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.



http://www.nature.com/nature/journal/v449/n7159/abs/nature06085.html

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.

Milankovitch is spot-on

This looks like a very interesting article. From reading the abstract the author makes a logical argument that, with respect to Milankovitch forcing (The change in incoming solar radiation due to changes in how the Earth is situated relative to the sun), one should consider ice volume dynamics (changes in ice volume) rather than ice volume total. In doing so the Milankovitch forcing is revealed without lag (there is often thousands of years of lag between forcing change and equivalent ice volume change). The author also finds that variations in CO2 appear to lag the rate of change in ice volume which gives CO2 a secondary role in ice volume change.



This finding helps make sense of the role that orbital changes play in ice volume. Up to this point there has been some confusion since ice volume has not always followed orbital parameters with any consistency. With this finding we see that orbital parameters play a role in how quickly the ice volume changes.



That CO2 has a secondary role to orbital parameters has been known to climate scientists for some time. The role of CO2 has been considered one of a positive feedback to orbital parameters with increased warming form changes in solar energy leading to more CO2 which leads to more warming.



Further research:

The impact of this research on how understanding of climate sensitivity.

GEOPHYSICAL RESEARCH LETTERS,VOL. 33, L24703, doi:10.1029/2006GL027817,2006

In defense of Milankovitch

Gerard Roe
Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA

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