Effect of increasing CO2 on the terrestrial carbon cycle |
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Authors: | David Schimel Britton B. Stephens Joshua B. Fisher |
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Affiliation: | aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91011; and;bEarth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, 80301 |
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Abstract: | Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO2 concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO2 effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO2 that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO2. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO2 concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO2 concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.In projections of future climate, the carbon cycle is second only to physical climate sensitivity itself in contributing uncertainty (1). Earth system model uncertainty has increased as more mechanisms have been incorporated into a growing number of increasingly sophisticated models. Terrestrial ecosystem feedbacks to atmospheric CO2 concentration result from two mechanisms, direct effects of CO2 on photosynthesis and effects of climate change on photosynthesis, respiration, and disturbance (2). The CO2 effect, used here to describe the effect of increasing atmospheric CO2 on terrestrial carbon storage by increasing photosynthetic rates, is also known as the β effect (3, 4). The effects of CO2 on carbon uptake occur at the enyzmatic and stomatal scales but impact the global carbon cycle.The CO2 effect on terrestrial carbon storage is a key potential negative feedback to future climate, and in models of the present, it is the largest carbon cycle feedback (5, 6). In simulations of the next century, the CO2 effect is four times larger than the climate effect on terrestrial carbon storage and twice as uncertain (4). Land use also creates large fluxes, but these are not driven by CO2 or climate directly and so are not feedbacks. In models of the future, the biosphere operates as a net sink, reducing the climate impact of fossil fuel and deforestation emissions, until positive feedbacks from climate change [reduced productivity, increased respiration, or dieback (7)] and land use emissions exceed the CO2 effect. The magnitude of this negative feedback is crucial to simulating future climate, but because observational constraints on the CO2 effect are limited, the effects of CO2 remain controversial. The effects of CO2 are known mainly from small-scale experimental studies, ranging from single-leaf experiments through to ecosystem-scale experiments with a spatial scale of hundreds of meters (8), but predictions from theory of a large tropical effect of CO2 have appeared to be inconsistent with global patterns of atmospheric CO2 (6).Photosynthesis increases with increasing CO2 following a Michaelis−Menton curve, and this effect grows stronger at higher temperatures, implying, all else being equal, larger effects in warmer climates (9–11), especially in the tropics. Many factors control the relationship between increased photosynthetic rate and carbon storage, including how fixed carbon is allocated to plant tissues and soils with different residence times, the development of progressive nitrogen limitation, interactions with water or light limitation, and many other biological responses (12). Theory and experiments agree in suggesting a CO2-driven net sink that should be roughly proportional to overall productivity (13) leading to a large sink in the tropics, a prediction that should be testable with global observations (11). |
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Keywords: | climate feedback carbon budget tropics atmospheric transport |
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