Productivity of land plants may be greater than previously thought

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.

The gross-exchange fluxes, such as photosynthesis, are however accessible only with difficulty. "Global estimates therefore depend upon a number of assumptions. This includes, for example, how many of the CO₂ molecules entering a plant are actually fixed by photosynthesis. The researchers of Lisa Welp's team assume that around 43 per cent of all CO₂ molecules entering a plant are taken up by the plant. If this were only 34%, the estimate would fall to about 120 billion tons of carbon -- that is, to the currently accepted value," for Matthias Cuntz reason of thought. In his opinion, the new findings do not completely upset the research to date. Nevertheless, they demonstrate an interesting new method for the determination of plant productivity over large areas. In future, the combination of several isotopic methods with conventional measurements represents a promising approach.

The now published study was carried out under the direction of Ralph F. Keeling, a professor of oceanography and the son of the late Charles David Keeling, after whom the so-called Keeling curve was named. This graph shows the concentration of CO₂ of the volcano Mauna Loa on Hawaii since the year 1957. In the 1950s the CO₂ fraction in Earth's atmosphere was still around 315 ppm. In 2011, by comparison, it has already increased to 390 ppm. With his measurements Keeling was able to show for the first time that the concentration of the greenhouse gas increases in relation to changing land use and the combustion of fossil fuels. This new study underscores the importance of long-term measurements of the isotope 18O in the carbon dioxide of the atmosphere from the scientific point of view, as this occupies a key position between the carbon cycle and the hydrogen cycle.