What was done
The authors used a coupled biosphere-atmosphere model to investigate the climatic impact of a realistic increase in the vegetative cover of the earth.  The vegetative change investigated was that which produced the difference between satellite observations of the maximum and minimum fractions of photosynthetically active radiation absorbed by green leaves over all land areas of the earth, except Antarctica, in each calendar month of the nine-year period 1982-90.

What was learned
In the tropics (14°S-14°N), going from the minimum to the maximum amount of vegetation observed over the period 1982-90 led to a year-round cooling of 0.8°C.  In northern latitudes (50.4-72°N), it led to a cooling of 1.8°C during the growing season and a slight warming in the winter.

What it means
In the words of the authors, "these results suggest that increases in vegetation density could partially compensate for parallel increases in greenhouse warming."  As an example of the strength of this phenomenon, they note that the globe's mean surface air temperature has been predicted to increase by up to 4.5°C in response to a doubling of atmospheric CO2 when interactions between physiological effects of plants and climate are ignored, which act of ignorance, they say, is typical of most global warming predictions.  However, they state that when using "biophysically-realistic coupled biosphere-atmosphere climate models," the global temperature increase is only about 2.6°C, which is only a little over half of the maximum warming predicted in the absence of a simultaneous increase in vegetation.  And in the case of the minimum warming predicted by the bulk of the general circulation models of the atmosphere in use today, this vegetative feedback would almost totally annihilate the predicted warming.

The bottom line of this analysis thus continues to substantiate what we have long claimed, i.e., that negative biologically-based feedbacks play major roles in earth's complex climate system and tend to thwart almost any impetus for dangerous global warming (see our Position Paper Carbon Dioxide and Global Warming: Where We Stand on the Issue).