Climate Change: Evidence & Causes 2020 - Royal Society
Climate ChangeEvidence & CausesUpdate 2020An overview from the Royal Society and theUS National Academy of Sciences
n sum m a r yForewordCLIMATE CHANGE IS ONE OF THE DEFINING ISSUES OF OUR TIME. It is now more certainthan ever, based on many lines of evidence, that humans are changing Earth’s climate. Theatmosphere and oceans have warmed, which has been accompanied by sea level rise, a strongdecline in Arctic sea ice, and other climate-related changes. The impacts of climate change onpeople and nature are increasingly apparent. Unprecedented flooding, heat waves, and wildfireshave cost billions in damages. Habitats are undergoing rapid shifts in response to changingtemperatures and precipitation patterns.The Royal Society and the US National Academy of Sciences, with their similar missions topromote the use of science to benefit society and to inform critical policy debates, produced theoriginal Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UK-USteam of leading climate scientists. This new edition, prepared by the same author team, has beenupdated with the most recent climate data and scientific analyses, all of which reinforce ourunderstanding of human-caused climate change.The evidence is clear. However, due to the nature of science, not every detail is ever totally settledor certain. Nor has every pertinent question yet been answered. Scientific evidence continues tobe gathered around the world. Some things have become clearer and new insights have emerged.For example, the period of slower warming during the 2000s and early 2010s has ended with adramatic jump to warmer temperatures between 2014 and 2015. Antarctic sea ice extent, whichhad 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 questionsaddressed in this booklet.Calls for action are getting louder. The 2020 Global Risks Perception Survey from the WorldEconomic Forum ranked climate change and related environmental issues as the top five globalrisks likely to occur within the next ten years. Yet, the international community still has far to go inshowing increased ambition on mitigation, adaptation, and other ways to tackle climate change.Scientific information is a vital component for society to make informed decisions about how toreduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as akey reference document for decision makers, policy makers, educators, and others seekingauthoritative 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 Presidentof 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. Ascurrent Presidents of these organizations, we are pleased to offer an update to this key reference,supported by the generosity of the Cicerone Family.Marcia McNuttPresident, National Academy of Sciences2Clim at e Ch a ngeVenki RamakrishnanPresident, Royal Society
For further readingFor more detailed discussion of the topics addressed in this document(including references to the underlying original research), see: NRC, 2013: Abrupt Impacts of Climate Change: AnticipatingSurprises [https://www.nap.edu/catalog/18373] Intergovernmental Panel on Climate Change (IPCC), 2019:Special Report on the Ocean and Cryosphere in a Changing Climate[https://www.ipcc.ch/srocc] NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations,and Impacts Over Decades to Millennia[https://www.nap.edu/catalog/12877] National Academies of Sciences, Engineering, and Medicine(NASEM), 2019: Negative Emissions Technologies and ReliableSequestration: A Research Agenda[https://www.nap.edu/catalog/25259] Royal Society 2010: Climate Change: A Summary of the lications/2010/climate-change-summary-science] Royal Society, 2018: Greenhouse gas NRC, 2010: America’s Climate Choices: Advancing the Scienceof Climate Change [https://www.nap.edu/catalog/12782] U.S. Global Change Research Program (USGCRP), 2018: FourthNational Climate Assessment Volume II: Impacts, Risks, andAdaptation in the United States [https://nca2018.globalchange.gov]Much of the original data underlying the scientific findingsdiscussed here are available at: https://data.ucar.edu/ https://climatedataguide.ucar.edu IPCC, 2018: Global Warming of 1.5 C [https://www.ipcc.ch/sr15] https://iridl.ldeo.columbia.edu USGCRP, 2017: Fourth National Climate Assessment Volume I: ClimateScience Special Reports [https://science2017.globalchange.gov] https://ess-dive.lbl.gov/ https://www.ncdc.noaa.gov/NASEM, 2016: Attribution of Extreme Weather Events in theContext of Climate Change [https://www.nap.edu/catalog/21852] https://www.esrl.noaa.gov/gmd/ccgg/trends/ http://scrippsco2.ucsd.eduIPCC, 2013: Fifth Assessment Report (AR5) Working Group 1.Climate Change 2013: The Physical Science Basis[https://www.ipcc.ch/report/ar5/wg1] http://hahana.soest.hawaii.edu/hot/ THE NATIONAL ACADEMY OF SCIENCES (NAS) was established to advise the UnitedStates on scientific and technical issues when President Lincoln signed a Congressionalcharter in 1863. The National Research Council, the operating arm of the National Academyof Sciences and the National Academy of Engineering, has issued numerous reports on thecauses of and potential responses to climate change. Climate change resources from theNational Research Council are available at nationalacademies.org/climate.THE ROYAL SOCIETY is a self-governing Fellowship of many of the world’s mostdistinguished scientists. Its members are drawn from all areas of science, engineering,and medicine. It is the national academy of science in the UK. The Society’s fundamentalpurpose, reflected in its founding Charters of the 1660s, is to recognise, promote, andsupport excellence in science, and to encourage the development and use of sciencefor the benefit of humanity. More information on the Society’s climate change work isavailable at royalsociety.org/policy/climate-changeE v i de n c e & C a u se s 2 0 2 03
