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Combined climate and carbon-cycle effects of large-scale deforestation

Author(s): Bala G, Caldeira K, Wickett M, Phillips TJ, Lobell DB, Delire C, Mirin A

Published: April, 2007

Publisher: Proceedings of the National Academy of Sciences

DOI: 10.1073/pnas.0608998104

Tags: Afforestation, Climate Modelling, Climate Science, Carbon Cycle

URL: http://www.pnas.org/content/104/16/6550.abstract

Abstract: The prevention of deforestation and promotion of afforestation have often been cited as strategies to slow global warming. Deforestation releases CO2 to the atmosphere, which exerts a warming influence on Earth's climate. However, biophysical effects of deforestation, which include changes in land surface albedo, evapotranspiration, and cloud cover also affect climate. Here we present results from several large-scale deforestation experiments performed with a three-dimensional coupled global carbon-cycle and climate model. These simulations were performed by using a fully three-dimensional model representing physical and biogeochemical interactions among land, atmosphere, and ocean. We find that global-scale deforestation has a net cooling influence on Earth's climate, because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.


Biogeophysical effects of CO2 fertilization on global climate

Author(s): Bala G, Caldeira K, Mirin A, Wickett M, Delire C, Phillips TJ

Published: June, 2006

Publisher: Tellus B

DOI: 10.1111/j.1600-0889.2006.00210.x

Tags: Terrestrial Carbon Storage, Afforestation, Climate Modelling, Climate Science

URL: http://adsabs.harvard.edu/full/2006TellB..58..620B

Abstract: CO2 fertilization affects plant growth, which modifies surface physical properties, altering the surface albedo, and fluxes of sensible and latent heat. We investigate how such CO2-fertilization effects on vegetation and surface properties would affect the climate system. Using a global three-dimensional climate-carbon model that simulates vegetation dynamics, we compare two multicentury simulations: a ‘Control’ simulation with no emissions and a ‘Physiol-noGHG’ simulation where physiological changes occur as a result of prescribed CO2 emissions, but where CO2-induced greenhouse warming is not included. In our simulations, CO2 fertilization produces warming; we obtain an annual- and global-mean warming of about 0.65 K (and land-only warming of 1.4 K) after 430 yr. This century-scale warming is mostly due to a decreased surface albedo associated with the expansion of the Northern Hemisphere boreal forests. On decadal timescales, the CO2 uptake by afforestation should produce a cooling effect that exceeds this albedo-based warming; but if the forests remain in place, the CO2-enhanced-greenhouse effect would diminish as the ocean equilibrates with the atmosphere, whereas the albedo effect would persist. Thus, on century timescales, there is the prospect for net warming from CO2 fertilization of the land biosphere. Further study is needed to confirm and better quantify our results.


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