O Climate Change

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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

- 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

- 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

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

to the conditions of 200 years ago? 22

THE BASICS OF CLIMATE CHANGE B1–B8

CONCLUSION 23

ACKNOWLEDGEMENTS 24

FOR FURTHER READING C3

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EVIDENCE & CAUSES 1

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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 about 40%, with more than half the increase occurring since 1970. Since 1900, the global average surface temperature has increased by about 0.8 °C (1.4 °F). This has been accompanied by warming of the ocean, a rise in sea level, a strong decline in Arctic sea ice, and many other associated climate effects. Much of this warming has occurred in the last four 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.

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2 CLIM ATE CHA NGE

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Q& A

1

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FIGURE 1A. Earth’s global average surface temperature has risen as shown in this plot of combined land and ocean measurements from 1850 to 2012, 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: IPCC AR5, data from the HadCRUT4 dataset (black), UK Met Office Hadley Centre, the NCDC MLOST dataset (orange), US National Oceanic and Atmospheric Administration, and the NASA GISS dataset (blue), US National Aeronautics and Space Administration.

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IS THE CLIMATE WARMING?

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Yes. Earth’s average surface air temperature has increased by about 0.8 °C (1.4 °F) since 1900, with much of this increase taking place 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. In some places, these records 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 1983 to 2012 was probably the warmest 30-year period in more than 800 years.

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 monitoring systems, which gives added confidence

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