Effect of increasing CO2 on the terrestrial carbon cycle

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Abstract

Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO ₂ concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO ₂ effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO ₂ 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 CO ₂. 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 CO ₂ concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO ₂ 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.

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A large and persistent carbon sink in the world's forests.

Yude Pan, Richard A Birdsey, Jingyun Fang … (2011)

The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year(-1)) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year(-1) from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year(-1) partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year(-1). Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year(-1), with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.

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Observational contrains on the global atmospheric co2 budget.

P Tans, I. Fung, T Takahashi (1990)

Observed atmospheric concentrations of CO(2) and data on the partial pressures of CO(2) in surface ocean waters are combined to identify globally significant sources and sinks of CO(2). The atmospheric data are compared with boundary layer concentrations calculated with the transport fields generated by a general circulation model (GCM) for specified source-sink distributions. In the model the observed north-south atmospheric concentration gradient can be maintained only if sinks for CO(2) are greater in the Northern than in the Southern Hemisphere. The observed differences between the partial pressure of CO(2) in the surface waters of the Northern Hemisphere and the atmosphere are too small for the oceans to be the major sink of fossil fuel CO(2). Therefore, a large amount of the CO(2) is apparently absorbed on the continents by terrestrial ecosystems.

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Forest response to elevated CO2 is conserved across a broad range of productivity.

Richard Norby, Evan Delucia, Birgit Gielen … (2005)

Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO(2)] ("CO(2) fertilization"), thereby slowing the rate of increase in atmospheric [CO(2)]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO(2) fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO(2)] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO(2) ( approximately 550 ppm) in four free-air CO(2) enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO(2)] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 +/- 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO(2)] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.