Based on worldwide local measurements and data-driven model simulations, an international team of researchers led by Christian Beer of the Max Planck Institute for Biogeochemistry in Jena has for the first time provided an observation-based estimate of the largest global flux of carbon between land and atmosphere and of its climate dependencies
The researchers evaluated the result against spatially explicit process models including the leading model LPJmL from PIK. Tropical ecosystems such as rain forests and savannas account for almost two thirds of the CO2 uptake, they report in an article published by the journal “Science” on the “Science Express” web site today.
“Our results confirm the major role of rainfall for the global carbon cycle: It controls plant uptake of CO2 for over forty percent of all vegetated land,” says Alberte Bondeau of the Potsdam Institute for Climate Impact Research (PIK), who contributed computer simulations of global vegetation to the study. Patterns of precipitation are expected to undergo substantial changes with global warming. “This could profoundly affect the productivity of land ecosystems,” Bondeau notes.
The researchers used information from a global network of observation stations to quantify the exchange of CO2 between different ecosystems and the atmosphere. More than 250 observation towers provide continuous measurements. The team used these data to train diagnostic computer models which were then used to compute the value of total worldwide uptake of CO2 by vegetation, the so-called gross primary production of the terrestrial biosphere.
The value of ca. 450 billion tonnes per year describes the total volume of fixation of CO2 in the process of photosynthesis on land and is a function of environmental conditions, particularly of climate variables and vegetation properties. The carbon removed from the atmosphere in this manner later returns to it on various time scales: a large fraction returns quickly due to plant respiration, another large fraction more slowly when leaves decay or woody material decomposes in the soil, or through wildfires. The delay in the return of some of the CO2 fixed by plants to the atmosphere is an important factor controlling the rate of climate change due to human emissions.
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