Past climate and the role of carbon dioxide

One of the ways in which we know that atmospheric carbon dioxide is important in controlling global climate is through the study of our past climate. Over the last two and half million years the Earth's climate has cycled between the great ice ages, with ice sheets over 3 km thick over North America and Europe, to conditions that were even milder than they are today. These changes are extremely rapid if compared to other geological variations, such as the movement of continents around the globe, where we are looking at a time period of millions of years. But how do we know about these massive ice ages and the role of carbon dioxide? The evidence mainly comes from ice cores drilled in both Antarctica and Greenland. As snow falls, it is light and fluffy and contains a lot of air. When this is slowly compacted to form ice, some of this air is trapped. By extracting these air bubbles trapped in the ancient ice, scientists can measure the percentage of greenhouse gases that were present in the past atmosphere. Scientists have drilled over two miles down into both the Greenland and Antarctic ice sheets, which has enabled them to reconstruct the amount of greenhouse gases that occurred in the atmosphere over the last half a million years. By examining the oxygen and hydrogen isotopes in the ice core, it is possible to estimate the temperature at which the ice was formed. The results are striking, as greenhouse gases such as atmospheric carbon dioxide (CO2) and methane (CH4) co-vary with temperatures over the last 400,000 years (see Figure 2). This strongly supports the

4 glacial cycles recorded in the Vostok ice core

4 glacial cycles recorded in the Vostok ice core

150 200 250 Age (kyr BP)

2. Greenhouse gases and temperature for the last four glacial cycles recorded in the Vostok ice core

150 200 250 Age (kyr BP)

2. Greenhouse gases and temperature for the last four glacial cycles recorded in the Vostok ice core idea that the carbon dioxide content in the atmosphere and global temperature are closely linked, i.e. when CO2 and CH4 increase, the temperature is found to increase and vice versa. This is our greatest concern for future climate: if levels of greenhouse gases continue to rise, so will the temperature of our atmosphere. The study of past climate, as we will see throughout this book, provides many clues about what could happen in the future. One of the most worrying results from the study of ice cores, and lake and deep-sea sediments, is that past climate has varied regionally by at least 5°C in a few decades, suggesting that climate follows a non-linear path. Hence we should expect sudden and dramatic surprises when greenhouse gas levels reach an as yet unknown trigger point in the future.

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