O Climate Change

[pic 1]CLIMATE CHANGE

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EVIDENCE & CAUSES

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

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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, accompanied by sea-level rise, a strong decline in Arctic sea ice, and other climate-related changes.

The evidence is clear. However, due to the nature of science, not every single detail is ever totally settled or completely certain. Nor has every pertinent question yet been answered. Scientific evidence continues to be gathered around the world, and assumptions and findings about climate change are continually analysed and tested. Some areas of active debate and ongoing research include the link between ocean heat content and the rate of warming, estimates of how much warming to expect in the future, and the connections between climate change and extreme weather events.

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, offer this new publication as a key reference document for decision makers, policy makers, educators, and other individuals seeking authoritative answers about the current state of climate-change science. The publication makes clear what is well established, where consensus is growing, and where there is still uncertainty. It is written and reviewed by a UK-US team of leading climate scientists. It echoes and builds upon the long history of climate-related work from both national science academies, as well as the newest climate-change assessment from the United Nations’ Intergovernmental Panel on Climate Change.

Scientific information is a vital component of the evidence required for societies to make sensible policy decisions. Climate-change science will continue to help society make informed decisions about how to reduce the magnitude of climate change and to adapt to its impacts. The Royal Society and the US National Academy of Sciences will continue to support the use of robust science toward these critical goals.

In 2008 Raymond and Beverly Sackler established the USA-UK Scientific Forum to help the scientists of the United Kingdom and the United States forge an enduring partnership on topics of worldwide scientific concern. As Presidents of the Royal Society and National Academy of Sciences, we are pleased to introduce the latest piece of work supported by the Sacklers’ inspired generosity.

Dr. Ralph J. Cicerone

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Sir Paul Nurse

President, National Academy of Sciences

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President, Royal Society

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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

1. 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

1. 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 Does the recent slowdown of warming mean that climate change is no longer happening?        12

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 is not?        14

1. 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

1. What are scientists doing to address key uncertainties

in our understanding of the climate system?        19

1. Are disaster scenarios about tipping points like ‘turning off the Gulf Stream’

and release of methane from the Arctic a cause for concern?        21

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