The kinds of increases in atmospheric carbon dioxide taking place today could have a significantly larger effect on global temperatures than previously thought, according to a new study led by Yale University geologists. Their findings appear 20 December in the advanced online edition of Nature Geoscience.
The team demonstrated that only a relatively small rise in atmospheric carbon dioxide (CO2) was associated with a period of substantial warming in the mid- and early-Pliocene era, between three to five million years ago, when temperatures were approximately 3 to 4 degrees Celsius warmer than they are today.
Climate sensitivity - the mean global temperature response to a doubling of the concentration of atmospheric CO2 - is estimated to be 1.5 to 4.5 degrees Celsius, using current models.
'These models take into account only relatively fast feedbacks, such as changes in atmospheric water vapour and the distribution of sea ice, clouds and aerosols,' said Mark Pagani, associate professor of geology and geophysics at Yale and lead author of the paper. 'We wanted to look at Earth-system climate sensitivity, which includes the effects of long-term feedbacks such as change in continental ice-sheets, terrestrial ecosystems and greenhouse gases other than CO2.'
To do this, the team focused on the most recent episode of sustained global warmth with geography similar to today's. Their reconstructed CO2 concentrations for the past five million years was used to estimate Earth-system climate sensitivity for a fully equilibrated state of the planet, and found that a relatively small rise in CO2 levels was associated with substantial global warming 4.5 million years ago. They also found that the global temperature was 2 to 3 degrees Celsius higher than today while CO2 levels were only between about 365 and 415 parts per million (ppm) - similar to today's concentration of about 386 ppm.
'This work and other ancient climate reconstructions reveal that Earth's climate is more sensitive to atmospheric carbon dioxide than is discussed in policy circles,' Pagani said. 'Since there is no indication that the future will behave differently than the past, we should expect a couple of degrees of continued warming even if we held CO2 concentrations at the current level.'