Sunday, November 27, 2005

Record ice core reveals Earth's ancient atmosphere

There are several important points in these recent results:

Record ice core reveals Earth's ancient atmosphere
The longest ice-core record of climate history ever obtained has hugely extended the detailed history of Earth's atmosphere, and shows that levels of greenhouse gases really do march in lockstep with changes in temperature.

The frozen record of the Earth's atmosphere is 3270 metres long and covers the last 650,000 years – 50% longer than before. It was obtained from the tiny air bubbles trapped in a deep ice core from Antarctica.

The tight coupling between temperatures and the greenhouse gas levels revealed by the core matches the predictions from climate models used to forecast future global warming. It also bears some good news: the warm interglacial periods between ice ages can last a long time, contrary to the view that we may already be due for the onset of the next ice age.

First point: Levels of greenhouse gases and changes in temperature are correlated.

Second point: There's a lot of variability in ice age cycles. The lengths of colder and warmer periods vary over a wide range.
...during all that time, the atmosphere has never had anywhere near the levels of greenhouse gases seen today.

Today's level of 380 parts per million of carbon dioxide is 27% above its previous peaks of about 300 ppm, according to the team led by Thomas Stocker of the University of Bern in Switzerland.

Third point: We're in uncharted territory. There is no record of any time in the last 650,000 years when the Earth's atmosphere has had the levels of greenhouse gases that it does now. Where we go from here is impossible to predict from this data... but the trend is anything but encouraging.

On the other hand, 650,000 years is just an eyeblink in Earth's history -- about .014% to be more exact.

There is further reporting on this research, and a vigorous debate in the comments about its meaning, in this article at Real Climate: 650,000 years of greenhouse gas concentrations.

The most contentions issue is with respect to the correlation of temperatures and greenhouse gas (GHG) concentrations. It appears that changes in temperature precede rather than follow GHG by perhaps 600 years. However, this lag can be explained as the result of higher temperatures (from other causes, such as variations in Earth's orbit) causing the release of GHGs from the oceans, leading to a positive feedback loop as the GHGs drive a greenhouse effect.

This discussion shows the complexity of the situation. A number of different feedback loops need to be incorporated in climate models. When done carefully, the resulting models accurately represent the historical climate record.

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