Nenita Mangulabnan Sulit

 Deutsche Telekom Company Book: zretyping job climate Change 36 pages Given date: November25,2025 Due date: Nov 27,2025 Submitted to: Roman Clinton Supervisor Dare Submitted:zNov 27, 2025  $Php rate $1.58.78 = =- Project by Nenita Sulit CTB / Virtual Assistant/ Freelancer7 Climate Change Evidence&Causes Update 2020 An overview from the Royal Society and the US National Academy of Sciences Foreword CLIMATECHANGEISONEOFTHEDEFININGISSUESOFOURTIME.Itisnowmorecertain than ever, based on many lines of evidence, that humans are changing Earth’s climate. The atmosphere and oceans have warmed, which has been accompanied by sea level rise, a strong decline in Arctic sea ice, and other climate-related changes. The impacts of climate change on people and nature are increasingly apparent. Unprecedented flooding, heat waves, and wildfires havecostbillionsindamages.Habitatsareundergoingrapidshiftsinresponsetochanging temperatures and precipitation patterns. The Royal Society and the US National Academy of Sciences, with their similar missions to promotetheuseofsciencetobenefitsocietyandtoinformcriticalpolicydebates,producedthe original Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UKUS teamofleadingclimatescientists.Thisnewedition,preparedbythesameauthorteam,has been updatedwiththemostrecentclimatedataandscientificanalyses,allofwhichreinforceour understanding of human-caused climate change. The evidence is clear. However, due to the nature of science, not every detail is ever totally settled or certain. Nor has every pertinent question yet been answered. Scientific evidence continues to be gathered around the world. Some things have become clearer and new insights have emerged. Forexample,theperiodofslowerwarmingduringthe2000sandearly2010shasendedwitha dramatic jump to warmer temperatures between 2014 and 2015. Antarctic sea ice extent, which had been increasing, began to decline in 2014, reaching a record low in 2017 that has persisted. These and other recent observations have been woven into the discussions of the questions addressed in this booklet. Calls for action are getting louder. The 2020 Global Risks Perception Survey from the World Economic Forum ranked climate change and related environmental issues as the top five global riskslikelytooccurwithinthenextten years.Yet,theinternationalcommunitystillhasfartogoin showing increased ambition on mitigation, adaptation, and other ways to tackle climate change. Scientificinformationisavitalcomponentforsocietytomakeinformeddecisionsabouthowto reducethemagnitudeofclimatechangeandhowtoadapttoitsimpacts.Thisbookletservesasa key reference document for decision makers, policy makers, educators, and others seeking authoritative answers about the current state of climate-change science. We are grateful that six years ago, under the leadership of Dr. Ralph J. Cicerone, former President of the National Academy of Sciences, and Sir Paul Nurse, former President of the Royal Society, these two organizations partnered to produce a high-level overview of climate change science. As current Presidents of these organizations, we are pleased to offer an update to this key reference, supported by the generosity of the Cicerone Family For further reading Formore detailed discussion ofthe topics addressed in this document (includingreferencestotheunderlyingoriginalresearch),see: ■ Intergovernmental Panel on ClimateChange(IPCC), 2019: Special Report on the Ocean and Cryosphere in a Changing Climate [https://www.ipcc.ch/srocc] ■ NationalAcademiesof Sciences, Engineering, andMedicine (NASEM), 2019: Negative Emissions Technologies and Reliable Sequestration: A Research Agenda [https://www.nap.edu/catalog/25259] ■ RoyalSociety,2018:Greenhousegasremoval [https://raeng.org.uk/greenhousegasremoval] ■ U.S.Global Change ResearchProgram(USGCRP),2018:Fourth National Climate Assessment Volume II: Impacts, Risks, and Adaptation in the United States [https://nca2018.globalchange.gov] ■ IPCC, 2018: Global Warming of 1.5°C [https://www.ipcc.ch/sr15] ■ USGCRP,2017:Fourth NationalClimateAssessmentVolumeI:Climate Science Special Reports [https://science2017.globalchange.gov] ■ NASEM,2016: AttributionofExtremeWeatherEventsinthe Context of Climate Change [https://www.nap.edu/catalog/21852] ■ IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. ClimateChange2013:ThePhysicalScienceBasis [https://www.ipcc.ch/report/ar5/wg1] ■ NRC, 2013: AbruptImpacts of Climate Change:Anticipating Surprises [https://www.nap.edu/catalog/18373] ■ NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia [https://www.nap.edu/catalog/12877] ■ Royal Society 2010: Climate Change:A Summary of theScience [https://royalsociety.org/topics-policy/publications/2010/ climate-change-summary-science] ■ NRC,2010:America’sClimateChoices: AdvancingtheScience of Climate Change [https://www.nap.edu/catalog/12782] Muchoftheoriginaldataunderlyingthescientific findings discussedhereareavailableat: ■ https://data.ucar.edu/ ■ https://climatedataguide.ucar.edu ■ https://iridl.ldeo.columbia.edu ■ https://ess-dive.lbl.gov/ ■ https://www.ncdc.noaa.gov/ ■ https://www.esrl.noaa.gov/gmd/ccgg/trends/ ■ http://scrippsco2.ucsd.edu ■ http://hahana.soest.hawaii.edu/hot/ THENATIONALACADEMYOFSCIENCES(NAS)wasestablishedtoadvisetheUnited StatesonscientificandtechnicalissueswhenPresidentLincolnsigneda Congressional charterin1863.TheNationalResearchCouncil,theoperatingarmoftheNationalAcademy ofSciencesandtheNationalAcademyofEngineering,hasissuednumerousreportsonthe causesofandpotentialresponsestoclimatechange.Climatechangeresourcesfromthe National Research Council are available at nationalacademies.org/climate. THEROYALSOCIETYisaself-governingFellowshipofmanyoftheworld’smost distinguishedscientists.Itsmembers aredrawnfromallareasofscience,engineering, andmedicine.ItisthenationalacademyofscienceintheUK. The Society’sfundamental purpose,refl cted initsfoundingCharters ofthe 1660s,istorecognise,promote, and supportexcellenceinscience,andtoencouragethedevelopmentanduseofscience forthebenefitofhumanity.MoreinformationontheSociety’sclimatechangeworkis available at royalsociety.org/policy/climate-change Content Summary....................................................................................... 2 Climate Change Q& A 1 Is the climate arming?.............................................................................. 3 2 Howdo knowthatrecent climatechangeis largely caused by humanactivities? ....... 5 3 CO2 is already in the atmosphere naturally, so why are emissions from human activity significant?..........................................................................6 4 Whatrole has the Sun played in climate change in recent decades?............................7 5 What do changes in the vertical structure of atmospheric temperature—from the surfaceuptothestratosphere—tellusaboutthecausesofrecentclimatechange?...........8 6 Climate is always changing. Why is climate change of concern now? ..........................9 7 IsthecurrentlevelofatmosphericCO2concentrationunprecedentedinEarth’shistory?.....9 8 Is there a point atwhich adding moreCO2will not cause furtherwarming?.................. 10 9 Does the rate of warming vary from one decade to another?...................................11 10 Didtheslowdownofwarmingduringthe2000s toearly2010s mean that climate change is no longer happening?............................................. 12 The Basics of Climate Change ............................................................................................. B1–B8 Climate Change Q&A (continued) 11 Iftheworldiswarming,why are somewinters and summers still very cold?................. 13 12 Why isArctic sea ice decreasingwhileAntarctic sea icehas changedlittle? ................. 14 13 Howdoes climatechange affectthe strength and frequency of floods, droughts, hurricanes, and tornadoes?.....................................................15 14 How fastis sea level rising?........................................................................ 16 15 Whatis ocean acidification and why does it matter?................................................17 16 Howconfident are scientists that Earth will warm further overthe coming century? ........... 18 17 Are climate changes of a few degrees a cause for concern? ........................................19 18 What are scientists doing to address key uncertainties in our understanding ofthe climate system?.................................................... 19 19 Are disaster scenarios about tipping points like “turning off the Gulf Stream” and release of methane from the Arctic a cause for concern?................................. 21 20 If emissions of greenhouse gases were stopped, would the climate return to the conditions of 200 years ago?............................................................... 22 Conclusion ........................................................................................ 23 Acknowledgements.............................................................................. 24 Summary GREENHOUSEGASESsuchascarbondioxide(CO2)absorbheat(infraredradiation) emitted from Earth’s surface. Increases in the atmospheric concentrations of these gases cause Earth to warm by trapping more of this heat. Human activities—especially theburningoffossilfuelssincethestartoftheIndustrialRevolution—haveincreased atmospheric CO2 concentrations by more than 40%, with over half the increase occurring since1970.Since1900, the global average surface temperature has increased by about 1 °C (1.8 °F). This has been accompanied by warming of the ocean, a rise in sea level, a strong decline in Arctic sea ice, widespread increases in the frequency and intensity of heatwaves, and many other associated climate effects. Much of this warming has occurred in the last five decades. Detailed analyses have shown that the warming duringthisperiodismainlyaresultoftheincreasedconcentrationsofCO2 andother greenhouse gases. Continued emissions of these gases will cause further climate change, including substantial increases in global average surface temperature and important changesinregionalclimate.Themagnitudeandtimingofthesechangeswilldependon many factors, and slowdowns and accelerations in warming lasting a decade or more will continue to occur. However, long-term climate change over many decades will depend mainlyonthetotalamountofCO2andothergreenhousegasesemittedasaresultof human activities. 