Climate change Evidence &Causes

 Climate Change

Evidence & Causes

Update 2020

An overview from the Royal Society and the US National Academy of Sciences

Foreword

CLIMATE CHANGE IS ONE OF THE DEFINING ISSUES OF OUR TIME. It is now more certain

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 have cost billions in damages. Habitats are undergoing rapid shifts in response to changing temperatures and precipitation patterns.

The Royal Society and the US National Academy of Sciences, with their similar missions to promote the use of science to benefit society and to inform critical policy debates, produced the original Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UK-US team of leading climate scientists. This new edition, prepared by the same author team, has been updated with the most recent climate data and scientific analyses, all of which reinforce our 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. For example, the period of slower warming during the 2000s and early 2010s has ended with a 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 risks likely to occur within the next ten years. Yet, the international community still has far to go in showing 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 to reduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as a 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.

Marcia McNutt

President, National Academy of Sciences

Venki Ramakrishnan

President, Royal Society

For further reading

For more detailed discussion of the topics addressed in this document (including references to the underlying original research), see:

Intergovernmental Panel on Climate Change (IPCC), 2019: Special Report on the Ocean and Cryosphere in a Changing Climate [https://www.ipcc.ch/srocc]

National Academies of Sciences, Engineering, and Medicine (NASEM), 2019: Negative Emissions Technologies and Reliable Sequestration: A Research Agenda [https://www.nap.edu/catalog/25259]

Royal Society, 2018: Greenhouse gas removal

[https://raeng.org.uk/greenhousegasremoval]

U.S. Global Change Research Program (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 National Climate Assessment Volume I: Climate Science Special Reports [https://science2017.globalchange.gov]

NASEM, 2016: Attribution of Extreme Weather Events in the Context of Climate Change [https://www.nap.edu/catalog/21852]

IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. Climate Change 2013: The Physical Science Basis [https://www.ipcc.ch/report/ar5/wg1]

NRC, 2013: Abrupt Impacts 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 the Science [https://royalsociety.org/topics-policy/publications/2010/ climate-change-summary-science]

NRC, 2010: America’s Climate Choices: Advancing the Science of Climate Change [https://www.nap.edu/catalog/12782]

Much of the original data underlying the scientific findings discussed here are available at:

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/

THE NATIONAL ACADEMY OF SCIENCES (NAS) was established to advise the United States on scientific and technical issues when President Lincoln signed a Congressional charter in 1863. The National Research Council, the operating arm of the National Academy of Sciences and the National Academy of Engineering, has issued numerous reports on the causes of and potential responses to climate change. Climate change resources from the National Research Council are available at nationalacademies.org/climate.

THE ROYAL SOCIETY is a self-governing Fellowship of many of the world’s most distinguished 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 fundamental purpose, reflected in its founding Charters of the 1660s, is to recognise, promote, and support excellence in science, and to encourage the development and use of science

for the benefit of humanity. More information on the Society’s climate change work is available at royalsociety.org/policy/climate-change

Summary 2

Climate Change Q& A

Is the climate warming? 3

How do scientists know that recent climate change is largely caused by human activities? 5

CO2 is already in the atmosphere naturally, so why are emissions from

human activity significant? 6

What role has the Sun played in climate change in recent decades? 7

What do changes in the vertical structure of atmospheric temperature—from the

surface up to the stratosphere—tell us about the causes of recent climate change? 8

Climate is always changing. Why is climate change of concern now? 9

Is the current level of atmospheric CO2 concentration unprecedented in Earth’s history? 9

Is there a point at which adding more CO2 will not cause further warming? 10

Does the rate of warming vary from one decade to another? 11

Did the slowdown of warming during the 2000s to early 2010s

mean that climate change is no longer happening? 12

The Basics of Climate Change B1–B8

Climate Change Q& A (continued)

If the world is warming, why are some winters and summers still very cold? 13

Why is Arctic sea ice decreasing while Antarctic sea ice has changed little? 14

How does climate change affect the strength and frequency

of floods, droughts, hurricanes, and tornadoes? 15

How fast is sea level rising? 16

What is ocean acidification and why does it matter? 17

How confident are scientists that Earth will warm further over the coming century? 18

Are climate changes of a few degrees a cause for concern? 19

What are scientists doing to address key uncertainties

in our understanding of the climate system? 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

If emissions of greenhouse gases were stopped, would the climate return

to the conditions of 200 years ago? 22

Conclusion 23

Acknowledgements 24

GREENHOUSE GASES such as carbon dioxide (CO2) absorb heat (infrared radiation) 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 the burning of fossil fuels since the start of the Industrial Revolution—have increased

atmospheric CO2 concentrations by more than 40%, with over half the increase occurring since 1970. Since 1900, 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 during this period is mainly a result of the increased concentrations of CO2 and other

greenhouse gases. Continued emissions of these gases will cause further climate change, including substantial increases in global average surface temperature and important changes in regional climate. The magnitude and timing of these changes will depend on 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 mainly on the total amount of CO2 and other greenhouse gases emitted as a result of human activities.

