Climate Change

Evidence & Causes

Update 2020
An overview from the Royal Society and the
US National Academy of Sciences
n sum m a r y

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 Venki Ramakrishnan

President, National Academy of Sciences President, Royal Society

2 Clim ate Ch a nge

For further reading

For more detailed discussion of the topics addressed in this document ■ NRC, 2013: Abrupt Impacts of Climate Change: Anticipating
(including references to the underlying original research), see: Surprises [https://www.nap.edu/catalog/18373]

■ Intergovernmental Panel on Climate Change (IPCC), 2019: ■ NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations,
Special Report on the Ocean and Cryosphere in a Changing Climate and Impacts Over Decades to Millennia
[https://www.ipcc.ch/srocc] [https://www.nap.edu/catalog/12877]

■ National Academies of Sciences, Engineering, and Medicine ■ Royal Society 2010: Climate Change: A Summary of the Science
(NASEM), 2019: Negative Emissions Technologies and Reliable [https://royalsociety.org/topics-policy/publications/2010/
Sequestration: A Research Agenda climate-change-summary-science]
[https://www.nap.edu/catalog/25259]
■ NRC, 2010: America’s Climate Choices: Advancing the Science
■ Royal Society, 2018: Greenhouse gas removal of Climate Change [https://www.nap.edu/catalog/12782]
[https://raeng.org.uk/greenhousegasremoval]
Much of the original data underlying the scientific findings
■ U.S. Global Change Research Program (USGCRP), 2018: Fourth
discussed here are available at:
National Climate Assessment Volume II: Impacts, Risks, and
■ https://data.ucar.edu/
Adaptation in the United States [https://nca2018.globalchange.gov]
■ 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: Climate ■ https://ess-dive.lbl.gov/
Science Special Reports [https://science2017.globalchange.gov] ■ https://www.ncdc.noaa.gov/
■ NASEM, 2016: Attribution of Extreme Weather Events in the ■ https://www.esrl.noaa.gov/gmd/ccgg/trends/
Context of Climate Change [https://www.nap.edu/catalog/21852] ■ http://scrippsco2.ucsd.edu

■ IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. ■ http://hahana.soest.hawaii.edu/hot/

Climate Change 2013: The Physical Science Basis
[https://www.ipcc.ch/report/ar5/wg1]

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
E v i de n c e & C a u se s 2 0 2 0 3
contents

Summary ........................................................................................................................................................ 2

Climate Change Q& A

1 Is the climate warming? ........................................................................................................................... 3
2 How do scientists know that recent climate change is largely caused by human activities? ............. 5
3 CO2 is already in the atmosphere naturally, so why are emissions from
human activity significant? ...................................................................................................................... 6
4 What role has the Sun played in climate change in recent decades? ................................................... 7
5 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
6 Climate is always changing. Why is climate change of concern now? ................................................. 9
7 Is the current level of atmospheric CO2 concentration unprecedented in Earth’s history? ................ 9
8 Is there a point at which adding more CO2 will not cause further warming? .................................... 10
9 Does the rate of warming vary from one decade to another? ............................................................. 11
10 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)

11 If the world is warming, why are some winters and summers still very cold? .................................... 13
12 Why is Arctic sea ice decreasing while Antarctic sea ice has changed little? ..................................... 14
13 How does climate change affect the strength and frequency
of floods, droughts, hurricanes, and tornadoes? ................................................................................. 15
14 How fast is sea level rising? .................................................................................................................. 16
15 What is ocean acidification and why does it matter? ........................................................................... 17
16 How confident are scientists that Earth will warm further over the 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 of the 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

E v i de n c e & C a u se s 2 0 2 0 1
 Summary

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.

2 Clim ate Ch a nge

 Q& A

Is the climate warming?

1 Yes. Earth’s average surface air temperature has increased by about 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
ocean heat content) and indications from the natural world (such as poleward shifts
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
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 Annual global surface temperature (1850−2019)
surface temperature has risen as Hadley Centre (UK Met)
1.0
shown in this plot of combined
Difference from average temperature (°C)

NASA (GISS)
0.8
land and ocean measurements NOAA (NCEI)
from 1850 to 2019, derived from 0.6
three independent analyses of the 0.4
available data sets. The temperature
changes are relative to the global 0.2

average surface temperature of 0 1961−1990 average

1961−1990. Source: NOAA Climate.
-0.2
gov; data from UK Met Office Hadley
Centre (maroon), US National -0.4

Aeronautics and Space Administration -0.6

Goddard Institute for Space Studies
-0.8
(red), and US National Oceanic 1850 1870 1890 1910 1930 1950 1970 1990 2010
and Atmospheric Administration Year
National Centers for Environmental
Information (orange).

E v i de n c e & C a u se s 2 0 2 0 3
n Q& A

Arctic 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)
Arctic sea ice extent in winter and summer (1979−2019)
Figur e 1b. A large amount of 20
(percent) 20
observational evidence besides 20
(percent)
20
(percent)

surface temperature records shows 1981−2010

(percent)

0 1981−2010
average
that Earth’s climate is changing. 1981−2010
average

0 average
1981−2010
average

0 average
average

For example, additional evidence 0 average March

average

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maximum
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from

September maximum
March
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Difference

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summer minimum (which occurs -40

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1985 1955 1965 1975 1995 2005 2015
June snow cover in the Northern 1985 1955 1965 1975 1995 2005 2015
Year
1985 1955 1965 1975 1995 2005 2015 NSIDC
Data:
Hemisphere, the increases in the Year
1985 1955 1965 1975 1995 2005 2015 NSIDC
Data:
Year Data: NSIDC
Year
Northern Hemisphere June snow cover (1967−2019)
global average upper ocean (upper Northern Hemisphere June snow cover (1967−2019)
Data: NSIDC

700 m or 2300 feet) heat content Northern

6 Hemisphere June snow cover (1967−2019)
Northern Hemisphere June snow cover (1967−2019)
) 2) 2)

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4 Clim ate Ch a nge
 Q& A n

How do scientists know that recent

2 climate change is largely caused by
human activities?
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
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.

E v i de n c e & C a u se s 2 0 2 0 5
n Q& A

CO2 is already in the atmosphere

3 natur ally, so why are emissions from
human activity significant?
Human activities have significantly disturbed the natural carbon cycle by extracting long-
buried fossil fuels and burning them for energy, thus releasing CO2 to the atmosphere.

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).

6 Clim ate Ch a nge