Will growing forests make the global warming problem better or worse?

Details Will growing forests make the global warming problem better or worse? Will growing forests make the global warming problem better or worse?

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.b31d..03c&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #B31D-03

Physics

0428 Carbon Cycling (4806), 0429 Climate Dynamics (1620), 1615 Biogeochemical Cycles, Processes, And Modeling (0412, 0414, 0793, 4805, 4912), 1622 Earth System Modeling (1225), 1632 Land Cover Change

Scientific paper

Carbon storage in forests has been promoted as a means to slow global warming. However, forests affect climate not only through the carbon cycle; forests also affect both the absorption of solar radiation and evapotranspiration. Previously, it has been shown that boreal forests have the potential to warm the planet, offsetting the benefits of carbon storage in boreal forests (Betts, Nature 408, 187-190, 2000). Here, we show that direct climate effects of forest growth in mid-latitudes also have the potential to offset benefits of carbon storage. This suggests that mid-latitude afforestation projects must be evaluated very carefully, taking direct climate effects into account. In contrast, low-latitude tropical forests appear to cool the planet both by storing carbon and by increasing evapotranspiration; thus, slowing or reversing tropical deforestation is a win/win strategy from both carbon storage and direct climate perspectives. Evaluation of costs and benefits of afforestation depends on the time scales under consideration. On the shortest time scale, each unit of CO2 taken up by a plant is removed from the atmosphere. However, over centuries most of this CO2 taken up from the atmosphere by plants is replaced by outgassing from the ocean. On the longest time scales, atmospheric carbon dioxide content is controlled by the carbonate-silicate cycle, so the amount of carbon stored in a forest is not relevant to long-term climate change. While atmospheric CO2 impacts of afforestation diminish over time, the direct effects on climate (and silicate weathering) persist, so these effects become more important as the time scale of concern lengthens. In some cases, afforestation is predicted to lead to cooling on the time scale of decades followed by warming on the time scale of centuries. Our study involves simulations using the NCAR CAM3 atmospheric general circulation model with a slab ocean to perform idealized (and extreme) land-cover change simulations. We explore the time-dependent carbon-cycle/climate implications of these results using a schematic model of the long-term carbon cycle and climate.

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