In a new study in Nature Geoscience, Daniel Lunt, et. al. provide disconcerting evidence, based on paleoclimatic data, that the IPCC may be substantially underestimating the response of the Earth system to elevated atmospheric carbon dioxide concentrations. The researchers contrast the Intergovernmental Panel on Climate Change’s emphasis on so-called fast short-term feedback mechanisms )the so-called Charney sensitivity), e.g. water, vapor, snow albedo in its effort to ascertain climate sensitivity (usually defined as the increase in global mean temperature owing to a doubling of carbon dioxide) with what they term “Earth system sensitivity,” which they defined “the long-term equilibrium surface temperature change given an increase in CO2, including all Earth system feedbacks, but neglecting processes associated with the carbon cycle itself, such as marine productivity or weathering.”
Lunt, et al., conclude that Earth sensitivity may be different, and significantly greater than Charney sensitivity. To test this hypothesis, the researchers looked at the paleo proxy record during the mid-Pliocene warm period, about 3.3-3 million years ago, a pertinent period to look at since atmospheric carbon dioxide concentrations and temperatures were higher than pre-industrial times, climatic fluctuations were reduced compared to the Quaternary, and there are data sets. The researchers used a couple atmosphere-ocean general circulation model to simulate the climate of the mid-Pliocene warm period. In comparing the simulation with proxy records of mid-Pliocene sea surface temperature, the study concludes that the response of the Earth system to elevated carbon dioxide concentrations is 30-50% greater than the response based on fast-adjusting components of the climate system.
The researchers conclude that the IPCC should focus on Earth system sensitivity rather than the traditional Charney sensitivity in estimating the impacts of long-term greenhouse-gas stabilization scenarios. Should additional research confirm the results of this study, the clear implication is that stabilizing atmospheric concentrations in the range of 400-450ppm CO2e will result in a substantial overshoot of the 2C limit that the Parties to the UNFCCC focused upon in Copenhagen.
Citation: Daniel J. Lunt, et al., Earth System Sensitivity Inferred from Pliocene Modelling and Data, 3 Nature Geoscience 60-64 (2009) (subscription required)