Geologists from Yale University reconstructed CO2 concentrations for the past five million years to estimate Earth-system climate sensitivity.
They found that about 4.5 million years ago 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 387 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," said Mark Pagani, Associate Professor of Geology and Geophysics at Yale and lead author of the paper. "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."
Climate sensitivity is the mean global temperature response to a doubling of the concentration of atmospheric CO2. It 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 vapor and the distribution of sea ice, clouds and aerosols," said Mark Pagani, "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."
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.
This research was released just after the Copenhagen COP15 climate negotiations ended, which failed to produce substantial binding emission cuts, but did produce an aspirational "Copenhagen Accord" which countries may sign on to which aims to keep global temperature rise below 2 degrees.
"For the first time, all the major greenhouse gas emitters have agreed to be part of a global accord to tackle the problem. But it still needs to be turned into a treaty with legal force. More importantly, the developed nations - including Australia - have to put forward serious plans for the scale of emissions cuts needed, toward 40 per cent by 2020. As US President Obama said last night, the targets being put forward today are not sufficient and the science demands more aggressive action." said Ian Lowe, Emeritus professor of science, technology and society at Griffith University, in Australia and President of the Australian Conservation Foundation commenting on the outcome of COP15.
A draft UN document indicated a global temperature rise of 3 degrees is in store with current national commitments on carbon reduction. But some climate models estimate emission cuts already promised by countries would lead to a global temperature rise of 3.9 degrees by the end of the century.
This new research into climate sensitivity would seem to indicate the temperature rise in store arising from atmospheric CO2 may be a couple of degrees higher than what present models would indicate.
The study used samples provided by the Integrated Ocean Drilling Program and was funded by the National Science Foundation and the Yale Climate and Energy Institute. The findings appear December 20 in the advanced online edition of Nature Geoscience.
Anyone for a climate emergency?
- Yale University Media Release December 20, 2009 - Global Temperatures Could Rise More Than Expected, New Study Shows
- Australian Science Media Centre - Quote from Professor Ian Lowe