1.) Is the climate warming? Yes.Earth’saveragesurface airtemperaturehas increasedbyabout 1 °C (1.8°F) since 1900, with over half of the increase occurring since the mid-1970s [Figure 1a]. A wide range of other observations (such as reduced Arctic sea ice extent and increased oceanheatcontent)andindicationsfromthenaturalworld(suchaspolewardshifts of temperature-sensitive species of fish, mammals, insects, etc.) together provide incontrovertible evidence of planetary-scale warming The clearest evidence for surface warming comes from widespread thermometer records that, in some places, extend back to the late 19th century. Today, temperatures are monitored at many thousands of locations, over both the land and ocean surface. Indirect estimates of temperature change from such sourcesastreeringsandicecoreshelptoplacerecenttemperaturechangesinthecontextofthepast.In terms of Earth's average surface temperature, these indirect estimates show that 1989 to 2019 was very likely the warmest 30-year period in more than 800 years; the most recent decade,-, is the warmest decade in the instrumental record so far (since 1850). A wide range of other observations provides a more comprehensive picture of warming throughout the climate system. For example, the lower atmosphere and the upper layers of the ocean have also warmed, snow and ice cover are decreasing in the Northern Hemisphere, the Greenland ice sheet is shrinking, and sealevel is rising[Figure1b].These measurements aremade with a varietyof land-,ocean-,and space-based monitoring systems, which gives added confidence in the reality of global-scale warming of Earth’s climate. Figure 1a. Earth’s global average surface temperaturehas risen as shown in thisplotof combined land andocean measurements from 1850 to 2019, derived from three independent analysesof the availabledata sets.The temperature changes are relative to the global Annual global surface temperature (1850−- Hadley Centre (UK Met) NASA (GISS) NOAA (NCEI) averagesurfacetemperatureof 1961−1990. Source: NOAA Climate. gov;data fromUKMetOfficeHadley Centre (maroon), US National AeronauticsandSpaceAdministration GoddardInstitute forSpaceStudies (red), and USNational Oceanic andAtmosphericAdministration NationalCentersforEnvironmental Information (orange). Figure 1b. A large amount of observationalevidencebesides surface temperature records shows that Earth’s climate is changing. Arctic sea ice extent in winter and summer (1979−-−2010 For example, additional evidence ofa warmingtrendcan befound inthedramaticdecreaseinthe extentofArcticseaiceatits summerminimum(whichoccurs in September), the decrease in June snow cover in the Northern Hemisphere, the increases in the- global average upper ocean (upper 700 m or 2300feet) heat content (shown relative to the- average), and the rise in global sea level. Source: NOAA Climate.gov 2.)How do scientists know that recent climate change is largely caused by humanactivities? Scientists know that recent climate change is largely caused by human activities from an understanding of basic physics, comparing observations with models, and fingerprinting the detailed patterns of climate change caused by different human and natural influences Since the mid-1800s, scientists have known that CO2 is one of the main greenhouse gases of importance to Earth’s energy balance. Direct measurements of CO2 in the atmosphere and in air trapped in ice show thatatmosphericCO2increasedbymorethan40%from1800to2019.Measurementsofdifferentforms of carbon (isotopes, see Question 3) reveal that this increase is due to human activities. Other greenhouse gases (notably methane and nitrous oxide) are also increasing as a consequence of human activities. The observed global surface temperature rise since 1900 is consistent with detailed calculations of the impacts of the observed increase in atmospheric greenhouse gases (and other human-induced changes) on Earth’s energy balance. Different influences on climate have different signatures in climate records. These unique fingerprints are easierto see by probing beyond a single number (such as the average temperature ofEarth’s surface), and by looking instead at the geographical and seasonal patterns of climate change.The observed patterns of surface warming, temperature changes through the atmosphere, increases in ocean heat content, increases in atmospheric moisture, sea level rise, and increased melting of land and sea ice also match the patterns scientists expect to see due to human activities (see Question 5). The expected changes in climate are based on our understanding of howgreenhouse gases trap heat.Both this fundamental understanding of the physics of greenhouse gases and pattern-based fingerprint studies show that natural causes alone are inadequate to explain the recent observed changes in climate. Natural causesincludevariationsintheSun’soutputandinEarth’sorbitaroundtheSun,volcaniceruptions,and internalfluctuationsintheclimatesystem(suchasElNiñoandLaNiña).Calculationsusingclimatemodels (see infobox, p. 20) have been used to simulate what would have happened to global temperatures if only natural factors were influencing the climate system. These simulations yield little surface warming, or even a slightcooling,overthe20thcenturyandintothe21st.Onlywhenmodelsincludehumaninfluencesonthe composition of the atmosphere are the resulting temperature changes consistent with observed changes. 3.)CO is already in the atmosphere 2 naturally, so why are emissions from human activity significant? Human activities have significantly disturbed the natural carbon cycle by extracting long- buried fossil fuels and buming them forenergy, thus releasing CO, tothe atmosphere in nature, CO, is exchanged continually between the atmosphere, plants, and animals through photosynthesis, respiration, and decomposition, and between the atmosphere and ocean through gas exchange. A very small amount of CO, (roughly 16 of the emission rate from fossil fuel combustionjin alsoemitted involcaniceruptions. This is balanced by an equivalentamount thatisremoved bychemical weathering of rocks The CO, level in 2019 was more than 40% higher than it was in the 19th century. Most of this CO, increase has taken place since 1970, about the time when global energy consumption accelerated. Measured decreasesinthefraction of other forms of carbon (the isotopes "Cand "Cjandasmalldecrease in atmospheric oxygen concentration (observations of which have been available since 1990) show that therise in CO, is largely from combustionoffossil fuels (whichhave low Cfractions and no "C). Deforestation and other land use changes have also released carbon from the biosphere (Iving world) where it normallyresides for decadestocenturies. The additional CO, from fossil fuel buming and deforestation has disturbed the balance of the carbon cycle, because the natural processes that could restore the balance are too slowcomparedtotherates at whichhuman activities are adding CO, tothe atmosphere. As a result, a substantial fractionofthe CO, emittedfromhumanactivities accumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of years. Comparison with the CO, levels measured in air extracted from ice cores indicates that the current concentrations are substantially higher than they have been in at least 800,000 years (see Question 6) 4.)What role has the Sun played in climate change in recent decades? The Sun provides the primary source of energy driving Earth's climate system, but its variations have played very little role in the climate changes observed in recent decades. Dinect satellite measurements since the late 1970s show no net increase in the Sun's out- put, while at the same time global surface temperatures have increased Figure 21. because thechanges are infenedfromindirectsources-including the number of sunspots and the abundance of certain forms (isotopes) of carbonorberyllium atoms, whoseproduction rates in Earth's atmosphere are influenced by variations in the Sun. There is evidence that the 11-year solar cycle, during which the Sun's energy output varies by roughly 0.1%, can influence ozone concentrations, temperatures, and winds in the stratosphere (the layerin the atmosphereabove the troposphere, typically from 12to 50km above earth's surface, depending on latitude and season), These stratospheric changes may have a small effect on surface climate over the 11-year cycle. However, the available evidence does not indicate pronouncedlong-term changes in the Sun'soutput over the past century, during which time human induced increases in CO, concentrations have been the dominant influence on the long-term global surface temperature increase. Further evidence that cument warming is not a result of solar changes can be found in the temperature trends at differentalliludes in the atmosphere (see Question 5) Sun's energy inccent on Earth show nonetinomasein solar forcing during the past 40 years. and therefore this cannot be responsible for warming during thatperiod. The data show only small periodic ampiltide variations associated with the Sur's 11-year cycle Source TS/data from Physikalisch-Meteorologisches Observatorium Devos, Stand onthenew WRGOscaleom 1978 mid-2018 mparture da for same ime period fromthe HadCRUT state / Met Office Hadley Centre . 