Vegetation may Double Greenhouse Effect

In a study published in the March 8 issue of Science, NASA Goddard's Piers Sellers and colleagues there and at the Carnegie Institution, Colorado State Univ., and the Univ. of Victoria, report that standard general atmospheric general circulation models (AGCMs) -- an important class of models that simulate the motions of the earth's atmosphere -- generally ignore the physiological effects of vegetation.

Typically, AGCMs predict that a doubling of CO2 could increase the globally averaged surface air temperature by 1.5 to 3.5 degrees C, initiated primarily or solely by changes in the radiation balance, i.e., the greenhouse effect.

However, in new computer analyses using an AGCM that incorporates vegetation physiological responses, the team says that the temperature could rise as much as an additional 0.9 degrees C in the tropics, with lower effects in less-densely vegetated regions.

These models are based on years of work exploring the relationship between the biochemistry of photosynthesis and the physiology of stomatal conductance. (Stomates are small pores on a leaf's surface which regulate the uptake of CO2 and the release of water, or transpiration.)

The AGCM that includes SiB2 (described more fully in a forthcoming issue of J. Climate) was developed by David Randall of Colorado State Univ. in support of NASA's mission to planet earth. SiB2 combines satellite and surface data to define the type, density, and physiological properties of worldwide vegetation.

Results from simulations with SiB and the AGCM in coupled mode indicate that greenhouse warming is only part of the story. Additional warming caused by closing plant stomates under doubled atmospheric carbon dioxide amplifies the greenhouse warming by up to 25 percent, with the largest effects in the summer, in regions with dense vegetation. This is in sharp contrast to the greenhouse efffect, which is largest in the winter at high latitudes.

Stomatal closure is further exaggerated if increased CO2 leads to decreased photosynthetic capacity, as is observed in some experimental studies; in such case, the warming would be even greater -- up to 50 percent above that attributable to the greenhouse effect alone. The actual extents of this effect are not yet known, but the simulations reported in the Science paper bracket the likely range of responses.

A temperature increase of 25-50 percent above that due to the greenhouse effect is up to 1 degree C. Though this is not large in absolute terms, it is an entirely new component of global climate change, not included in previous analyses.

Climatic effects of stomatal closure are probably smaller than, but of the same order of magnitude as, the greenhouse effect or the effect of deforesting a large region like the Amazon. The earth's vegetation clearly responds to changes in climate. This study indicates that vegetation also plays a major role in determining climate.