Wednesday, September 12, 2007

Palaeozoic CO2 and Temperature: coupled again?

Though there are many other influences on climate such as contintental
configuration that can change over the period of hundreds of millions
of years, GHGs are still considered an primary driver of average
surface temperature. Which is why the publication by Veizer et al in
2000 (ref 4 in abstract below) of a sea surface reconstruction of
temperatures from the Palaeozoic era had led to some confusion. What
they had found was that during a period of high CO2 the sea surface
temperature had not been greatly affected. Or, there had been a
decoupling of CO2 and temperature. In today's Nature a new
reconstruction has been presented that uses a different proxy method
and finds sea surface was indeed much warmer during high CO2 periods
than during low CO2 periods. This then throws into question CO2
decoupling during the Palaeozoic era.

Nature 449, 198-201 (13 September 2007) | doi:10.1038/nature06085;
Received 15 April 2007; Accepted 3 July 2007

Coupling of surface temperatures and atmospheric CO2 concentrations
during the Palaeozoic era

Rosemarie E. Came1, John M. Eiler1, Ján Veizer2, Karem Azmy3, Uwe
Brand4 & Christopher R. Weidman5

Atmospheric carbon dioxide concentrations seem to have been several
times modern levels during much of the Palaeozoic era (543–248 million
years ago), but decreased during the Carboniferous period to
concentrations similar to that of today1, 2, 3. Given that carbon
dioxide is a greenhouse gas, it has been proposed that surface
temperatures were significantly higher during the earlier portions of
the Palaeozoic era1. A reconstruction of tropical sea surface
temperatures based on the delta18O of carbonate fossils indicates,
however, that the magnitude of temperature variability throughout this
period was small4, suggesting that global climate may be independent
of variations in atmospheric carbon dioxide concentration. Here we
present estimates of sea surface temperatures that were obtained from
fossil brachiopod and mollusc shells using the 'carbonate clumped
isotope' method5—an approach that, unlike the delta18O method, does
not require independent estimates of the isotopic composition of the
Palaeozoic ocean. Our results indicate that tropical sea surface
temperatures were significantly higher than today during the Early
Silurian period (443–423 Myr ago), when carbon dioxide concentrations
are thought to have been relatively high, and were broadly similar to
today during the Late Carboniferous period (314–300 Myr ago), when
carbon dioxide concentrations are thought to have been similar to the
present-day value. Our results are consistent with the proposal that
increased atmospheric carbon dioxide concentrations drive or amplify
increased global temperatures1, 6.

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