The results of the study show that the models currently used to forecast climate change often neglect components of the Earth's climate system that vary over long timescales and have an important effect on temperature sensitivity--such as land-ice and vegetation.
The researchers compared results from a global climate model to temperature reconstructions of the Earth's environment three million years ago when global temperatures and carbon dioxide concentrations were relatively high. The temperature reconstructions were derived using data from three million-year-old sediments on the ocean floor.
"We found that, given the concentrations of carbon dioxide prevailing three million years ago, the model originally predicted a significantly smaller temperature increase than that indicated by the reconstructions. This led us to review what was missing from the model," said Daniel Lunt, one of the study's co-athors.
The authors argue that the increased temperatures indicated by the reconstructions can be explained if factors that vary over long timescales, such as land-ice and vegetation, are included in the model. This is primarily because changes in vegetation and ice lead to more sunlight being absorbed, which in turn increases warming.
Including these long-term processes in the model resulted in an increased temperature response of the Earth to carbon dioxide, indicating that the Earth's temperature is more sensitive to carbon dioxide than previously recognized. Climate models used by bodies such as the IPCC often do not fully include these long-term processes, thus these models do not entirely represent the sensitivity of the Earth's temperature to carbon dioxide.
Alan Haywood, a co-author on the study from the University of Leeds, said "If we want to avoid dangerous climate change, this high sensitivity of the Earth to carbon dioxide should be taken into account when defining targets for the long-term stabilisation of atmospheric greenhouse-gas concentrations".
Lunt added: "This study has shown that studying past climates can provide important insights into how the Earth might change in the future."
Citation: Daniel J. Lunt1, Alan M. Haywood, Gavin A. Schmidt, Ulrich Salzmann, Paul J. Valdes, Harry J. Dowsett, 'Earth system sensitivity inferred from Pliocene modelling and data', Nature Geoscience, 2009, doi: 10.1038/ngeo706