5.) Whatdochangesinthevertical structure of atmospheric temperature —from the surfaceup to the stratosphere—tellusabout the causesofrecent climatechange? The observed warming in the lower atmosphere and cooling in the upper atmosphere provide us with keyinsightsinto the underlying causes of climate change and reveal that natural factors alone cannot explain the observed changes Inthe early 1960s, results frommathematical/physicalmodelsofthe dimustesystemfirst showed that human-induced increases in CO, would be expected to lead to gradual warming of the lower almosphere (the troposphere) and cooling of higher levels of the atmosphere (the stratosphere). In contrast, increases in the Sun's output would warm both the troposphere and the full vertical extent of the stratosphere At that lime, there wasinsufficientobservational data to test this prediction, buttemperature measurements from weather balioons and satellites have since confirmed these early forecasts. His now known that the observed pattem of tropospheric warming and stratospheric cooling over the past 40 years is broadly consistent with computer model simulations that include incresses in CO, and decreases in stroriospheric ozone, each caused by human activities. The observed pattem is s not consistent with purely natural changes in the Suri's energy output, volcanic activity, or naturalicimatevariations suchas ElNiñoard La Niña Despite this agreement between the global-scale paltems of modelled and observed atmospheric temperature change, there are still some differences. The most noticeable differences are in the tropics, where models currently show more warming in the tropospherethanhas beenobserved, andinthe Arctic where the observedwarmingofthetroposphere is greater than in most models 6.) Climate isalways changing.Why is climate change of concern now? All major climate changes, including natural ones, are disruptive.Past climate changes led to extinction of many species, population migrations, and pronounced changes in the land surface and ocean circulation.The speed of the current climate change is fasterthan most of thepastevents,makingitmoredifficultforhumansocietiesandthenaturalworldtoadapt The largest global-scale climate variations in Earth’s recent geological past are the ice age cycles (see infobox, p.B4),whicharecoldglacialperiods followedby shorterwarm periods [Figure 3].Thelast fewof these natural cycles have recurred roughly every 100,000 years.They are mainly paced by slow changes in Earth’s orbit,whichalterthewaytheSun’senergyisdistributedwithlatitudeandbyseasononEarth.Theseorbital changes are very small over the last several hundred years, and alone are not sufficient to cause the observed magnitudeofchangeintemperaturesincetheIndustrialRevolution,nortoactonthewholeEarth.On ice-age timescales, these gradual orbital variations have led to changes in the extent of ice sheets and in the abundance of CO2 and other greenhouse gases, which in turn have amplified the initial temperature change. Recent estimates of the increase in global average temperature since the end of the last ice age are 4 to 5°C(7to9°F).Thatchangeoccurredoveraperiodofabout7,000years,starting18,000yearsago.CO2 hasrisenmorethan40%injustthepast 200years,muchofthissincethe1970s, contributingtohuman alterationoftheplanet’senergybudgetthathassofarwarmedEarthbyabout1°C(1.8°F).IftheriseinCO2 continues unchecked, warming of the same magnitude as the increase out of the ice age can be expected bytheendofthiscenturyor soonafter.This speedofwarmingismorethantentimes that at theend ofan ice age, the fastest known natural sustained change on a global scale 7.)Is the current level of atmospheric CO concentration unprecedented in Earth’s history? The present level of atmospheric CO, concentration is almost certainly unprecedented inthepastmillion years, during which time modern humansevolved and societies developed. The atmospheric CO, concentration was however higher in Earth's more distantpast(many millions of years ago), at which time pallaeoclimaticandgeological data indicate that temperatures and sea levels were also higher than they are today. Measurements of air in ice cores show that for the past 800.000 years up until the 20th century, the atmosphericCO concentrationstayed withinthe range- parts per million(ppm), making therecent rapid rise to more than 400 ppm over 200 years particularly remarkable During the glacial cycles of the past 800,000 years both CO, and methane have acted as important amplifiers of the climate changes triggered by variations in Earth's orbit around the Sun. As Earth wormed from the last ice age, temperature andCO2 startedtoriseatapproximatelythesametimeandcontinuedtoriseintandemfromabout 18,000 to11,000yearsago.Changesinoceantemperature,circulation,chemistry,andbiology causedCO2 tobe releasedtotheatmosphere,whichcombinedwithotherfeedbackstopushEarth intoanevenwarmerstate. Forearliergeologicaltimes,CO2 concentrationsandtemperatureshavebeeninferredfromless direct methods.Those suggest that the concentration of CO2 last approached 400 ppm about 3 to 5 million yearsago,a period when globalaveragesurfacetemperatureisestimatedto havebeen about2 to3.5°C higherthan in the pre-industrial period. At 50 million years ago, CO2 may have reached 1000 ppm, and globalaveragetemperaturewasprobablyabout10°C warmerthantoday.Underthoseconditions,Earth hadlittleice,andsealevelwasatleast60 metreshigherthancurrentlevels Figure 3.Datafromicecores have been used to reconstruct Antarctic temperaturesand atmospheric CO2 concentrations over the past 800,000 years.Temperatureis based on measurements of the isotopiccontentofwaterinthe DomeCicecore.CO2 ismeasured in air trapped in ice, and is a composite of the Dome C and Vostokicecore. The current CO2 concentration (blue dot)is from atmospheric measurements.Thecyclical pattern of temperature variations constitutes the ice ag 8.) Is there a point at which adding more CO will not cause further warming? No.AddingmoreCO2 totheatmospherewillcausesurfacetemperaturesto continueto increase.AstheatmosphericconcentrationsofCO2 increase,the additionofextraCO2 becomes progressivelylesseffective attrappingEarth’s energy, but surfacetemperature willstillrise Ourunderstandingof thephysicsbywhichCO2 affectsEarth’senergybalanceis confirmed by laboratory measurements, as well as by detailed satellite and surface observations of the emission and absorption ofinfraredenergyby theatmosphere.Greenhousegasesabsorb someoftheinfrared energy thatEarth emits in so-called bands of stronger absorption that occur CO2 (ppm) 8 No.AddingmoreCO2 totheatmospherewillcausesurfacetemperaturesto continueto increase.AstheatmosphericconcentrationsofCO2 increase,the additionofextraCO2 becomes progressivelylesseffective attrappingEarth’s energy, but surfacetemperature willstillrise. 10 Clim ate Ch ange at certain wavelengths. Different gases absorb energyatdifferentwavelengths.CO2 has its strongestheat-trappingband centredata wavelengthof15 micrometres (millionths of a metre), with absorption that spreadsout a few micrometres on either side. Therearealsomanyweaker absorptionbands.AsCO2 concentrationsincrease,theabsorptionatthecen- treofthestrong bandisalreadysointensethatitplayslittleroleincausingadditionalwarming.However, more energy is absorbed in the weaker bands and away from the centre of the strong band, causing the surfaceandloweratmospheretowarmfurther. 9.) Does the rate of warming vary from onedecadetoanother? Yes.The observed warming rate has varied from yearto year, decade to decade, and place to place, as is expected from our understanding of the climate system. These shortertermvariationsaremostlyduetonaturalcauses,anddonotcontradictourfundamental understandingthatthelong-termwarmingtrendisprimarilyduetohuman-induced changes in the atmospheric levels of CO2 and other greenhouse gases. Even as CO2 is rising steadily in the atmosphere, leading to gradual warming of Earth’s surface, many natural factors are modulating this long-term warming. Large volcanic eruptions increase the number of small particles in the stratosphere. These particles reflect sunlight, leading to short-term surface cooling lasting typically two to three years, followed by a slow recovery. Ocean circulation and mixing vary naturally on many time scales, causing variations in sea surface temperatures as well as changes in the rate at which heat is transported to greater depths. For example, the tropical Pacific swings between warm El Niño and cooler La Niña events on timescales of two to seven years. Scientists study many different types of climate variations, such as those on decadal and multi-decadal timescales in the Pacific and North Atlantic Oceans.Each type of variation has its own unique characteristics. These oceanic variations are associated with significant regionalandglobalshiftsintemperatureandrainfallpatternsthatareevidentintheobservations. Figure4.Theclimatesystemvaries naturallyfromyeartoyearandfrom decadetodecade.Tomakereliable inferences about human-induced climate change, multi-decadal and longerrecords are typically used. Calculating a “running average” over these longer timescales allows one to more easily see long-term trends. Forthe global average temperature for the period- (using the datafromtheUKMetOfficeHadley Centre relative to the1961-90 Warming from decade to decade can also be affected by human factors such as variations in emissions of greenhouse gases and aerosols (airborne particles that can have both warming and cooling effects) from coal-fired power plants and other pollution sources. These variations in the temperature trend are clearly evident in the observed temperature record [Figure 4]. Short-term natural climatevariations could also affect the long-term human-induced climate change signal and vice-versa, because climate variations on different space and timescales can interact withoneanother.Itispartlyforthisreasonthat climatechangeprojectionsaremadeusingclimate models(seeinfobox,p.20)thatcanaccountformanydifferenttypesofclimatevariationsandtheir interactions. Reliable inferences about human-induced climate change must be made with a longer view, using records that cover many decades. Figure4.Theclimatesystemvaries naturallyfromyeartoyearandfrom decadetodecade.Tomakereliable inferences about human-induced climate change, multi-decadal and longerrecords are typically used. Calculating a “running average” over these longer timescales allows one to more easily see long-term trends. Forthe global average temperature for the period- (using the datafromtheUKMetOfficeHadley Centre relative to the1961-90 average)theplotsshow(top)the average and range of uncertainty for annually averaged data; (2nd plot) theannualaveragetemperature for the ten years centred on any given date; (3rd plot) the equivalent picturefor30-year;and(4thplot) the60-yearaverages. Source:Met Office Hadley Centre, based on the HadCRUT4datasetfromtheMetOffice andClimaticResearchUnit(Moriceet al.,2012). 