged the scien-tic evidence, argued that global warming will have ben-ets, and asserted that proposed solutions would do moreharm than good.[225]Some people dispute aspects of climate changescience.[216][226]Organizations suchas thelibertarianCompetitive Enterprise Institute, conservative com-mentators, andsomecompaniessuchasExxonMobilhave challenged IPCC climate change scenarios, fundedscientists whodisagreewiththescienticconsensus,andprovidedtheir ownprojections of theeconomiccost of stricter controls.[227][228][229][230]Some fossilfuel companies have scaled back their eorts in recentyears,[231]or even called for policies to reduce globalwarming.[232]8.3.1 Surveys of public opinionMain article: Public opinion on climate changeA2010 poll by the Oce for National Statistics found that75% of UK respondents were at least fairly convincedthat the worlds climate is changing, compared to 87% ina similar survey in 2006.[233] A January 2011 ICM poll inthe UKfound 83%of respondents viewed climate changeas a current or imminent threat, while 14% said it was nothreat. Opinion was unchanged froman August 2009 pollasking the same question, though there had been a slightpolarisation of opposing views.[234]By 2010, with 111 countries surveyed, Gallup determinedthat there was a substantial decrease since 200708 in thenumber of Americans and Europeans who viewed globalwarming as a serious threat.In the US, just a little overhalf the population (53%) now viewed it as a serious con-cern for either themselves or their families; this was 10points belowthe 2008 poll (63%). Latin America had thebiggest rise in concern: 73%said global warming is a seri-ous threat to their families.[235] This global poll also foundthat people are more likely to attribute global warmingto human activities than to natural causes, except in theUS where nearly half (47%) of the population attributedglobal warming to natural causes.[236]A MarchMay 2013 survey by Pew Research Center forthe People & the Press polled 39 countries about globalthreats. According to 54% of those questioned, globalwarming featured top of the perceived global threats.[237]In a January 2013 survey, Pew found that 69% of Amer-icans say there is solid evidence that the Earths aver-age temperature has been getting warmer over the pastfew decades, up six points since November 2011 and 12points since 2009.[238]9 EtymologyIn the 1950s, research suggested increasing temperatures,and a 1952 newspaper reported climate change. Thisphrase next appeared in a November 1957 report in TheHammondTimes which described Roger Revelle's re-search into the eects of increasing human-caused CO2emissions on the greenhouse eect, a large scale globalwarming, with radical climate changes may result. Bothphrases were only used occasionally until 1975,whenWallace Smith Broecker published a scientic paper onthe topic; Climatic Change: Are We on the Brink ofa Pronounced Global Warming?" The phrase began tocome into common use, and in 1976 Mikhail Budyko'sstatement that a global warming up has started waswidely reported.[239] Other studies, such as a 1971 MITreport, referredtothehumanimpact asinadvertentclimate modication, but an inuential 1979 NationalAcademy of Sciences study headed by Jule Charney fol-lowed Broecker in using global warming for rising sur-face temperatures, while describing the wider eects ofincreased CO2 as climate change.[240]In 1986 and November 1987, NASA climate scientistJamesHansengavetestimonytoCongressonglobalwarming, but gained little attention. There were increas-ing heatwaves and drought problems in the summer of1988, and when Hansen testied in the Senate on 23June he sparked worldwide interest.[241] He said: globalwarming has reached a level such that we can ascribewith a high degree of condence a cause and eect rela-tionship between the greenhouse eect and the observedwarming.[242] Public attention increased over the sum-mer, and global warming became the dominant popularterm, commonly used both by the press and in publicdiscourse.[240]In a 2008 NASA article on usage, Erik M. Conway de-ned Global warming as the increase in Earths average13surface temperature due to rising levels of greenhousegases, while Climate change was a long-term change inthe Earths climate, or of a region on Earth.As eects suchas changingpatterns of rainfall andrising sea levels would probably have more impactthan temperatures alone, he considered global climatechange a more scientically accurate term, and like theIntergovernmental Panel on Climate Change, the NASAwebsite would emphasise this wider context.[240]10 See alsoClimate change and agricultureEects of global warming on oceansEnvironmental impact of the coal industryGeologic temperature recordGlobal coolingGlossary of climate changeGreenhouse gas emissions accountingHistory of climate change scienceIndex of climate change articlesScientic opinion on climate change11 Notes[1] Scientic journals use global warming to describe an in-creasing global average temperature just at earths surface,and most of these authorities further limit global warm-ing to such increases caused by human activities or in-creasing greenhouse gases.