Climate News Climate CO2 Sensitivity Overestimated
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September 04 2010
It is well known that carbon dioxide cannot directly account for the observed increase in global temperature over the past century. This has led climate scientists to theorize that many feedback relationships exists within the climate system, serving to amplify the impact of rising CO2 levels. One of these is the impact of rising temperature on the ability of the ecosystem to absorb CO2. The temperature sensitivity of ecosystem respiratory processes (referred to as Q10) is a key determinant of the interaction between climate and the carbon cycle.
New research, recently published in the journal Science, shows that the Q10 of ecosystem respiration is invariant with respect to mean annual temperature, and independent of the analyzed ecosystem type. This newly discovered temperature insensitivity suggests that climate sensitivity to CO2 is much smaller than assumed by climate models.
Climate sensitivity is generally given as how much temperature rise would result from a doubling of atmospheric CO2 levels. Using IPCC figures for radiative forcing, a doubling of CO2 would lead to a temperature rise of about half a degree (see “Another Look at Climate Sensitivity”). Yet the UN IPCC Fourth Assessment Report (AR4) gives a much higher value for climate sensitivity. It claims a 2°C to 4.5°C rise for a CO2 doubling, or from four to nine times higher than what is see in the real climate system. Why? Climate models assume that there are large positive feedbacks as Earth warms. Among these feedbacks is the impact of rising temperature on emission and absorption of CO2 by Earth's biota.
Accurately predicting future levels of atmospheric CO2 requires a clear understanding of how land and atmosphere exchange CO2. Each year, photosynthesizing land plants remove (fix) one in eight molecules of atmospheric CO2. Land plants and soil organisms return a similar amount of the dreaded greenhouse gas. The balance between removal and respiration determines whether terrestrial ecosystems are a net carbon sink or source. Two papers in the August 13, 2010, issue of Science bring a new understanding of land-atmosphere CO2 exchange.
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