If current emission rates remain unchanged, the amount of CO2 in the atmosphere will rise to around 900–1,000 parts per million by the end of the century. Using recently published research, Jeffrey Kiehl of the National Center for Atmospheric Research calculated that the last time that CO2 levels were this high was 35 million years ago. He then calculated that at that time, the Earth’s average temperature was about 16°C higher than the pre-industrial average.

Kiehl’s analysis also showed that in the distant past, CO2 may have had at least twice as great an effect on global temperatures as currently projected by climate models. Most leading models suggest that a doubling of CO2 in the atmosphere would have a heating impact of 0.5–1°C watts per square metre. But Kiehl’s results suggest that 35 million years ago, the impact was 2°C watts per square metre. ‘This analysis shows that on longer time scales, our planet may be much more sensitive to greenhouse gases than we thought,’ he said. ‘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.’

March 2011