Human‐InducedClimateChangeRequiresUrgentAction
Humanityisthemajorinfluenceontheglobalclimatechangeobservedoverthepast50years.
Rapidsocietalresponsescansignificantlylessennegativeoutcomes.
HumanactivitiesarechangingEarth’sclimate.Atthegloballevel,atmospheric
concentrationsofcarbondioxideandotherheat‐trappinggreenhousegaseshaveincreased
sharplysincetheIndustrialRevolution.Fossilfuelburningdominatesthisincrease.
Human‐causedincreasesingreenhousegasesareresponsibleformostoftheobserved
globalaveragesurfacewarmingofroughly0.8°C(1.5°F)overthepast140years.Because
naturalprocessescannotquicklyremovesomeofthesegases(notablycarbondioxide)
fromtheatmosphere,ourpast,present,andfutureemissionswillinfluencetheclimate
systemformillennia.
Extensive,independentobservationsconfirmtherealityofglobalwarming.These
observationsshowlarge‐scaleincreasesinairandseatemperatures,sealevel,and
atmosphericwatervapor;theydocumentdecreasesintheextentofmountainglaciers,
snowcover,permafrost,andArcticseaice.Thesechangesarebroadlyconsistentwithlong‐
understoodphysicsandpredictionsofhowtheclimatesystemisexpectedtorespondto
human‐causedincreasesingreenhousegases.Thechangesareinconsistentwith
explanationsofclimatechangethatrelyonknownnaturalinfluences.
Climatemodelspredictthatglobaltemperatureswillcontinuetorise,withtheamountof
warmingprimarilydeterminedbythelevelofemissions.Higheremissionsofgreenhouse
gaseswillleadtolargerwarming,andgreaterriskstosocietyandecosystems.Some
additionalwarmingisunavoidableduetopastemissions.
Climatechangeisnotexpectedtobeuniformoverspaceortime.Deforestation,
urbanization,andparticulatepollutioncanhavecomplexgeographical,seasonal,and
longer‐termeffectsontemperature,precipitation,andcloudproperties.Inaddition,
human‐inducedclimatechangemayalteratmosphericcirculation,dislocatinghistorical
patternsofnaturalvariabilityandstorminess.
Inthecurrentclimate,weatherexperiencedatagivenlocationorregionvariesfromyear
toyear;inachangingclimate,boththenatureofthatvariabilityandthebasicpatternsof
weatherexperiencedcanchange,sometimesincounterintuitiveways‐‐someareasmay
experiencecooling,forinstance.Thisraisesnochallengetotherealityofhuman‐induced
climatechange.
Impactsharmfultosociety,includingincreasedextremesofheat,precipitation,andcoastal
highwaterarecurrentlybeingexperienced,andareprojectedtoincrease.Otherprojected
outcomesinvolvethreatstopublichealth,wateravailability,agriculturalproductivity
(particularlyinlow‐latitudedevelopingcountries),andcoastalinfrastructure,thoughsome
benefitsmaybeseenatsometimesandplaces.Biodiversitylossisexpectedtoaccelerate
duetobothclimatechangeandacidificationoftheoceans,whichisadirectresultof
increasingcarbondioxidelevels.
Whileimportantscientificuncertaintiesremainastowhichparticularimpactswillbe
experiencedwhere,nouncertaintiesareknownthatcouldmaketheimpactsofclimate
changeinconsequential.Furthermore,surpriseoutcomes,suchastheunexpectedlyrapid
lossofArcticsummerseaice,mayentailevenmoredramaticchangesthananticipated.
Actionsthatcoulddiminishthethreatsposedbyclimatechangetosocietyandecosystems
includesubstantialemissionscutstoreducethemagnitudeofclimatechange,aswellas
preparingforchangesthatarenowunavoidable.Thecommunityofscientistshas
responsibilitiestoimproveoverallunderstandingofclimatechangeanditsimpacts.
Improvementswillcomefrompursuingtheresearchneededtounderstandclimatechange,
workingwithstakeholderstoidentifyrelevantinformation,andconveyingunderstanding
clearlyandaccurately,bothtodecisionmakersandtothegeneralpublic.
AdoptedbytheAmericanGeophysicalUnionDecember2003;RevisedandReaffirmed
December2007,February2012,August2013.
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Human‐Induced Climate Change Requires Urgent Action Humanity is the major influence on the global climate change observed over the past 50 years. Rapid societal responses can significantly lessen negative outcomes.