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Greenhouse history revealed

The Earth’s greenhouse history of the last 800,000 years is an open book now, thanks to years of detective work by two large international teams of climate scientists.

Nature has two papers this week, here and here, about the levels of atmospheric concentration of the two main greenhouse gases – carbon dioxide and methane – as derived from air entrapped in the EPICA Dome C ice core from Antarctica. Here is an editor’s summary.

The first and foremost results: The present day concentration of both gases is higher than has ever been the case in the past 800,000 years. Also, the ups and downs in carbon dioxide and methane curves follows the succession of cold glacial climates and relatively warm periods (such as ours) in between. An 800,000 year temperature record had been reconstructed previously from Antarctic ice cores.

Together, the results provide powerful evidence for a strong link between greenhouse gases and climate. During most of the Earth’s history greenhouse gas concentrations have fluctuated in the absence of humans burning coal and other fossil fuels. But the unprecedented rise of greenhouse gases in the modern atmosphere, to concentrations which threaten to unhinge vital components of the Earth’s climate system, is clearly the result of human activity.

In the past, greenhouse gas concentrations have varied owing to subtle feedbacks between orbital changes and oceanic and terrestrial carbon cylcles. Carbon dioxide concentrations depend on oceanic uptake, whereas methane is linked with the size and distribution of wetlands releasing the gas.

Like all good science, the new data will raise many new questions. One is why the amplitude of the 100,000-year oscillation in methane and carbon dioxide concentrations (which correlates with the 100,000 year temperature cycle) has changes so markedly around 450,000 years ago. Warm periods in the more recent history of the Earth seem to have been warmer than the interglacials prior to 450,000 years ago. Carbon dioxide and methane concentrations mirror this trend, which might hint to the existence of a longer-term cycle not visible in the existing record.

Another question is how greenhouse gas concentrations measured in Antarctic ice cores relate to episodes of rapid warming and cooling in Greenland and the northern hemisphere. It seems that more that 70 such temperature jumps, perhaps as a result of changes in ocean circulation, have occured over the last 800,000 years.

“These new benchmark data for greenhouse-gas variability pose questions as to what a much longer record might show,” writes Ed Brook in a news and views article. The search for the best drilling site which could produce such a record is beginning.

Quirin Schiermeier


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