10.) Did the slowdown of warming during the2000s toearly2010smeanthat climatechangeisnolongerhappening? No. After the very warm year 1998 that followed the strong 1997-98 El Niño, the Increase in average surface temperature slowed relative to the previous decade of rapid temperature increases. Despite the slower rate of warming, the 2000s were warmer than the 1990s. The limited period of slower warming ended with a dramatic jump to warmer temperatures between 2014 and 2015, with all the years from- warmer than anypreceding yearin the instrumental record.Ashort-termslowdown in the warming of Earth's surface does not invalidate our understanding of long-term changes in global temperature arising from human-induced changes in greenhouse gases. Decades of slow warming as well as decades of accelerated warming occur naturally in the climate system. Decades that are cold or warm compared to the long-term trendare seen in the observatioris of the past 150 years and are also captured by climate models. Because the atmosphere stores very little heat, surface temperatures can be rapidly affected by heat uptake elsewhere in the climate system and by changes in externalinfluences.onclimate (such as particles formed from material lofted high into the atmosphere from volcanic eruptions) More than 90% ofthe heat added to the Earth systeminrecentdecades has been absorbed by the oceans and penetrates onlyslowly into deepwater. Afasterrate of heat penetration into the deeperoceanwili slow the warming seen at the surface and in the atmosphere, but by itself it will not change the long-term warming thatwill occurfroma given amount of CO., Forexample, recent studies show that some heat comes out of the ocean into the atmosphere during warm El Niñoevents, and moreheat penetratesto ocean depths in cold La Niñas. Such changes occur repeatedly over timescales of decades and longer. An example is the major El Niño event in 1997-98 when the globally averaged air temperature soared to the highest level in the 20th century as the ocean lost heat to the atmosphere, mainlyby evaporation, Even during the slowdown in the rise of average surface temperature, a longer-term warming trend sstil evident (see Figure 4). Over that period, for example, record heatwaves were documented in Europe (summer 2003), in Russia (summer 2010), in the USA (July 2012), and in Australia (January 2013) Each of the last four decades was warmer than any previous decade since widespread thermometer measurements were introduced in the 1850s. The continuing effects of the warming dimate are seen in the increasingtrends in ocean heat contentand sea level as well as in the continued meltingof Arcticsea ice, glaciers and the Greenlandice sheet TheBasicsof ClimateChange GreenhousegasesaffectEarth’senergybalanceandclimate. The Sun serves as the primary energy source forEarth’s climate.Some of the incoming sunlight is refl cted directly back into space, especially by bright surfaces such as ice and clouds, and the rest is absorbed by the surface and the atmosphere. Much of this absorbed solar energy is re-emitted as heat (longwave or infrared radiation). The atmosphere in turn absorbs and re-radiates heat, some of which escapes to space.Any disturbance to this balance of incoming and outgoing energy will affect the climate. For example, small changes intheoutputofenergyfromtheSunwillaffectthisbalancedirectly. If all heat energy emitted from the surface passed through the atmosphere directly into space,Earth’saveragesurfacetemperaturewouldbe tensof degrees colderthantoday. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, act to make the surface much warmer than this because they absorb and emit heat energy in all directions (including downwards), keeping Earth’s surface and lower atmospherewarm [Figure B1].Without thisgreenhouseeffect, lifeas we knowit could not have evolved on our planet.Adding more greenhouse gases to the atmosphere makes it even more effective at preventing heat from escaping into space. When the energy leaving is less than the energy entering,Earth warms until a new balance is established. the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, absorb heat energy and emititin alldirections THE GREENHOUSE EFFECT (including downwards), keeping Earth’s surface and lower atmosphere warm. Adding more greenhouse gases to the atmosphere enhances the effect, making Earth’s Some solar radiation is reflected by Earth and the atmosphere Some of the infrared radiation passes through the atmosphere. Some is absorbed by greenhouse gases and re-emitted in all directions by the atmosphere. The effect of this is to warm Earth’s surface and lower atmosphere even warmer. Image based on a figure from US Environmental Protection Agency Greenhouse gases emitted by human activities alter Earth’s energy balance and thus itsclimate.Humansalsoaffectclimatebychangingthenatureofthelandsurfaces(for examplebyclearingforestsforfarming)andthroughtheemissionofpollutantsthat affecttheamountandtypeofparticlesintheatmosphere. Scientists have determined that, when all human and natural factors are considered, Earth’s climate balance has been altered towards warming, with the biggest contributor being increases in CO2. Humanactivitieshaveaddedgreenhousegases to theatmosphere. The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began. In the case of carbon dioxide, the average concentration measured at the Mauna Loa Observatory in Hawaii has risen from316partspermillion(ppm)1 in1959(thefirstfullyearofdataavailable)tomore than411ppmin2019[Figure B2].Thesameratesofincreasehavesincebeenrecorded at numerous other stations worldwide. Since preindustrial times, the atmospheric concentration of CO2 has increased by over 40%, methane has increased by more than 150%, and nitrous oxide has increased by roughly 20%. More than half of the increase in CO2 has occurred since 1970. Increases in all three gases contribute to warming of Earth, with the increase in CO2 playing the largest role. See page B3 to learn about the sources of humanemitted greenhouse gases. Scientists have examined greenhouse gases in the context of the past. Analysis of air trappedinsideicethathasbeenaccumulatingovertimeinAntarcticashowsthattheCO2 Figure B2. Measurements of atmospheric CO2 since 1958 from the Mauna Loa Observatory in Hawaii(black)andfromtheSouth Pole (red) show a steady annual increase in atmospheric CO2 concentration. The measurements are made at remote places like thesebecausetheyarenot greatly influenced by local processes, so thereforetheyare representative of the background atmosphere. The small up-and-down saw-tooth pattern reflects seasonalchanges inthereleaseanduptakeofCO2 by plants.Source: Scripps CO2 Progra concentration began to increase significantly in the 19 th century [Figure B3], after staying in the range of 260 to 280 ppm forthe previous 10,000 years. Ice core records extending back 800,000 years showthatduringthattime,CO2 concentrations remainedwithinthe range of 170 to 300 ppm throughout many “ice age” cycles—see infobox, pg.B4 to learn about the ice ages—and no concentration above 300 ppm is seen in ice core records untilthepast200 years. Figure B3. CO2variations during the past 1,000 years, obtained fromanalysisofairtrappedinan ice core extracted from Antarctica (redsquares),showasharprisein atmosphericCO2 startinginthelate 19th century. Modern atmospheric measurements from Mauna Loa are superimposedingray.Source:figure byEricWolff, data from Etheridge et al., 1996; MacFarling Meure et al., 2006; Scripps CO2Program. Learnaboutthesourcesofhuman-emittedgreenhousegases: ■ Carbondioxide(CO2 )hasboth natural and human sources, but CO2 levels are increasing primarily becauseofthecombustionoffossil fuels, cement production, deforestation (which reduces the CO2 taken up by trees and increases the CO2 released by decomposition of thedetritus),andotherlanduse changes. Increases in CO2 are the singlelargestcontributortoglobal warming. ■ Methane (CH4 ) has both human and natural sources, and levels have risen significantly since pre-industrialtimesduetohuman activities such as raising livestock, growingpaddyrice,fillinglandfills, andusingnaturalgas(whichis mostlyCH4 ,someofwhichmay be released when it is extracted, transported, and used). ■ Nitrousoxide (N2O) concentrations have risen primarily because ofagriculturalactivitiessuchasthe use of nitrogen-based fertilisers and landusechanges. ■ Halocarbons, including chlorofluorocarbons (CFCs),arechemicals used as refrigerants and fire retardants.Inadditiontobeing potent greenhouse gases, CFCs also damagethe ozone layer.The productionofmostCFCshasnow beenbanned,sotheirimpactis starting to decline. However, many CFCreplacements are also potent greenhousegases and their concentrations and the concentrations ofotherhalocarbonscontinueto increase Measurements of the forms (isotopes) of carbon in the modern atmosphere show a clear fingerprintof the addition of“old” carbon (depletedin naturalradioactive 14C) coming from the combustion of fossil fuels (as opposed to “newer” carbon coming from living systems).Inaddition,itisknownthathumanactivities (excludinglandusechanges) currently emit an estimated 10 billion tonnes of carbon each year, mostly by burning fossil fuels, which is more than enough to explain the observed increasein concentration. Theseandotherlinesofevidencepointconclusivelytothefactthattheelevated CO2 concentration in our atmosphereis the result of human activities. Climate recordsshow a warming trend. Estimating global average surface air temperature increase requires careful analysis of millionsofmeasurementsfromaroundtheworld,includingfrom land stations, ships, and satellites. Despite the many complications of synthesising such data, multiple independent teams have concluded separately and unanimously that global average surface airtemperature has risen by about 1 °C (1.8°F) since1900 [Figure B4].Although the record shows several pausesand accelerationsintheincreasing trend,each ofthelastfour decadeshasbeenwarmerthananyotherdecadeinthe instrumentalrecordsince1850. Goingfurtherbackintimebeforeaccuratethermometerswerewidelyavailable, temperaturescanbereconstructedusingclimate-sensitiveindicators“proxies” Learn about the ice ages: Detailed analyses of ocean sediments, icecores,andotherdata showthatforat leastthelast2.6 millionyears,Earthhas gone throughextendedperiodswhen temperatures were much lower than todayandthickblanketsofice covered largeareasoftheNorthern Hemisphere. Theselongcold spells,lastinginthe mostrecent cyclesforaround100,000 years, were interrupted by shorter warm ‘interglacial’ periods, including the past 10,000 years. Throughacombinationoftheory, observations, and modelling, scientists have deducedthat the ice ages * are triggeredbyrecurringvariationsin Earth’sorbitthatprimarilyalterthe regional and seasonal distribution of solarenergyreaching Earth.These relatively small changes in solar energy arereinforcedoverthousandsofyearsby gradual changes inEarth’s ice cover(the cryosphere), especially over the Northern Hemisphere, and in atmospheric composition, eventuallyleadingtolarge hanges in global temperature. Theaverageglobaltemperaturechange duringanice-agecycleisestimatedas5 °C±1°C(9°F±2°F). *NotethatingeologicaltermsEarthhas beeninaniceageeversincethe Antarctic IceSheetlastformedabout36 million yearsago.However,inthis documentwe haveusedtheterminits morecolloquial