Decoupling of atmospheric CO₂ and global temperatures.

In a previous post you may recall that I looked a paper by Retallack (2002) titled “Carbon Dioxide and Climate over the Past 300 Myr”, in it the idea of decoupling of atmospheric CO₂ and global temperatures was touched upon.  As I plan to move on to look at present situations shortly, I felt it may be useful to have a look at the evidence for decoupling.  As a result I came across a paper by Veizer et al (2000) published in Nature titled “Evidence fordecoupling of atmospheric CO2 and global climate during the Phanerozoic eon”. 

The paper focuses on the Phanerozoic eon and Veizer et al (2000) present a reconstruction of tropical sea surface temperatures.  I highly recommend following the link on the paper title and reading it in detail for yourself as I will only be covering the basic details.  It was found that “large oscillations of tropical sea surface temperatures in phase with the cold ± warm cycles, thus favouring the idea of climate variability as a global phenomenon”  (Veizer et al, 2000), however when this data was compared with a mass balance model reconstructing temperature which was produced using atmospheric CO₂ there was a conflict in findings.  In order for the results to be reconciled it would be necessary to assume that CO₂ was “not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable” (Veizer et al, 2000). 

As we have seen previously, it is generally accepted that increases in CO₂ result in increased global temperatures.  So surely it should also follow that lower CO₂ levels in the atmosphere would see lower global temperatures.  The Permo/Carboniferour and Cenozoic icehouse episode follow this idea and show low partial pressures of CO₂ in the atmosphere.  However biochemical models for the late Ordovician/earliest Silurian and late Jurassic/early Cretaceous show high values of CO₂ despite them being classed as icehouse episodes.  Veizer et al (2000) explain that theoretical models resolve this by “advocating the development of permanent high-latitude ice sheets at more than 10 times present-day CO₂ levels have been proposed” (Veizer et al, 2000) and then go on to offer their own alternative interpretation by mean of experimental evidence to “suggest large variations in tropical sea surface temperatures (SSTs; up to 9°C) between the peaks of greenhouse/icehouse modes” (Veizer et al, 2000). 

There are three possible finding suggested as a result of Veizer et al (2000) study, the first is that past CO₂ reconstructions are at least in part incorrect, the second is that for a least two of the icehouse episodes of the Phanerozoic pCO₂ was not the driving force of climatic changes and finally, that climate models are not able to correctly represent past climates as they are calibrated to the present interstadial . 



The Phanerozoic eon. 

Roy Shepherd (2002). Phanerozoic eon [online]. Available from: <http://fossilsandfossils.weebly.com/fossil-periods.html>. [Accessed 08th January 2016].

 

Reference;          

Veizer, J. Godderis, Y. and François, L.M. (2002). Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon. Nature. 408, pp.698-701.