If carbon dioxide emissions continue at their current rate through to the end of this century, atmospheric concentrations of the greenhouse gas will reach levels that existed about 30 million to 100 million years ago, according to Jeffrey Kiehl from the US National Centre for Atmospheric Research (NCAR). 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.
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http://environmentalresearchweb.org/cws/article/news/44990