usage indicating the regular occurrence of extensive ice sheets over North America and northern Eurasia. inmaterialssuchastreerings,icecores,andmarinesediments.Comparisonsofthe thermometer record with these proxy measurements suggest that the time since the early 1980shasbeenthewarmest40-yearperiodinatleasteightcenturies,andthatglobal temperatureisrisingtowardspeaktemperatureslastseen5,000to10,000yearsagointhe warmest part of our current interglacial period. Manyotherimpactsassociatedwiththewarmingtrendhavebecomeevidentinrecent years.Arcticsummerseaicecoverhasshrunkdramatically.Theheat contentoftheocean hasincreased.Global averagesealevelhasrisenby approximately16cm(6inches)since 1901,duebothtotheexpansionofwarmeroceanwaterandtotheadditionofmeltwaters fromglaciersandicesheetsonland.Warmingandprecipitationchangesarealteringthe geographicalrangesofmanyplant andanimalspeciesandthetimingoftheirlifecycles. In addition tothe effects on climate,someofthe excess CO2 in the atmosphereisbeing takenupbytheocean, changingits chemical composition(causingoceanacidification). Figure B4. Earth’s global average surface temperature has risen, as showninthisplotofcombined land and ocean measurements from 1850 to 2019 derived from threeindependent analyses ofthe available data sets. The top panel shows annual average values from the three analyses, and the bottom panel shows decadal average values,includingtheuncertainty range (grey bars) forthe maroon (HadCRUT4) dataset. The tem- -0.2 -0.4 -0.6 Decadal average perature changes are relative to the global average surface temperature, averaged from 1961−1990. Source: NOAAClimate.gov,basedonIPCCAR5. Data from UKMet Office HadleyCentre (maroon), US National Aeronautics and SpaceAdministrationGoddardInstitute for Space Studies(red), and USNational OceanicandAtmosphericAdministration NationalCentersforEnvironmentalInfor Many complex processesshape our climate. BasedjustonthephysicsoftheamountofenergythatCO2 absorbsandemits,adoublingof atmosphericCO2 concentrationfrompre-industriallevels(uptoabout560ppm)wouldby itself cause a global average temperature increase of about 1 °C (1.8 °F). In the overall climate system, however, things are more complex; warming leads to further effects (feedbacks) that eitheramplifyordiminishtheinitialwarming. The most important feedbacks involve various forms of water. A warmer atmosphere generally contains more water vapour. Water vapour is a potent greenhouse gas, thus causing more warming;itsshortlifetimeintheatmospherekeepsitsincreaselargelyinstepwithwarming. Thus,watervapouristreatedasanamplifier,andnotadriver,ofclimatechange.Higher temperatures in the polarregions melt sea ice and reduce seasonal snow cover,exposing a darker ocean and land surface that can absorb more heat, causing further warming. Another important but uncertain feedback concerns changes in clouds. Warming and increases in water vapourtogether may cause cloud coverto increase or decrease which can either amplify or dampen temperature change depending on the changes in the horizontal extent, altitude, and properties of clouds. The latest assessment of the science indicates that the overall net global effectofcloudchangesislikelytobetoamplifywarming. The ocean moderates climatechange.The oceanisa hugeheatreservoir,but it isdifficult to heat its fulldepthbecausewarmwatertendsto staynearthe surface.The rateatwhichheat istransferredtothedeepoceanisthereforeslow;it variesfromyeartoyearandfromdecade todecade,andithelpstodeterminethepaceofwarmingatthesurface.Observationsofthe sub-surfaceoceanarelimitedpriortoabout1970,butsincethen,warmingoftheupper700m (2,300 feet) is readily apparent, and deeper warming is also clearly observed since about 1990. Surface temperatures and rainfall in most regions vary greatly from the global average because of geographical location, in particular latitude and continental position. Both the average values of temperature, rainfall, and their extremes (which generally have the largest impacts on natural systems and human infrastructure), are also strongly affected by local patterns of winds. Estimating the effects of feedback processes, the pace of the warming, and regional climate change requires the use of mathematical models of the atmosphere, ocean, land, and ice (thecryosphere)builtuponestablishedlawsofphysicsandthelatestunderstandingofthe physical, chemical and biological processes affecting climate, and run on powerful computers. Modelsvaryintheirprojectionsofhowmuchadditionalwarmingtoexpect(dependingonthe type of model and on assumptions used in simulating certain climate processes, particularly cloudformationandoceanmixing),butallsuchmodelsagreethattheoverallneteffectof feedbacks is to amplify warming Human activities are changing the climate. Rigorousanalysisofalldataandlinesofevidenceshowsthatmostoftheobservedglobal warming over the past 50 years or so cannot be explained by natural causes and instead requiresasignificantrolefortheinfluenceofhumanactivities. Inordertodiscernthehumaninfluenceonclimate,scientistsmustconsidermanynatural variationsthataffecttemperature,precipitation,andotheraspectsofclimatefromlocalto global scale, on timescales from days to decades and longer. One natural variation is the El Niño Southern Oscillation (ENSO), an irregular alternation between warming and cooling (lasting about two to seven years)in the equatorial Pacific Ocean that causes significant year-to-yearregional and global shifts in temperature and rainfall patterns. Volcanic eruptionsalsoalterclimate,inpartincreasingtheamountofsmall(aerosol)particlesinthe stratospherethatrefl ctorabsorbsunlight,leadingtoashort-termsurfacecoolinglasting typically about two to three years. Over hundreds of thousands of years, slow, recurring variationsinEarth’sorbitaroundtheSun,whichalterthedistributionofsolarenergy received byEarth, have been enough to triggerthe ice age cycles of the past 800,000 years. Fingerprinting is a powerful way of studying the causes of climate change. Different influences on climate lead to different patterns seen in climate records. This becomes obviouswhen scientistsprobe beyond changes in the averagetemperatureof the planet and look more closelyat geographicalandtemporalpatternsof climatechange.Forexample,an increase intheSun’senergyoutputwillleadtoaverydifferentpatternoftemperaturechange(across Earth’ssurfaceandverticallyintheatmosphere)comparedtothatinducedbyanincreasein CO2 concentration.Observed atmospheric temperature changes show a fingerprint much Learn more about other human causes of climate change: Inadditiontoemittinggreenhouse gases, human activities have also altered Earth’s energy balance through, forexample: ■ Changes in land use. Changes in the way people use land—for example, forforests,farms,orcities—can leadtobothwarmingand coolingeffects locally by changing the reflectivity of Earth’s surfaces (affecting how much sunlightissentbackintospace)and by changing how wet a region is. ■ Emissions of pollutants (other than greenhousegases).Someindustrial and agricultural processes emit pollutantsthatproduceaerosols (small droplets or particles suspended in the atmosphere). Most aerosols cool Earth by reflecting sunlight back to space.Someaerosolsalsoaffect theformationofclouds,which can have a warming or cooling effect depending on their type and location. Black carbon particles (or “soot”)producedwhenfossilfuels or vegetationare burnedgenerally haveawarmingeffectbecausethey absorb incoming solar radiation closertothatofa long-termCO2 increasethantothatofa fluctuatingSunalone. Scientists routinelytestwhetherpurelynaturalchangesintheSun,volcanicactivity, orinternalclimate variabilitycouldplausiblyexplainthepatternsofchangetheyhave observedinmanydifferent aspectsoftheclimatesystem.Theseanalyseshave shownthattheobservedclimatechanges ofthepastseveraldecadescannotbe explainedjustbynaturalfactors. Figure B5. Theamount andrateof warming expected forthe 21st century dependson the totalamountof greenhouse gases that humankind emits. Models project the temperature increasefor a business-as-usual emissionsscenario(inred)and aggressive emission reductions, fallingclosetozero50yearsfrom now(inblue).Blackisthemodelled estimate of past warming. Each solid line represents the average ofdifferent modelrunsusingthe same emissions scenario, and the shaded areasprovide a measure of the spread (one standard deviation) between the temperaturechanges projected by the different models. All data are relative to a reference period (set to zero) of-. Source: Based on IPCC AR5 How will climate change in the future? Scientists have made major advances in theobservations, theory, and modelling of Earth’s climatesystem, and theseadvanceshaveenabled them to projectfuture climatechange withincreasingconfidence.Nevertheless, severalmajorissues makeitimpossibletogive preciseestimatesofhowglobalorregionaltemperature trendswillevolvedecadebydecade intothefuture.Firstly,wecannotpredicthow muchCO2 humanactivitieswillemit,asthis dependsonfactorssuchashowthe globaleconomydevelopsandhowsociety’sproduction and consumptionof energychangesin thecoming decades.Secondly,withcurrent understandingof thecomplexitiesofhowclimatefeedbacksoperate,thereisa rangeof possible outcomes,evenfora particularscenarioofCO2 emissions.Finally,overtimescales ofa decadeor so, natural variability can modulatetheeffects of an underlyingtrend in temperature.Takentogether,allmodelprojectionsindicatethatEarthwillcontinue towarm considerablymoreoverthenextfewdecadestocenturies.Iftherewereno technologicalor policychangestoreduceemissiontrendsfromtheircurrent trajectory,thenfurtherglobally- averagedwarmingof2.6to4.8 °C(4.7to8.6°F)in additiontothatwhichhasalready occurredwouldbeexpectedduringthe21 stcentury[Figure B5].Projectingwhatthose ranges willmeanfortheclimateexperiencedatany particularlocationis a challenging scientific problem,butestimatesarecontinuingtoimproveas regional andlocal-scale modelsadvance Figure B5. Theamount andrateof warming expected forthe 21st century dependson the totalamountof greenhouse gases that humankind emits. Models project the temperature increasefor a business-as-usual emissionsscenario(inred)and aggressive emission reductions, fallingclosetozero50yearsfrom now(inblue).Blackisthemodelled estimate of past warming. Each solid line represents the average ofdifferent modelrunsusingthe same emissions scenario, and the shaded areasprovide a measure of the spread (one standard deviation) between the temperaturechanges projected by the different