Earth’s equilibrium climate sensitivity—a measure of how much the global average surface temperature will rise in response to a doubling of greenhouse gases in the atmosphere—is a key metric that is widely used in economic and policy assessments of global warming. In a 2013 report, the Intergovernmental Panel on Climate Change estimated that climate sensitivity is likely between 1.5°C and 4.5°C.

In numerous studies, scientists have attempted to refine estimates of this metric by characterizing climate sensitivity from the geological record during Earth’s recent past. Such research, however, has been limited by the fact that during the past 5 million years, carbon dioxide concentrations have not exceeded 560 parts per million by volume (ppmv), a level that is lower than what is projected in many scenarios representing the end of the 21st century.

To develop more relevant climate sensitivity estimates, researchers have begun investigating intervals of time more than 5 million years ago when Earth’s atmospheric carbon dioxide concentrations exceeded 1,000 ppmv. But because factors other than carbon dioxide—such as the brightness of the Sun and the configuration of the continents—can affect climate over such long timescales, it is unclear how applicable the results of these recent investigations are for forecasting Earth’s future.

In a new study, Farnsworth et al. try to more fully explore the planet’s climate sensitivity over geologic timescales by utilizing an ensemble of 19 climate model simulations to examine Earth between about 150 million and 35 million years ago. In the coupled ocean-atmosphere-vegetation simulations, the researchers varied Earth’s paleogeography and the Sun’s brightness as appropriate for each geologic interval from the Early Cretaceous to the late Eocene and incorporated atmospheric carbon dioxide concentrations set at double (560 ppmv) and quadruple (1,120 ppmv) the preindustrial level.

The results suggested that climate sensitivity during this time frame ranged from approximately 3.5°C to 5.5°C. The authors attribute the observed variation to a combination of factors, including the gradually increasing brightness of the Sun as well as changes in the arrangement of the continents, which in turn influenced ocean circulation and, because of differences in ocean surface area, the planet’s albedo.

The findings indicate that over long timescales, climate sensitivity is strongly correlated to Earth’s continental configuration, the Sun’s strength, and the background carbon dioxide concentration. This research suggests that within the context of the past 150 million years, the modern estimate of climate sensitivity is relatively low and that, ultimately, Earth’s past climate sensitivity may not provide a perfect analogue for potential future conditions. (Geophysical Research Letters, https://doi.org/10.1029/2019GL083574, 2019)