contentsSummary . 2Climate Change Q& A1 Is the climate warming? . 32 How do scientists know that recent climate change is largely caused by human activities? . 53 CO2 is already in the atmosphere naturally, so why are emissions fromhuman activity significant? . 64 What role has the Sun played in climate change in recent decades? . 75 What do changes in the vertical structure of atmospheric temperature—from thesurface up to the stratosphere—tell us about the causes of recent climate change? . 86 Climate is always changing. Why is climate change of concern now? . 97 Is the current level of atmospheric CO2 concentration unprecedented in Earth’s history? . 98 Is there a point at which adding more CO2 will not cause further warming? . 109 Does the rate of warming vary from one decade to another? . 1110 Did the slowdown of warming during the 2000s to early 2010smean that climate change is no longer happening? . 12The Basics of Climate Change . B1–B8Climate Change Q& A (continued)11 If the world is warming, why are some winters and summers still very cold? . 1312 Why is Arctic sea ice decreasing while Antarctic sea ice has changed little? . 1413 How does climate change affect the strength and frequencyof floods, droughts, hurricanes, and tornadoes? . 1514 How fast is sea level rising? . 1615 What is ocean acidification and why does it matter? . 1716 How confident are scientists that Earth will warm further over the coming century? . 1817 Are climate changes of a few degrees a cause for concern? . 1918 What are scientists doing to address key uncertaintiesin our understanding of the climate system? . 1919 Are disaster scenarios about tipping points like “turning off the Gulf Stream”and release of methane from the Arctic a cause for concern? . 2120 If emissions of greenhouse gases were stopped, would the climate returnto the conditions of 200 years ago? . 22Conclusion . 23Acknowledgements . 24E v i de n c e & C a u se s 2 0 2 01
SummaryGREENHOUSE GASES such as carbon dioxide (CO2) absorb heat (infrared radiation)emitted from Earth’s surface. Increases in the atmospheric concentrations of thesegases cause Earth to warm by trapping more of this heat. Human activities—especiallythe burning of fossil fuels since the start of the Industrial Revolution—have increasedatmospheric CO2 concentrations by more than 40%, with over half the increase occurringsince 1970. Since 1900, the global average surface temperature has increased by about1 C (1.8 F). This has been accompanied by warming of the ocean, a rise in sea level, astrong decline in Arctic sea ice, widespread increases in the frequency and intensityof heatwaves, and many other associated climate effects. Much of this warming hasoccurred in the last five decades. Detailed analyses have shown that the warmingduring this period is mainly a result of the increased concentrations of CO2 and othergreenhouse gases. Continued emissions of these gases will cause further climate change,including substantial increases in global average surface temperature and importantchanges in regional climate. The magnitude and timing of these changes will depend onmany factors, and slowdowns and accelerations in warming lasting a decade or more willcontinue to occur. However, long-term climate change over many decades will dependmainly on the total amount of CO2 and other greenhouse gases emitted as a result ofhuman activities.2Clim at e Ch a nge
Q& A1Is the climate warming?Yes. Earth’s average surface air temperature has increased by about 1 C (1.8 F) since1900, with over half of the increase occurring since the mid-1970s [Figure 1a]. A widerange of other observations (such as reduced Arctic sea ice extent and increasedocean heat content) and indications from the natural world (such as poleward shiftsof temperature-sensitive species of fish, mammals, insects, etc.) together provideincontrovertible evidence of planetary-scale warming.The clearest evidence for surface warming comes from widespread thermometer records that, in someplaces, extend back to the late 19th century. Today, temperatures are monitored at many thousands oflocations, over both the land and ocean surface. Indirect estimates of temperature change from suchsources as tree rings and ice cores help to place recent temperature changes in the context of the past. Interms of the average surface temperature of Earth, these indirect estimates show that 1989 to 2019 wasvery likely the warmest 30-year period in more than 800 years; the most recent decade, 2010-2019, is thewarmest decade in the instrumental record so far (since 1850).A wide range of other observations provides a more comprehensive picture of warming throughout theclimate 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, andsea level is rising [Figure 1b]. These measurements are made with a variety of land-, ocean-, and space-basedmonitoring systems, which gives added confidence in the reality of global-scale warming of Earth’s climate.Annual global surface temperature (1850 2019)Difference from average temperature ( C)Figure 1a. Earth’s global averagesurface temperature has risen asshown in this plot of combinedland and ocean measurementsfrom 1850 to 2019, derived fromthree independent analyses of theavailable data sets. The temperaturechanges are relative to the globalaverage surface temperature of1961 1990. Source: NOAA Climate.gov; data from UK Met Office HadleyCentre (maroon), US NationalAeronautics and Space AdministrationGoddard Institute for Space Studies(red), and US National Oceanicand Atmospheric AdministrationNational Centers for EnvironmentalInformation (orange).Hadley Centre (UK Met)1.0NASA (GISS)NOAA (NCEI)0.80.60.40.201961 1990 0197019902010E v i de n c e & C a u se s 2 0 2 03