models. All data are relative to a reference period (set to zero) of-. Source: Based on IPCC AR 11.) If the world is warming, why are some winters and summers still very cold? Globalwarmingisalong-termtrend,butthatdoesnotmeanthateveryyearwillbe warmerthan the previous one. Day-to-day and year-to-year changes in weather patterns willcontinuetoproducesomeunusuallycolddaysandnightsandwintersandsummers, even as the climate warms. Climate change means not only changes in globally averaged surface temperature, but also changes in atmosphericcirculation,inthesizeandpatternsofnaturalclimatevariations,andinlocalweather.La Niña events shift weather patterns so that some regions are made wetter, and wet summers are generally cooler.Strongerwindsfrompolarregionscancontributetoanoccasionalcolderwinter.Inasimilarway, thepersistenceofonephaseofanatmosphericcirculationpatternknownastheNorthAtlanticOscillationhascontributedtoseveralrecentcoldwintersinEurope,easternNorthAmerica,andnorthernAsia. Atmospheric and ocean circulation patterns will evolve as Earth warms and will influence storm tracks andmanyotheraspectsoftheweather.Globalwarmingtiltstheoddsinfavourofmorewarmdaysand seasons and fewer cold days and seasons. For example, across the continental United States in the 1960s thereweremoredailyrecordlowtemperaturesthanrecordhighs,butinthe2000sthereweremorethan twiceasmanyrecordhighsasrecordlows.Anotherimportantexampleoftiltingtheoddsisthatover recent decades heatwaves have increased in frequency in large parts ofEurope,Asia,South America, and Australia. Marine heat waves are also increasing 12.) Why is Arctic sea ice decreasing while Antarctic sea ice has changed little? Sea ice extent is affected by winds and ocean currents as well as temperature. Sea ice in the partly-enclosed Arctic Ocean seems to be responding directly to warming, while changesinwindsandintheoceanseemtobedominatingthepatternsofclimateandsea ice change in the ocean around Antarctica. Some differences in seasonal sea ice extent between the Arctic and Antarctic are due to basic geography and its influence on atmospheric and oceanic circulation. The Arctic is an ocean basin surrounded largely bymountainouscontinentallandmasses,andAntarcticaisacontinentsurroundedbyocean.Inthe Arctic,seaiceextentislimitedbythesurroundinglandmasses.IntheSouthernOceanwinter,seaicecan expandfreelyintothesurroundingocean,withitssouthernboundarysetbythecoastlineofAntarctica. BecauseAntarcticseaiceformsatlatitudesfurtherfromtheSouthPole(andclosertotheequator),less ice survives the summer. Sea ice extent in both poles changes seasonally; however, longer-term variability insummerandwintericeextentisdifferentineachhemisphere,dueinparttothesebasicgeographical differences. Sea ice in the Arctic has decreased dramatically since the late 1970s, particularly in summer and autumn. Since the satellite record began in 1978, the yearly minimum Arctic sea ice extent (which occurs in September)hasdecreasedbyabout40%[Figure 5]. IcecoverexpandsagaineachArcticwinter,but theiceisthinnerthanitusedtobe.Estimatesofpastseaiceextent suggestthatthisdeclinemaybe unprecedented in at least the past 1,450 years. Because sea ice is highly reflective, warming is amplified as the ice decreases and more sunshine is absorbed by the darker underlying ocean surface. Figure 5. TheArcticsummer sea ice extent in 2012,(measured in September) was a record low, shown (inwhite)compared tothe median summer sea ice extent for 1979 to 2000 (in orange outline). In 2013,Arctic summer sea ice extent rebounded somewhat, but was still the sixth smallest extent on record. In2019,seaiceextenteffectively tiedforthe secondlowest minimum inthesatelliterecord,alongwith 2007 and 2016—behind only 2012, which is still the record minimum. The 13lowest ice extents in the satelliteerahavealloccurredinthe last 13 years. Source: National Snow and Ice Data Center Sea ice in the Antarctic showed a slight increase in overall extent from 1979 to 2014, although some areas, such as that to the west of the Antarctic Peninsula experienced a decrease. Short-term trends in theSouthern Ocean, such as those observed, canreadilyoccurfromnaturalvariabilityofthe atmosphere,oceanandseaicesystem.Changes insurfacewindpatternsaroundthecontinent contributedtotheAntarcticpatternofseaice change; ocean factors such as the addition of cool fresh water from melting ice shelves may also have played a role. However,after 2014,Antarctic ice extent began to decline,reaching a record low (withinthe40yearsofsatellitedata)in2017,and remaininglowinthefollowingtwoyears Figure 5. TheArcticsummer sea ice extent in 2012,(measured in September) was a record low, shown (inwhite)compared tothe median summer sea ice extent for 1979 to 2000 (in orange outline). In 2013,Arctic summer sea ice extent rebounded somewhat, but was still the sixth smallest extent on record. In2019,seaiceextenteffectively tiedforthe secondlowest minimum inthesatelliterecord,alongwith 2007 and 2016—behind only 2012, which is still the record minimum. The 13lowest ice extents in the satelliteerahavealloccurredinthe last 13 years. Source: National Snow and Ice Data Center 13.) How does climate change affect the strength and frequency of floods, droughts, hurricanes, and tornadoes? Earth's lower atmosphere is becoming warmer and moister as a result of human-caused greenhouse gas emissions. This gives the potential for more energy for storms and certain extreme weather events. Consistent with theoretical expectations, the types of events most closely related to temperature, such as heatwaves and extremely hot days, are becoming more likely, Heavy rainfall and snowfall events (which increase the risk of flooding) are also generally becoming more frequent As Earth's dimale has warmed, more frequent and more intense weather events have both been observed around the world. Scentists typically identify these weather events as "extreme" if they are unlike 90% or 95% of similar weather events that happened before in the same region. Many factors contribute to any individual extreme weather event-including patterns of natural climate veriability, such as El Niño and La Niña- making it challenging to attribute any particular extreme event to human-caused climate change. However, studies can show whether the warming climate made an event more severe or more likely to happen. A warming climate can contribute to the intensity of heat waves by increasing the chances of very hot days and nights. Climate warming also increases evaporation on land, which can worsari drought and creute conditions more prone to wildfire and a longer wildfire season. A warming atmosphere is also associated with heavier precipitationevents (rainand snowstorms) throughincreases intheair'scapacity to hold moisture. Niño events favour drought in many tropical and subtropical land areas, while La Niña events promote weller conditions in many places. These short-term and regional variations are expected to become more extreme in a waming dimate Earth's warmer and moister atmosphere and warmer oceans make it likely that the strongest hurricanes will be more intense, produce more rainfall, affect new areas, and possibly be larger and longer-lived. This is supported by available observational evidence in the North Atlantic, in addition, sea level rise (see Question 14) increases the amount of seawater that is pushed onto shore during coastalstorms, which, along with more rainfall produced by the storms, can result in more destructive storm surges and flooding While global warmingis likely making hurricanes moreintense, the change in the number of hurricanes each year is quite uncertain. This remains a subject of ongoing research. Some conditions favourable for strong thunderstorms that spawn tornadoes are expected to increase with warming, butuncertaintyexists in otherfactors that affecttomadoformation, suchas changesinthevertical and horizontal varañions of winds 14.) How fast is sea level rising? Long-term measurements of tide gauges and recent satellite data show that global sea levelis rising, with the bestestimate of the rate of global-average rise over the last decade being 3.6 mm per year (0.14 inches per year). The rate of sea level rise has increased since measurementsusing altimetry from space were started in 1992; the dominantfactorin global-average sea level rise since 1970 is human-caused warming. The overall observed rise since 1902 is about 16cm (6 inches) [Figure] Thissealevelrisehasbeendrivenbyexpansionofwatervolumeastheoceanwarms,meltingofmountain glaciersinallregionsoftheworld,andmasslossesfromtheGreenlandandAntarcticicesheets.Allof theseresultfromawarmingclimate.Fluctuationsinsealevelalsooccurduetochangesintheamountsof water stored on land. The amount of sea level change experienced at any given location also depends on a variety of other factors, including whether regional geological processes and rebound of the land weighted downbypreviousicesheetsarecausingthelanditselftoriseorsink,andwhetherchangesinwindsand currents are piling ocean water against some coasts or moving water away. Figure 6. Observationsshow thatthe global average sea level hasrisenbyabout16cm(6inches) since the late 19 th century.Sea level is rising faster in recent decades; measurements from tide gauges (blue)andsatellites(red)indicate thatthebestestimateforthe average sea levelriseoverthe last decadeiscentredon3.6mmper year(0.14inchesper year).The shadedarearepresentsthesealevel uncertainty, which hasdecreased asthenumberofgaugesitesused in calculating the global averages andthenumberofdatapointshave increased. Source: Shum and Kuo (2011) The effects of rising sea level are felt most acutely in the increased frequency and intensity of occasional stormsurges.IfCO2andothergreenhousegasescontinuetoincreaseontheircurrenttrajectories,itis projectedthatsealevelmayrise,atminimum,byafurther 0.4to0.8m (1.3to2.6feet)by2100,although future ice sheet melt could make these values considerably higher. Moreover, rising sea levels will not stopin2100;sealevelswillbemuchhigherinthefollowingcenturiesastheseacontinuestotakeup heatandglacierscontinuetoretreat.ItremainsdifficulttopredictthedetailsofhowtheGreenlandand AntarcticIceSheetswillrespondtocontinuedwarming,butitisthoughtthatGreenlandandperhaps WestAntarcticawillcontinuetolosemass,whereasthecolderpartsofAntarcticacouldgainmassas theyreceivemoresnowfallfromwarmerairthatcontainsmoremoisture.Sealevelinthelastinterglacial (warm)periodaround125,000yearsagopeakedatprobably5to10mabovethepresentlevel.Duringthis