Nature's Journal Club

Eric Wolff

British Antarctic Survey, Cambridge, UK

An ice-core scientist wonders what makes the Earth run hot and cold.

In the past 800,000 years, Earth has seen long, cold phases punctuated every 100,000 years by short, warm interglacials. If I claim to understand climate, then I should know why these cycles occur and why we are in a warm phase today.

The most obvious external controls on our climate are small changes in Earth’s orbit. These affect the variation of incoming sunlight (insolation) with season and latitude. ‘Milankovitch theory’ says that this in turn controls the occurrence of glaciations.

There is one obvious problem: although 100,000 years is the period of eccentricity of Earth’s orbit, insolation shows much stronger effects at shorter periods, such as 41,000 and 23,000 years.

A recent paper (E. Tziperman et al. Paleoceanography 21, PA4206; 2006) suggests a way around this. It uses a model in which climate varies with an average period controlled by internal features — such as the time needed for ice-sheet growth — on a 100,000-year timescale.

However, the exact timing of climate changes is paced by orbital cycles at shorter periods. The result is that a wide range of plausible internal controls on climate can give similar predictions of how climate has evolved with time, all of them with a ‘Milankovitch imprint’.

This frees us from the apparent misconception that we need an external forcing with a period of 100,000 years, but it does not identify the internal mechanisms responsible.

I used to think this was a problem for others to solve, but as part of the team that extended the ice-core record back 800,000 years, I have the tantalizing hope that the clues we need might be locked in our cold room.

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