Carbon at 20 million year high

By environment correspondent Alex Kirby

Two British scientists say the level of carbon dioxide (CO2) in the Earth's atmosphere is higher than for 20 million years.

But their study of levels over the last 60 million years suggests that the gas was once even more abundant than it is today.

Carbon dioxide is the main gas caused by human activity that has been linked to global warming. Concentrations now are about 360 ppm (parts per million), but will continue to rise as emissions increase.

Based on current trends, by 2030 the total climatic impact of rising levels of all greenhouse gases will be equal to that caused by the doubling of pre-industrial CO2 concentrations. By 2100, the effect would be trebled.

The research, by Dr Paul Pearson of the University of Bristol and Professor Martin Palmer, of Imperial College, London, is reported in the journal Nature.

Shelling out

They used a new technique to establish CO2 levels in the ancient atmosphere, analysing the shells of planktonic organisms that once lived near the surface of the ocean.

This enabled them to establish past seawater acidity, which in turn was dictated by the amount of atmospheric CO2.

The researchers estimate that between about 60 and 52 million years ago, CO2 concentrations reached more than 2,000 ppm.

But from about 55 to 40 million years ago, there was "an erratic decline", which may have been caused by a reduction in CO2 emissions from ocean ridges and volcanoes, and by increased carbon burial.

Since about 24 million years ago, concentrations appear to have remained below 500 ppm and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 million years ago.

"Our observations put the modern greenhouse effect into a long-term perspective," said Dr Pearson.

Back to the future

Commenting on the prospect suggested by climate models, that the 2100 CO2 level could be as high as that last seen in the Eocene period, 50 million years ago, he said: "This does not necessarily mean we will recreate Eocene-type conditions.

"There are still too many unknowns involved in climate prediction. But the sweltering ice-free world of the Eocene does warn us of what might happen if a runaway greenhouse effects sets in."

Some researchers still doubt that human activities are inducing rapid climate change. They highlight the inconsistencies between the temperature records taken at the Earth's surface, which show rapid warming over the last century, and the data produced by satellite and balloon studies.

These show little if any warming, in the last two decades, of the low to mid-troposphere - the atmospheric layer extending up to about 8 km from the Earth's surface.

Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming recorded at ground level.