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CO2 emissions off the hook?



A new study suggests that carbon dioxide has not always been the main driver of climate change. But how accurate are the data, and how relevant are the findings to the current climate change debate?

An article by German geoscientist Jan Veizer and Canadian colleagues published in this week's Nature has found that there were periods during the Phanerozoic era (up to 570 million years ago) when atmospheric CO2 levels did not accord with climate change, suggesting CO2 may not be the problem it has been made out to be.

"Such a conclusion deserves close scrutiny, because the policy implications are huge," writes Lee R. Kump in an accompanying commentary.

"The geological record is our best hope of establishing a correspondence between atmospheric levels of CO2 and climate and understanding the likely consequences of fossil-fuel burning. If large changes in atmospheric CO2 in the past have not produced the climate response we thought they had, that undermines the case for reducing fossil-fuel emissions."

However, Kump isn't so convinced, pointing out that it may not be appropriate to draw such a conclusion based on the methods used - something the authors themselves admit.

Going back in time helps climate change scientists to test the models used to predict future changes, however according to Dr David Etheridge of CSIRO's Division of Atmospheric Research, going back too far in time can be tricky.

Humans have only been directly measuring temperature for 150 years and CO2 since about 1960. Ice cores can give quite direct information dating back to 0.5 million years ago, however the further back in time scientists go, the more they have to rely on indirect measurements. The use of such "proxies" is at the heart of the debate raised by this new research.

While Kump describes the new research as presenting "an exquisite temperature record" based on the oxygen isotopic composition of tropical marine fossils, he is not so sure about their use of carbon isotopes proxies used for measuring CO2 levels. Neither is Etheridge.

"500 million years ago Earth was a very different planet. Among other things, there were different continents, mountains and oceans which all impacted on climate change. The Sun may also have been much weaker," Etheridge told ABC Science Online.

"It's important to remember that fossilised organic material doesn't just sit around without interacting with its environment, so the record from such proxies can be incomplete and there needs to be a lot of extrapolation and interpretation".

"There were only a couple of exceptions in the record where Veizer and team found CO2 did not accord with climate change, and these could have been at times when the proxies being used for CO2 were inadequate," says Etheridge.

"Clearly, factors other than atmospheric CO2, can influence climate on geological timescales," concludes Kump. "But when we put everything we know into models of the carbon cycle, though, we predict changes in atmospheric CO2 that largely parallel inferred climate shifts."

"So the lack of close correspondence between climate change and proxy indicators of atmospheric CO2, may force us to re-evaluate the proxies rather than disavow the notion that substantially increased atmospheric CO2 will indeed lead to marked warming in the future."

"There is an argument from the field of geology that claims looking back millions of years is essential for understanding climate change," says Etheridge.

"But for the purposes of informing policy, it's more important to look at the more recent history where we have much better data," he says.

Etheridge believes the real strength of this study is in its contribution to our understanding of what ancient climates were like - a matter of basic scientific curiosity, rather than policy.