period,thepolarregionswerewarmerthantheyaretoday.Thissuggeststhat,overmillennia,longperiods ofincreasedwarmthwillleadtoverysignificantlossofpartsoftheGreenlandandAntarcticIceSheets Figure 6. Observationsshow thatthe global average sea level hasrisenbyabout16cm(6inches) since the late 19 th century.Sea level is rising faster in recent decades; measurements from tide gauges (blue)andsatellites(red)indicate thatthebestestimateforthe average sea levelriseoverthe last decadeiscentredon3.6mmper year(0.14inchesper year).The shadedarearepresentsthesealevel uncertainty, which hasdecreased asthenumberofgaugesitesused in calculating the global averages andthenumberofdatapointshave increased. Source: Shum and Kuo (2011) 15.) What is ocean acidification and why does it matter? Direct observations of ocean chemistry have shown that the chemical balance of seawater has shifted to a more acidic state (lower pH). Some marine organisms (such as corals and some shellfish) have shells composed of calcium carbonate, which dissolves more readilyin acid. As the acidity of seawaterincreases, it becomes more difficult for these organisms to form or maintain their shells. CO, dissolves in water to form a weak acid, and the oceanshave absorbed about a third of the CO.resulting from human activities, leading to a steady decrease in ocean pH levels. With increasing atmospheric CO this chemical balance will change even more during the next century. Laboratory and other experiments show that under high CO, and in more acidic waters, some marine species have misshapen shells und lower growth rates, although the effect varies among species. Acidification also alters the cycling of nutrients and many other elements and compounds in the ocean, and it is likely to shift the competitive advantage among species, with as-yet-to-be-determined impacts on marine ecosystems and the food web figure 7. As CO2 in the air has increased,therehasbeen an increaseintheCO2 contentofthe surface ocean (upper box), and a decrease in the seawater pH (lower box). Source: adapted from Dore et al. (2009) and Bates et al. (2012) 16.) How confident are scientists that Earth will warm further over the coming century? Very confident. If emissions continue on their present trajectory, without either technolog calor regulatory abatement, then warming of 2.6 to 4.8°C (4.7 to 8.6°F) in addition to that which has already occurred would be expected during the 21" century ( Figure 8) Warmingduetotheadditionoflargeamountsofgreenhousegasestotheatmospherecanbeunderstood intermsofverybasicpropertiesofgreenhousegases.Itwillinturnleadtomanychangesinnatural climateprocesses,withaneteffectofamplifyingthewarming.Thesizeofthewarmingthatwillbe experienced depends largely on the amount of greenhouse gases accumulating in the atmosphere and hence on the trajectory of emissions. If the total cumulative emissions since 1875 are kept below about 900 gigatonnes(900billiontonnes)ofcarbon,thenthereisatwo-thirdschanceof keepingtherisein global average temperature since the pre-industrial period below 2 °C (3.6 °F). However, two-thirds of this amount has already been emitted. A target of keeping global average temperature rise below 1.5 °C (2.7 °F) would allow for even less total cumulative emissions since 1875. BasedjustontheestablishedphysicsoftheamountofheatCO2 absorbsandemits,adoublingof atmosphericCO2 concentrationfrompreindustriallevels(uptoabout560ppm)wouldbyitself,without amplification by any other effects, cause a global average temperature increase of about 1 °C (1.8 °F). However,thetotalamountofwarmingfromagivenamountofemissionsdependsonchainsofeffects (feedbacks)thatcanindividuallyeitheramplifyordiminishtheinitialwarming. The most important amplifying feedback is caused by water vapour, which is a potent greenhouse gas. As CO2 increasesandwarmstheatmosphere,thewarmeraircanholdmoremoistureandtrapmoreheatin theloweratmosphere.Also,asArcticseaiceandglaciersmelt,moresunlightisabsorbedintothedarker underlyinglandandoceansurfaces,causingfurtherwarmingandfurthermeltingoficeandsnow.The biggest uncertainty in our understanding of feedbacks relates to clouds (which can have both positive and negative feedbacks), and how the properties of clouds will change in response to climate change. Other important feedbacks involve the carbon cycle. Currently the land and oceans togetherabsorbabouthalfoftheCO2emitted fromhumanactivities,butthecapacitiesof landandoceantostoreadditionalcarbonare expected to decrease with additional warming, leading to faster increases in atmospheric CO2andfasterwarming.Modelsvaryintheir projectionsofhowmuchadditionalwarming toexpect,butallsuchmodelsagreethatthe overall net effect of feedbacks is to amplify the warming figure 8. If emissions continue ontheirpresenttrajectory,without either technological or regulatory abatement, then the best estimate is that global average temperature willwarma further 2.6 to 4.8 °C (4.7to 8.6 °F) by the end ofthe century (right). Land areas are projectedtowarmmorethanocean areas and hence more than the globalmean.Thefigureontheleft shows projected warming with very aggressive emissions reductions. The figures represent multi-model estimates of temperature averages for- compared to-.Source:IPCCAR5 17.) Are climate changes of a few degrees a cause for concern? Yes. Even though an increase of a few degrees in global average temperature does not sound like much, globalaverage temperature during the lastice age was only about4 to 5°C(7109°F) colder than now, Global warming of just a few degrees will be associated with widespread changes in regional and local temperature and precipitation as well as with increases in some types of extreme weather events. These and other changes (such assealevelrise and storm surge) will have serious impactsonhuman societies and the natural world Both theory and direct observations have confirmed that global warming is associated with greater warming over land than oceans, moistening of the atmosphere, shifts in regional precipitation patterns, increases in extreme weather events, ocean acidification, melting glaciers,and risingsea levels (which increases the riskof coastal inundation and storm surge).Already,record high temperatures are on average significantly outpacing record lowtemperatures,wet areasare becomingwetter as dry areasare becoming drier,heavy rainstorms have become heavier,and snowpacks (an important sourceof freshwaterfor many regions) are decreasing. These impacts are expected to increase with greater warming and will threaten food production, freshwater supplies, coastal infrastructure, and especially the welfare of the huge population currently living in low-lying areas. Even though certain regions may realise some local benefit from the warming, the long-term consequences overall will be disruptive. Itisnotonlyanincreaseofafewdegreesinglobalaveragetemperaturethatiscauseforconcern—thepace at which this warming occurs is also important (see Question 6). Rapid human-caused climate changes meanthatlesstimeisavailabletoallowforadaptationmeasurestobeputinplaceorforecosystemsto adapt, posing greater risks in areas vulnerable to more intense extreme weather events and rising sea levels. 18.) What are scientists doing to address key uncertainties in our understanding of theclimatesystem? Sciencs is a continual process of observation, understanding, modelling, testing, and prediction. The prediction of a long-termtrendin globalwarming from increasing greenhouse gases is robust and has been confirmed by a growing body of evidence. Nevertheless, understanding of certain aspects of dimate change remains incomplete. Examples include naturall dimate variations on decadal-to-centennial timescales and regional-to-local spatial scales and cloud responses to climate change, which are all areas of active research. Why are computer models usedto study climate change? The future evolution of Earth's dimate as trespondsto the present rapid rate of increasing atmospheric CO. has noprecise analoguesin the past, norcan it be properly understood through laboratory experiments. As we are also unable to carry out deliberate controlled experiments on Earth self, computer models are among the most importanttoolsusedto study Earth's climatesystem. Cimals models are based on mathematical equations that represent the best understanding of the basic laws of physics, chemistry, and biology that govem the behaviour of the atmosphere, ocean, land surface, ice andother parts of the climate system, as well as the interactions among them. The most comprehensive dimate models. Earth-System Models, are designed to simulate Earth's climate system with as much detall asis permittedbyour understandingandby available supercomputers The capability of climate models has improved steadily since the 1960s. Using physics-based equations, the models can be tested and are successful in simulating abroadrange of weather and climate variations, for example from individual stone, jet stream meanders. EiNiñoevents, and the climate of the last century Their projections of the most prominent features of the long-term human-induced climate change signal have remained robust, as gerverations of increasingly complex models yield richer details of the change. They are also usedfoperformexperimentsto isolate specific causes of dimate change andtoexplorethe consequences of different scenarios of future greenhouse gas emissions and other influences on climate Comparisonsofmodel predictions withobservations identify what is wel-understood and, at the same time, reveal uncertainties or gaps in ourunderstanding. This helps to setpriorities for new research, Vigilant monitoring of the entire climate system-the atmosphere, oceаль land, andice is therefore critical, as the climate-systemmay be fullof surprises Together, field and laboratory data and theoretical understanding are used to advance models of Earth's cimate system and to improve representation oflikey processes in them, especially those associated with clouds, aerosols, and transportofheatintotheoceans. Thisiacriticalfor accurately simulating climate change and associated changes in severe weather, especially at the regional and local scales important for policy decisions. Simulating how clouds will change with warming andintum mayaffect warming remains one of the major challenges for global dimate modeli, in part because different doud types have different impacts on climate and the many cloud processes occur on scales smaller than most cument models can resolve. Greater computer