Is the climate 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 sources as tree rings and ice cores help to place recent temperature changes in the context of the past. In terms of the average surface temperature of Earth, 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, 2010-2019, 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 sea level is rising [Figure 1b]. These measurements are made with a variety of 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 temperature has risen as shown in this plot of combined land and ocean measurements from 1850 to 2019, derived from three independent analyses of the

available data sets. The temperature changes are relative to the global average surface temperature of 1961−1990. Source: NOAA Climate. gov; data from UK Met Offfce Hadley Centre (maroon), US National Aeronautics and Space Administration Goddard Institute for Space Studies

Annual global surface temperature (1850−2019)

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Hadley Centre (UK Met)

(red), and US National Oceanic

and Atmospheric Administration National Centers for Environmental Information (orange).

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Figur e 1b. A large amount of observational evidence besides surface temperature records shows that Earth’s climate is changing.

For example, additional evidence of a warming trend can be found in the dramatic decrease in the extent of Arctic sea ice at its summer minimum (which occurs in September), the decrease in June snow cover in the Northern Hemisphere, the increases in the

Arctic sea ice extent in winter and summer (1979−2019)

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global average upper ocean (upper 700 m or 2300 feet) heat content (shown relative to the 1955–2006 average), and the rise in global sea level. Source: NOAA Climate.gov

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Data: NOAA NODC

1955 1965 1975 1985 1995 2005 2015

Year Data: Church & White 2011, UHSLC

How do scientists know that recent climate change is largely caused by human activities?

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 that atmospheric CO2 increased by more than 40% from 1800 to 2019. Measurements of different forms 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 easier to see by probing beyond a single number (such as the average temperature of Earth’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 how greenhouse 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 causes include variations in the Sun’s output and in Earth’s orbit around the Sun, volcanic eruptions, and internal 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 only natural factors were influencing the climate system. These simulations yield little surface warming, or even a slight cooling, over the 20th century and into the 21st. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes.

CO2 is already in the atmosphere naturally, so why are emissions from human activity significant?

In nature, CO2 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 CO2 (roughly 1% of the emission rate from fossil fuel combustion) is also emitted in volcanic eruptions. This is balanced by an equivalent amount that is removed by chemical weathering of rocks.

The CO2 level in 2019 was more than 40% higher than it was in the 19th century. Most of this CO2 increase has taken place since 1970, about the time when global energy consumption accelerated. Measured decreases in the fraction of other forms of carbon (the isotopes 14C and 13C) and a small decrease in atmospheric oxygen concentration (observations of which have been available since 1990) show that

the rise in CO2 is largely from combustion of fossil fuels (which have low 13C fractions and no 14C). Deforestation and other land use changes have also released carbon from the biosphere (living world) where it normally resides for decades to centuries. The additional CO2 from fossil fuel burning and deforestation has disturbed the balance of the carbon cycle, because the natural processes that could restore the balance are too slow compared to the rates at which human activities are adding CO2 to the atmosphere. As a result, a substantial fraction of the CO2 emitted from human activities accumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of

years. Comparison with the CO2 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).

For periods before the onset of satellite measurements, knowledge about solar changes is less certain because the changes are inferred from indirect sources — including the number of sunspots and the abundance of certain forms (isotopes) of carbon or beryllium atoms, whose production 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 layer in the atmosphere above the troposphere, typically from 12 to 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 pronounced long-term changes in the Sun’s output over the past century, during which time human- induced increases in CO2 concentrations have been the dominant influence on the long-term global surface temperature increase. Further evidence that current warming is not a result of solar changes can be found in the temperature trends at different altitudes in the atmosphere (see Question 5).

Figur e 2 . Measurements of the Sun’s energy incident on Earth show no net increase in solar forcing during the past 40 years, and therefore this cannot be responsible for warming during that period. The data show only small periodic amplitude variations associated with the Sun’s 11-year cycle. Source: TSI data from Physikalisch-Meteorologisches Observatorium Davos, Switzerland,

on the new VIRGO scale from 1978 to mid-2018; temperature data for same time period from the HadCRUT4 dataset, UK Met Offfce, Hadley Centre.

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What do changes in the vertical structure of atmospheric temperature

—from the surface up to the stratosphere—tell us about the

causes of recent climate change?

In the early 1960s, results from mathematical/physical models of the climate system first showed that human-induced increases in CO2 would be expected to lead to gradual warming of the lower atmosphere (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 time, there was insufficient observational data to test this prediction, but temperature measurements from weather balloons and satellites have since confirmed these early forecasts. It is now known that the observed pattern of tropospheric warming and stratospheric cooling over the past 40 years is broadly consistent with computer model simulations that include increases in CO2 and decreases in stratospheric ozone, each caused by human activities. The observed pattern is not consistent with purely natural changes in the Sun’s energy output, volcanic activity, or natural clima