001207-10 10.1038/nsu001207-10 7 December 2000 climate Force of change not necessarily CO2 HeikeLangenberg A glimpse at the deep past hints that carbon dioxide might not always have caused global warming, reports Heike Langenberg.

Back and forth: how big was the role of CO2 in the ice ages?
There is little doubt that increasing concentrations of carbon dioxide, and other greenhouse gasses, are to blame for the warming of the Earth over the past century. But new research suggests that today's main culprit may not always have driven the swings between greenhouse and ice-age climates on our planet since life colonized it 540 million years ago. This raises questions about the need to reduce CO2 emissions.

To reconstruct past temperatures, Jan Veizer of the Ruhr Universität Bochum and his colleagues collected a huge amount of marine fossils. When these organisms were alive, the composition of sea water -- and hence that of the atmosphere -- left its traces in their shells and skeletons.

Dug out and analysed today, the fossils give up their ancient secrets. Using this data pool, Veizer and colleagues have reconstructed the climate throughout the period of significant life on Earth.

They generated a second climate reconstruction for the same time period, by feeding a record of carbon dioxide concentrations into a computer model that translates changes in the concentrations of CO2 into temperature changes.

Over about two thirds of the past 540 million years, the two records match reasonably well.

But during two of the ice ages, around 440 and around 150 million years ago, the expectations based on the CO2 record contradict the fossil data. Tropical temperatures should have been higher than today in these two icy periods, according to the second reconstruction.

The researchers suggest two possible explanations for this dilemma. First, the carbon dioxide reconstruction may not be reliable. This is a strong possibility, given the immense difficulties in interpreting the main carbon indicators -- soil and leaf remnants that are several hundred million years old.

But the second explanation -- that carbon dioxide concentrations may not always have been the driving force behind climatic change -- begs the urgent question: is it still necessary to reduce CO2 emissions in order to prevent global warming?

"Clearly there are factors other than atmospheric CO2 concentrations that can force the climate system," says Lee Kump, of the Earth Systems Science Center at Pennsylvania State University.

Currently, our best explanation for the climatic changes in the absence of greenhouse forcing is changes in the distribution of continents and the positions of mountain belts. Such a different configuration of land and ocean influences the cycle of water on Earth. And changes in cloudiness or in the extent of the ice caps at the poles are powerful agents of climate change.

But over the short time span of the 20th century, the drift of continents is clearly far too small to have played any such role. So the results from Veizer's team do not absolve us from trying to reduce greenhouse gas emissions.

Veizer, J., Godderis, Y. & Francois, L M. Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon. Nature 408, 698701 2000.