n Q& nt)(percent)(percent)2020201981 20101981 2010average1981 2010average1981 0520152015199519951995199520052015200520152005 Data: Rutgers2015Snow Lab2005 Data: Rutgers2015Snow Lab2015 NSIDCData:2015 NSIDCData:Data: NSIDCData: NSIDC1967 20191967 2019average1967 2019average1967 2019averageaverageno datano datano datano 1985YearYearData: Rutgers Snow LabData: Rutgers Snow Lab1955 20061955 2006average1955 2006average1955 199519951995Global sea level (1955-2019)Global sea level (1955-2019)Globallevel (1955-2019)80 seaGlobal80 sea level (1955-2019)Clim at e Ch a nge1995199519951995Northern Hemisphere June snow cover (1967 2019)NorthernHemisphere June snow cover (1967 2019)6NorthernHemisphere June snow cover (1967 2019)6Upper ocean heat content (1955 2019)Upper ocean heat content (1955 2019)Upper20 ocean heat content (1955 2019)Upper20 ocean heat content (1955 1985YearYearNorthern Hemisphere June snow cover (1967 s(millions(millions(millionsofofkmofkmkm) 2km) 2) 2)Figur e 1b. A large amount ofobservational evidence besidessurface temperature records showsthat Earth’s climate is changing.For example, additional evidenceof a warming trend can be foundin the dramatic decrease in theextent of Arctic sea ice at itssummer minimum (which occursin September), the decrease inJune snow cover in the NorthernHemisphere, the increases in theglobal average upper ocean (upper700 m or 2300 feet) heat content(shown relative to the 1955–2006average), and the rise in global sealevel. Source: NOAA Climate.govArctic sea ice extent in winter and summer (1979 2019)Arctic sea ice extent in winter and summer (1979 2019)Arctic sea ice extent in winter and summer (1979 2019)Arcticsea ice extent in winter and summer (1979 2019)202005200520052005201520152015 NODCData: NOAA2015 NODCData: NOAAData: NOAA NODCData: NOAA NODC1993 20081993 2008average1993 2008average1993 20052015UHSLCData:Church & White 2011,20052015UHSLCData:Church & White 2011,Data: Church & White 2011, UHSLCData: Church & White 2011, UHSLC
Q& A n2How do scientists know that recentclimate change is largely caused byhuman activities?Scientists know that recent climate change is largely caused by human activities from anunderstanding of basic physics, comparing observations with models, and fingerprintingthe 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 importanceto Earth’s energy balance. Direct measurements of CO2 in the atmosphere and in air trapped in ice showthat atmospheric CO2 increased by more than 40% from 1800 to 2019. Measurements of different formsof carbon (isotopes, see Question 3) reveal that this increase is due to human activities. Other greenhousegases (notably methane and nitrous oxide) are also increasing as a consequence of human activities. Theobserved global surface temperature rise since 1900 is consistent with detailed calculations of the impactsof the observed increase in atmospheric greenhouse gases (and other human-induced changes) on Earth’senergy balance.Different influences on climate have different signatures in climate records. These unique fingerprints areeasier to see by probing beyond a single number (such as the average temperature of Earth’s surface), andby looking instead at the geographical and seasonal patterns of climate change. The observed patterns ofsurface warming, temperature changes through the atmosphere, increases in ocean heat content, increasesin atmospheric moisture, sea level rise, and increased melting of land and sea ice also match the patternsscientists expect to see due to human activities (see Question 5).The expected changes in climate are based on our understanding of how greenhouse gases trap heat. Boththis fundamental understanding of the physics of greenhouse gases and pattern-based fingerprint studiesshow that natural causes alone are inadequate to explain the recent observed changes in climate. Naturalcauses include variations in the Sun’s output and in Earth’s orbit around the Sun, volcanic eruptions, andinternal fluctuations in the climate system (such as El Niño and La Niña). Calculations using climate models(see infobox, p. 20) have been used to simulate what would have happened to global temperatures if onlynatural factors were influencing the climate system. These simulations yield little surface warming, or even aslight cooling, over the 20th century and into the 21st. Only when models include human influences on thecomposition of the atmosphere are the resulting temperature changes consistent with observed changes.E v i de n c e & C a u se s 2 0 2 05
n Q& A3CO2 is already in the atmospherenatur ally, so why are emiss
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