"Understanding this relationship on longer timescales is important for two reasons," Dana Royer of Wesleyan University told environmentalresearchweb. "They add more 'weight' to the estimates based on shorter timescales, and over the past 15,000 years, carbon dioxide and temperature have been similar to or lower than the present-day. Thus, data from this interval may not represent the best analogue for predicting future globally-warm climates."

Royer says that deep-time records are important because for large intervals over the past 420 million years, carbon dioxide and temperature have been higher than today. And, according to the scientists, looking at climate sensitivity values for the recent past tends to underestimate the size of large climate change events.

To get their results, Royer and colleagues tuned GEOCARBSULF, a geochemical model for calculating carbon dioxide levels over long timescales, to a compilation of carbon dioxide proxy records. The proxy records included 490 data points from a number of sources such as carbon isotopes in ancient ocean plankton material.

"We were able to accomplish this tuning because one of the factors in the geochemical model is the relationship between carbon dioxide and temperature," said Royer. "We found that the deep-time geological records exclude the possibility of weak climate sensitivities: we conclude that the amount of warming for every doubling of carbon dioxide must be at least 1.5 °C."

The modelling revealed that climate sensitivity has probably been between 1.5 and 6.2 °C over the past 420 million years. Royer says that the study's best fit figure, at 2.8 °C, also ties in well with the climate sensitivity suggested as most likely by the recent report from the Intergovernmental Panel on Climate Change (IPCC).

"Moreover, analyses by other authors of short-term intervals in the geologic past also exclude climate sensitivities lower than 1.5 °C," he said. "This all adds up to a consistent story. The climate sensitivity on Earth has probably been greater than 1.5°C, regardless of temporal scaling. This result places constraints on predictions of future global warming."

Now the team plans to try and quantify climate sensitivity for small intervals of time, such as every 10 or 20 million years over the last 420 million years. "A more temporally-resolved study such as this could address many questions, for example whether climate sensitivity varies between when the Earth is glaciated and when it is not," said Royer.

The researchers reported their work in Nature.