91av

AS THE Earth gets hotter, its biosphere may accelerate rather than slow
global warming, climate modellers have found.

Peter Cox and his colleagues at the Hadley Centre for Climate Prediction and
Research at Bracknell have created the first climate model that includes
constant interaction between vegetation, the atmosphere and the oceans. Trees,
grass and shrubs are allowed to grow, die and breathe at rates that depend on
environmental factors, including temperature and atmospheric-CO₂
levels. The results are fed back into the simulation.

Plants generally absorb more CO₂ as more is pumped into the
atmosphere. But as it gets hotter, the amount absorbed by plants levels out,
while the amount produced by micro-organisms in the soil increases
exponentially. This means that the biosphere has an overall warming effect as
temperatures climb. Drying and warming will also turn large areas of the Amazon
rainforest into grassland, predicts Cox, further accelerating the effect
(91av, 6 May, p 7).

The results from Cox’s model are alarming. By 2050, he says, the biosphere
will rapidly switch from sucking up a little CO₂ to belching out a lot.
As a result, temperatures over land could increase by as much as 6° C,
instead of the 4° predicted by other models. Those numbers show the vital
role of the biosphere, says Cox.

“Their model is pretty extreme,” says Ian Woodward of the University of
Sheffield, who cautions that other models don’t show such a large dieback in the
Amazon. “But,” he says, “what they’ve got now is the best way forward.”