[2] The 2001 joint statement was signed by the na-tional academies of science of Australia, Belgium,Brazil, Canada, the Caribbean, the Peoples Repub-lic of China, France, Germany, India, Indonesia, Ire-land, Italy, Malaysia, NewZealand, Sweden, andtheUK.[11]The 2005 statement added Japan, Russia, andthe U.S. The 2007 statement added Mexico and SouthAfrica. The Network of African Science Academies,and the Polish Academy of Sciences have issued sep-aratestatements. Professional scienticsocieties in-clude American Astronomical Society, American Chem-ical Society, AmericanGeophysical Union, AmericanInstitute of Physics, American Meteorological Society,American PhysicalSociety, American Quaternary As-sociation, Australian Meteorological and OceanographicSociety, CanadianFoundationforClimateandAtmo-spheric Sciences, Canadian Meteorological and Oceano-graphic Society, European Academy of Sciences and Arts,European Geosciences Union, European Science Founda-tion, Geological Society of America, Geological Societyof Australia, Geological Society of London-StratigraphyCommission, InterAcademy Council, International Unionof Geodesy and Geophysics, International Union for Qua-ternaryResearch, National AssociationofGeoscienceTeachers, National Research Council (US), Royal Mete-orological Society, and World Meteorological Organiza-tion.[3] Earth has already experienced almost 1/2 of the 2.0 C(3.6 F) described in the Cancn Agreement.In the last100 years, Earths average surface temperature increasedby about 0.8 C (1.4 F) with about two thirds of the in-crease occurring over just the last three decades.[27][4] The greenhouse eect produces an average worldwidetemperature increase of about 33 C (59 F) comparedto black body predictions without the greenhouse eect,not an average surface temperature of 33 C (91 F). Theaverage worldwide surface temperature is about 14 C(57F).[5] Arise in temperature from10 Cto 20 Cis not a doublingof absolute temperature; a rise from(273 + 10) K= 283 Kto (273 + 20) K= 293 Kis an increase of (293 283)/283= 3.5 %.12 Citations[1] 16 January 2015: NASA GISS: NASA GISS: NASA,NOAA Find 2014 Warmest Year in Modern Record, in:Research News. NASAGoddard Institute for Space Stud-ies, New York, NY, USA. Accessed 20 February 2015[2] Hartmann, D. L.; Klein Tank, A. M. G.; Rusticucci, M.(2013). FAQ 2.1 2: Observations: Atmosphere and Sur-face (PDF). IPCC WGI AR5 (Report). Evidence for awarming world comes from multiple independent climateindicators, fromhigh up in the atmosphere to the depths ofthe oceans. They include changes in surface, atmosphericand oceanic temperatures; glaciers; snow cover; sea ice;sea level and atmospheric water vapour. Scientists fromallover the world have independently veried this evidencemany times.[3] Myth vs Facts..... EPA (US). 2013.The U.S. GlobalChangeResearchProgram, theNational AcademyofSciences, and the IntergovernmentalPanelon ClimateChange (IPCC) have each independently concluded thatwarming of the climate system in recent decades is 'un-equivocal'. This conclusion is not drawn from any onesource of data but is based on multiple lines of evidence,including three worldwide temperature datasets showingnearly identical warming trends as well as numerous otherindependent indicators of global warming (e.g., rising sealevels, shrinking Arctic sea ice).[4] Rhein, M.; Rintoul, S.R.(2013). 3: Observations:Ocean (PDF). IPCC WGI AR5 (Report). p. 257. Oceanwarming dominates the global energy change inventory.Warming of the ocean accounts for about 93% of the in-crease in the Earths energy inventory between 1971 and2010 (high condence), with warming of the upper (0 to700 m) ocean accounting for about 64%of the total. Melt-ing ice (including Arctic sea ice, ice sheets and glaciers)14 12 CITATIONSand warming of the continents and atmosphere accountfor the remainder of the change in energy.[5] IPCC, Climate Change 2013: The Physical Science Basis- Summary for Policymakers, Observed Changes in theClimate System, p. 2, in IPCC AR5 WG1 2013. Warm-ing of the climate system is unequivocal, and since the1950s, many of the observed changes are unprecedentedover decades to millennia.[6] CLIMATE CHANGE 2014: Synthesis Report. Sum-mary for Policymakers (PDF). IPCC. Retrieved 7 March2015. The following terms have been used to indicatethe assessed likelihood of an outcome or a result:virtu-ally certain 99100% probability, very likely 90100%,likely 66100%, about as likely as not 3366%, unlikely033%, very unlikely 010%, exceptionally unlikely 01%. Additional terms (extremely likely: 95100%, morelikely than not >50100%, more unlikely than likely 0