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