power is already allowing for some of these processes to be resolved in the new generation of models Dozens of groups and research institutions work on climate models. andsoentists arenowable to analyseresults from essentially all of the world's major Earth-System Models and compare them with each other and with observations. Such opportunites are of tremendous benellt inbringing out the strengths and weaknesses of various modelsand diagnosing the causes of differences among models, so that research can focus on the relevant processes. Differences among modeis allow estimates to be made of the uncertainties in projections of future climate change. Additionally, large archives of results from many different models help scientists to identify aspects of climate change projections that are robust and that can be interpreted in terms of known physical mechanisms Studyinghowclimate responded to major changes in the pastis another way of checking that we understand how different processes work and that models are capable of performing reliably under a wide range of conditions 19.) Aredisasterscenariosabouttipping points like “turning off the Gulf Stream” andreleaseofmethanefrom the Arctic a cause for concern? Results from the best available dimate models do not predict an abrupt changein (or collapse of) the Atlantic Meridional Overtuming Circulation, which includes the Gulf Stream, in the near future. However, this and other potential high-riskabrupt changes, like the release of methane and carbon dioxide from thawing permafrost, remain active areas of scientific research. Some abrupt changes are already underway, such as the decrease in Arctic sea ice extent (see Question 12), and as warming increases, the possibility of other majorabrupt changes cannot be ruled out. The composition of the atmosphere is changing towards conditions that have not been experienced for millions of years, sowe areheaded forunknowntemitory, anduncertainty is large. The dimate system involvesmanycompeting processes that couldswitch the climate into a different state once a threshold has been exceeded. A well-known example is the south-north oosan overturning circulation, which is maintained by cold salty water sinking in the North Atlantic and involves the transport of extra heat to the North Allanticvia the Gulf Stream. During the last ice age, pulses of freshwater from the meltingice sheetover North America led to slowing down of this overturning circulation. This inturncaused widespread changesin dimale around the Northem Hemisphere. Freshening of the North Atlantic from the meltingof the Greenlandice sheet is gradual, however, and hence is not expected to cause abrupt changes. Another concern relates to the Arctic, where substantial warming could destabilise methane (a greenhouse gas) trapped in ocean sediments and permafrost, potentially leading to a rapid release of alarge amountof methane. Ifsucharapid release occurred, then major, fastclimate changes would ensue. Such high-risk changes are considered unlikely in this century, but are by definition hard to predict. Scientists are therefore continuing to study the possibility of exceeding such tipping points, beyond which we risk large and abrupt changes. In addition to abrupt changes in the dimate systemitself, steady climate change can cross thresholds that trigger abrupt changes in other systems. In human systema, for example, infrastructure has typically been built to accommodate the dimate variability at the time of construction. Gradual climate changes can cause abrupt changes in the utility of the infrastructure-such as when rising sea levels suddenly surpass sea walls, or when thawing permafrost causes the sudden collapse of pipelines, buildings, or roads. In natural systems, as air and water temperatures rise, some species-such as the mountain pika and many ocean corals-wilinolonger be ableto survive in their current habitats and will be forced to relocate (f possible) or rapidly adapt. Other species mayfare better in the newconditions, causing abrupt shifts in the balance of ecosystems, for example, warmer temperatures have allowed more bark beetles to survive over winter in some regions, where beetle outbreaks have destroyed forests 20.) stopped, would the climate return to the conditions of 200 years ago? No. Even if emissions of greenhouse gases were to suddenly stop, Earth's surface temperature would require thousands of years to coolandreturn to thellevel in the pre-industrialera. Ifemissions of CO, stopped altogether, it would take many thousands of years for atmosphericCO, to retum to "pre-industrial levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments. Surface temperatures would stay elevated for at least a thousand years, implying a long-term commitment to a warmerplanet dueto pastand current emissions. Sea level would likely continue to rise for many centuries even after temperature stopped increasing Figure of Significant cooling would berequiredtoreversemelling of glaciers and the Greenlandice sheet, which formed during past cold dimates. The current CO-induced warming of Earth is therefore essentially kreversible on human timescales. The amount and rate of further warming will dependalmost entirely on how much moreCO, humankind emits Scenarios of future dimate change incmasingly assume the use of technologies that can remove green- housegases from the atmosphere. Insuch negative emissions scenarios, itassumed thatatsomepoint in the future, widespread effort will be undertakerithat utilises such technologies to remove CO, from the atmosphere and lower ita atmospheric concentra Bon, thereby starting to reverse CO-driven warming on longer mescales. Deployment of such technologies at scale would require large decreases in their costs. Even if such technological fixes were practical, substantial reductions in CO, emissions would still be essential figure 9. If global emissions weretosuddenlystop,itwould takealongtimefor surfaceair temperatures and the ocean to begin to cool becausetheexcess CO2intheatmospherewould remaintherefor a long time and wouldcontinuetoexertawarming effect. Model projections show how atmospheric CO2 concentration (a), surface air temperature (b), and ocean thermal expansion (c) would respond following a scenario of business-as-usualemissions ceasing in 2300 (red), a scenario of aggressive emission reductions, fallingclosetozero50yearsfrom now (orange), and two intermediate emissions scenarios (green and blue). The small downward tick in temperature at 2300 is caused bytheeliminationofemissions of short-lived greenhousegases, including methane. Source: Zickfeld Conclusion This document explains that there are well-understood physical mechanisms by which changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and are still increasing rapidly, that dimate change is occurring, and that most of therecent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further dimate change is inevitable; if emissions of greenhouse gases continue unabated, future changes will substantially exceed those that have occurred so far. There remains a range of estimates of the magnitude and regional expression of future change, but increases in the extremes of climate that can adversely affect natural ecosystems and human activities and infrastructure are expected. Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limitemissions of greenhouse gasesandhencethe magnitude of climate changes, they can wait for changes to occur and accept the losses, damage, and suffering that arise, they can adapt to actual and expected changes as much as possible, or they can seek as yet unproven "geoengineering" solutions to counteract some of the climate changes that would otherwise occur. Each of these options has risks, attractions and costs, and whatis actually done may be a mixture of these different options. Different nations and communities will vary in their vulnerability and their capacitytoadapt. Thereis an important debate to be had about choices among these options, to decide what is best for each group or nation, and most importantly for the global population as a whole. The options have to be discussed at a global scale because inmanycases those communities that are most vulnerable control few of the emissions, either past or future. Our description of the science of dimate change, with both its facts andits uncertainties, is offered as a basis to inform that policy debate Acknowledgements Authors Thefollowingindividualsservedastheprimarywritingteamforthe2014and2020editionsof this document: ■ Eric Wolff FRS, (UK lead), University of Cambridge ■ Inez Fung (NAS, US lead), University of California, Berkeley ■ Brian Hoskins FRS, Grantham Institute for Climate Change ■ John F.B.MitchellFRS, UK Met Office ■ Tim Palmer FRS, University of Oxford ■ Benjamin Santer (NAS), Lawrence Livermore National Laboratory ■ JohnShepherdFRS,Universityof Southampton ■ KeithShineFRS,University ofReading. ■ Susan Solomon (NAS), Massachusetts Institute of Technology ■ Kevin Trenberth, National Center for Atmospheric Research ■ John Walsh,University ofAlaska, Fairbanks ■ Don Wuebbles, University of Illinois Staff support for the 2020 revision was provided by Richard Walker, Amanda Purcell, Nancy Huddleston, and Michael Hudson. We offer special thanks to Rebecca Lindsey and NOAA Climate.gov for providing data and figure updates. Reviewers The following individuals served as reviewers of the 2014 document in accordance with procedures approved by the RoyalSociety and the NationalAcademy ofSciences: ■ Richard Alley (NAS), Department of Geosciences, Pennsylvania State University ■ AlecBroersFRS,FormerPresidentoftheRoyal AcademyofEngineering ■ Harry Elderfield FRS, Department of Earth Sciences,University ofCambridge ■ Joanna HaighFRS,Professor ofAtmospheric Physics, Imperial College London ■ Isaac Held (NAS), NOAA Geophysical Fluid DynamicsLaboratory ■ John Kutzbach (NAS), Center for Climatic Research, University ofWisconsin ■ Jerry Meehl,Senior Scientist, National Center forAtmospheric Research ■ JohnPendryFRS,ImperialCollegeLondon ■ JohnPyleFRS,DepartmentofChemistry, University of Cambridge ■ Gavin Schmidt, NASA Goddard Space Flight Center ■ Emily Shuckburgh, British Antarctic Survey ■ GabrielleWalker,Journalist ■ Andrew Watson FRS, University ofEastAnglia Support The Support for the 2014 Edition was provided by NAS Endowment Funds. We offer sincere thanks to theRalphJ.andCarolM.CiceroneEndowmentforNASMissionsforsupportingtheproductionofthis 2020 Edition