CICS research suggests the tropical biosphere is a large CO2 source
Researchers at Princeton University and NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) collaborating through the Cooperative Institute for Climate Science (CICS) have found new evidence that suggests that there is not a significant terrestrial carbon sink due to CO2 fertilization or some other mechanism in the tropics. The story begins in the ocean, where 67,000 individual observations of dissolved inorganic carbon and nutrient observations were combined with ocean models from GFDL and other research groups to provide a new estimate of the geographical distribution of air-sea CO2 fluxes. The ocean inverse results were then combined with an analogous atmospheric inversion using atmospheric CO2 data from NOAA's Earth System Laboratory (ESRL). Combining the oceanic data with the atmospheric data in this joint inversion resulted in significant changes in the estimated terrestrial fluxes compared with previous atmospheric inversions. The largest changes occurred in the tropical and southern hemisphere regions, which are not well sampled by the atmospheric observations alone. Previous results had tended to give fluxes for this region that were not statistically different from zero. By contrast, the joint inversion finds that the tropical and southern hemisphere land regions are a source of carbon to the atmosphere, with a 77% chance that their total source exceeds 1 billion metric tons of carbon per year.
Background: The net terrestrial flux obtained by the inversions represents the sum of emissions due to tropical deforestation and a potential natural sink from plant fertilization via increasing atmospheric CO2 levels. The net source estimated by the joint inversion is about the same magnitude as independent estimates of the tropical carbon source due to deforestation and land use change alone. Unless emissions due to tropical land use change have been underestimated, this result implies that the proposed tropical carbon sink CO2 fertilization is less important a player in the global carbon cycle than previously thought.
Significance: Most of the current generation of climate models used to estimate the effects of climate change, including those being used for the International Panel on Climate Change (IPCC) Assessment Report 4, incorporate a substantial CO2 fertilization sink. This sink reduces the amount of mitigation required to stabilize atmospheric CO2 concentrations to about half what it would be without CO2. Without such a CO2 fertilization sink, the challenge of stabilizing atmospheric CO2 becomes much greater.
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