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.