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Ice cores and climate: Patterns and causes of change

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The last few hundred thousand years are critical for understanding the working of the Earth System because they show a wide range of climates under a geography similar to the present. Ice cores are an important palaeorecord because they record different aspects of the atmosphere (including trace gas concentrations) rather directly. Over recent decades, ice cores have provided several insights that have shaped the research agenda, and informed discussions about future climate change.

There are two very obvious patterns of climate variability that dominate the record seen, not only in ice cores, but also in records from other archives. At lower time resolution, the obvious feature of all late Quaternary (Quaternary = last ∼ 2 Ma) records is the occurrence of glacial-interglacial cycles with a period averaging around 100,000 years. Almost all parameters we measure in ice cores follow this pattern, with glacial periods having low Antarctic temperatures, low CO2 and CH4 concentrations, and high fluxes of dust and sea salt through the atmosphere and into the ice. The cold periods were also marked by the growth of ice sheets over North America and Europe, and the entire global climate system was considerably different. Most paleoclimate scientists agree that the pacing of these cycles is controlled by changes in the pattern of incoming solar radiation due to changes in Earths orbit and axis. And they agree that a combination of amplifying factors: changes in albedo, greenhouse gas concentrations, etc., lead to a very non-linear response. A large strand of research looks for the detailed physical and biogeochemical mechanisms that link the different factors (for example, why does CO2 increase in a warming world). An often separate strand wonders whether we really understand the 100 ka period, especially in the light of knowledge that the earlier parts of the Quaternary were dominated by 40 ka periodicity.

At higher resolution, the second dominant pattern of climate variability, also most clearly revealed by ice cores, is the series of warmings and coolings (known as Dansgaard-Oeschger events) that occurs during glacial periods. Studies of Greenland ice cores have shown that very rapid climate changes (several degrees in a few decades) can occur, again with a wide range of climate variables affected. The subdued counterparts seen in Antarctic ice cores support the idea that the probable mechanism is switches in the mode of ocean heat transport in the Atlantic Ocean. A particular interest of these events is the concern that, although the triggering mechanism must be different, such switches might be possible in a future warmer world, not just in a colder world. In both the cases discussed, one type of research appears to emphasise the pattern of variability, while almost ignoring the mechanisms, while another appears to make ever more complex models that describe the mechanisms but that find it hard to explain the pattern and pace of the variability. In the current audience it would be interesting to discuss how these strands should work together.

This talk is part of the British Antarctic Survey's Natural Complexity: Data and Theory in Dialogue series.

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