The nature of past Antarctic temperature lead over carbon dioxide is clarified
The analysis of past changes of Antarctic temperature and the concentration of greenhouse gases alone cannot reveal causal relationships in the climate system. A recent modeling study shows that several climate processes need to be taken into account to discern causes and consequences. In the journal “Quaternary Science Reviews” researchers provide an explanation for observed Antarctic temperature lead over carbon dioxide concentration for several recent glacial-interglacial transitions.
“Antarctic ice core data tell us that during glacial-interglacial transitions Antarctic temperature rose some thousand years before the concentration of carbon dioxide did,” says lead author Andrey Ganopolski of the Potsdam Institute for Climate Impact Research. This fact apparently contradicts the concept of carbon dioxide (CO2) as the primary driver of climate change and has been used to challenge the leading role of CO2 in past and future climate changes. “But our results demonstrate clearly that the Antarctic temperature lead over CO2 observed during past glacial cycles does not challenge the major role of greenhouse gases in driving climate change,” says Ganopolski.
Using a climate model, Ganopolski and Didier Roche of the French Laboratoire des Sciences du Climat et de l’Environnement confirmed that CO2 concentration change during glacial cycles is the most important factor for Antarctic temperature variations. In their simulations CO2 forcing alone explains more than half of the observed glacial-interglacial temperature variations in Antarctica. However, if only CO2 concentration is taken into consideration, then CO2 and temperature change approximately simultaneously. To explain the temperature lead, one has to take into account the fact that, although important, CO2 is not the only factor affecting temperature during glacial-interglacial transitions.
An additional factor is the so-called orbital forcing. Regular variations in Earth’s orbit around the sun, known as the Milankovitch cycles, are believed to determine the beginnings and ends of the glacial periods. The study shows that variations in the Earth’s orbital parameters have direct effect on Antarctic climate and contribute to the observed Antarctic temperature lead.
Additionally, there is an indirect effect: During the terminations of the glacial cycles due to orbital forcing, the vast ice sheets covering Northern America and Eurasia melted rapidly causing a large fresh water flux into the oceans sufficient to disrupt the Atlantic meridional overturning circulation (AMOC) over many thousand years. The disruption of AMOC, in turn, dramatically reduced the oceanic heat transport from the Southern to the Northern Atlantic and led to a delay in the Northern Hemisphere warming and, at the same time, a more abrupt and strong warming in the Southern Hemisphere. The latter is the primary mechanism explaining Antarctic temperature lead over CO2.
“The Southern Hemisphere temperature lead arises from the response to CO2, oceanic heat transport and changes of the Earth’s orbital parameters,” says Ganopolski. All these climate forcings have very different temporal dynamics and therefore it is not surprising that their combined effect can result in the lead of temperature changes over CO2 over certain periods of time. “The ultimate understanding of past climate changes will only be possible when using paleoclimate records in conjunction with comprehensive Earth system models,” the authors state.
Article: Ganopolski, A., Roche, D.M., On the nature of lead–lag relationships during glacial–interglacial climate transitions, Quaternary Science Reviews (2009), doi:10.1016/j.quascirev.2009.09.019
PIK Press Release (05/22/2006): Feedback between temperature and CO2 may enhance global warming beyond previous estimates
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