Productivity of Land Plants May Be Greater Than Previously Thought
ScienceDaily (Sep. 29, 2011) — The global uptake of carbon by land plants may be up to 45 per cent more than previously thought. This is the conclusion of an international team of scientists, based on the variability of heavy oxygen atoms in the carbon dioxide of the atmosphere driven by the El Niño effect. As the oxygen atoms in carbon dioxide were converted faster than expected during the El Niño years, current estimates for the uptake of carbon by plants are probably too low. These should be corrected upwards, say the researchers in the current issue of the journal Nature.
Instead of 120 petagrams of carbon, the annual global vegetation uptake probably lies between 150 and 175 petagrams of carbon. This value is a kind of gross national product for land plants and indicates how productive the biosphere of Earth is. The reworking of this so-called global primary productivity would have significant consequences for the coupled carbon cycle-climate model used in climate research to predict future climate change.
Lisa Welp of the Scripps Institution of Oceanography at the University of California in San Diego and her colleagues evaluated the data for the global isotopic composition of the greenhouse gas CO2 over the last 30 years. This analysis indicated regular fluctuations between years and a connection with the El Niño phenomenon in the Pacific. Overall, El Niño years are warmer. They are also characterised by greater precipitation in South America and less intensive monsoons in Southeast Asia. The researchers found a more rapid recovery of the isotopic ratios following the El Niño events than assumed, indicating a shorter conversion time for CO2 in the terrestrial biosphere. On the basis of these data, the authors calculate the so-called global primary productivity (GPP). They now propose correcting this in the global models from 120 to 150-175 petagrams) of carbon annually.
Since 1977 the isotopic ratios in the carbon dioxide of the atmosphere (18O/16O and 13C/12C) have been measured in order to better understand the global carbon cycle, as the exchange processes between the biosphere, the atmosphere and the oceans are reflected in these values. "We assume that the redistribution of moisture and rain in the tropics during El Niño raises the 18O/16O ratio in precipitation and plant water and then signals this to the atmospheric carbon dioxide," explains Lisa Welp the new approach of the researchers.
"Our atmosphere is a perfect blender. Changes in its levels of trace gases -- such as carbon dioxide -- reflect the overall release and uptake of trace gases from all sources. So if you measure the carbon exchange of a forest ecosystem, for example, you "only" get the net exchange of all the carbon taken up by the trees for photosynthesis and all the carbon released by the trees and soil ," writes Dr. Matthias Cuntz of the Helmholtz Centre for Environmental Research (UFZ) in his commentary in the same issue of Nature.
Article continues: http://www.sciencedaily.com/releases/2011/09/110929122804.htm