In a Perspective article in Science, Kiehl describes how he examined the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago. Global temperatures then averaged about 16 °C above pre-industrial levels.
The article pulls together several recent studies that look at various aspects of the climate system, while adding a mathematical approach by Kiehl to estimate average global temperatures in the distant past.
The study found that carbon dioxide may have two times or more the effect on global temperatures than currently projected by computer models of global climate. The world's leading computer models generally project that a doubling of carbon dioxide in the atmosphere would have a climate feedback factor (ratio of change in surface temperature to radiative forcing) in the range of 0.5 to 1.0 °C per watts per square metre.
However, the published data show that the comparable climate feedback factor of carbon dioxide 35 million years ago amounted to about 2 °C per watt per square metre.
"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 computer models. As these improvements come online, Kiehl believes that computer models and the paleoclimate record will be in closer agreement, showing that the impacts of carbon dioxide on climate over time are likely to be far more substantial than recent research has indicated.
"If we don't start seriously working toward a reduction of carbon emissions, we are putting our planet on a trajectory that the human species has never experienced," says Kiehl. "We will have committed human civilization to living in a different world for multiple generations."
Kiehl focused on a fundamental question: when was the last time Earth's atmosphere contained as much carbon dioxide as it may by the end of this century? If society continues its current pace of increasing the burning of fossil fuels, atmospheric levels of carbon dioxide are expected to reach about 900-1,000 parts per million (ppm) by the end of this century. That compares with current levels of about 390 ppm, and pre-industrial levels of about 280 ppm.
Kiehl drew on recently published research that, by analysing molecular structures in fossilized organic materials, showed that carbon dioxide levels likely reached 900-1,000 ppm about 35 million years ago.
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. He estimates that global temperatures may take centuries or millennia to fully adjust in response to the higher carbon dioxide levels.