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Thursday, 7 December, 2000, 19:00 GMT
Fossils nag at carbon's climate role
Drought, Namibia
The research will fuel climate arguments
By environment correspondent Alex Kirby

Researchers who have examined the fossil record say it questions the role of carbon dioxide (CO2) as the main force driving climate variability.

They first reconstructed temperatures throughout the period of significant life on Earth from the fossilised shells of marine organisms.

They then attempted a second climate reconstruction for the same time period, using a computer model.

And they found several significant mismatches between the two.

The researchers, Jan Veizer from the Ruhr University in Germany and the Ottawa-Carleton Geoscience Centre, Canada, and two colleagues from Liege University in Belgium, report their work in the journal Nature.

Their first reconstruction of tropical sea surface temperatures during the Phanerozoic eon, dating from 544 million years ago, used a database of oxygen isotopes in calcite and aragonite shells.

Conflicting data

"The data indicate large oscillations of tropical sea surface temperatures in phase with the cold-warm cycles [between glacial and interglacial periods], thus favouring the idea of climate variability as a global phenomenon," they write.

"But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric CO2 concentrations. The results can be reconciled if atmospheric CO2 concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed CO2 concentrations are not reliable."

It is during two periods, the glaciation of the Late Ordovician period around 440 million years ago, and the cool climate of the Jurassic and early Cretaceous (about 220-120 million years ago), that expectations based on the CO2 record contradict the fossil data.

According to the second reconstruction, tropical temperatures should have been higher than today during these two cold periods.

The researchers say their findings have several possible implications. It could be that the reconstructed past CO2 levels are partly incorrect. There is also the chance that climate models are calibrated to the present and are therefore unable to reproduce correctly past climate modes.

But they say there is a third possibility: that "the role of CO2 as the main driving force of past global (long-term) climate changes is questionable, at least during two of the four main cool climate modes of the Phanerozoic".

'Climate driver'

In an article in the same issue of Nature, Lee Kump, of the Earth Systems Science Center at Pennsylvania State University, US, discusses the researchers' findings.

"Clearly", he says, "factors other than atmospheric CO2 can influence climate on geological timescales."

He says their conclusion that CO2 "may not be the climate driver it has been made out to be deserves close scrutiny, because the policy implications are huge".

"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."

He considers the Jurassic mismatch "more persistent and problematic" than that of the Late Ordovician, though he says that neither of "these seeming paradoxes" is new.

But he concludes that it is perhaps not the role of CO2 in driving climate change that we need to question, but rather the value of "proxy" [indirect] measurements of ancient CO2 levels, based in this instance on the carbon isotopic composition of organic matter in marine sediments.

"When we put everything we know into models of the carbon cycle", Lee Kump writes, "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."

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17 Aug 00 | Sci/Tech
Carbon at 20 million year high
14 Nov 00 | Climate change
Questioning global warming
10 Nov 00 | Sci/Tech
Climate change glossary
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