SCIENCE

Twice the heat?

The study also found that carbon dioxide may have at least twice the effect on global temperatures than currently projected by computer models of global climate.

The world’s leading supercomputer models generally project that a doubling of carbon dioxide in the atmosphere would have a heating impact in the range of 0.5 to 1.0 degree C watts per square meter. (The unit is a measure of the sensitivity of Earth’s climate to changes in greenhouse gases.) However, the published data show that the comparable impact of carbon dioxide 35 million years ago amounted to about 2 degrees C watts per square meter.

SuperComputer models successfully capture the short-term effects of increasing carbon dioxide in the atmosphere. But the record from Earth's geologic past also encompasses longer-term effects, which accounts for the discrepancy in findings. The eventual melting of ice sheets, for example, leads to additional heating because exposed dark surfaces of land or water absorb more heat than ice sheets.

“This analysis shows that on longer time scales our planet may be much more sensitive to greenhouse gases than we thought,” Kiehl says.

Climate scientists are currently adding more sophisticated depictions of ice sheets and other factors to supercomputer models. As these improvements come on line, Kiehl believes that the supercomputer models and the paleoclimate record will be in closer agreement, showing that the impacts of carbon dioxide on climate over time will likely be far more substantial than recent research has indicated.

Because carbon dioxide is being pumped into the atmosphere at a rate that has never been experienced, Kiehl could not estimate how long it would take for the planet to fully heat up. However, a rapid warm-up would make it especially difficult for societies and ecosystems to adapt, he says.

If emissions continue on their current trajectory, “the human species and global ecosystems will be placed in a climate state never before experienced in human history,” the paper states.