快猫短视频

IT SEEMED too good to be true . . . and it was. A new study has overturned
last year鈥檚 controversial suggestion that the enormous emissions of carbon
dioxide by the US are almost entirely offset by the absorption of the gas by its
forests.

Half of the carbon pumped into the atmosphere by human activities seems to
disappear. And that鈥檚 a problem for climate modellers who are trying to forecast
the rate of future global warming, because they can鈥檛 make accurate predictions
without knowing how nature performs this vanishing trick and how long it will
last.

Less than a year ago, a team known as the Climate Modeling Consortium
suggested that vast quantities of the missing carbon were being absorbed by
forests in the North America鈥攅nough, in fact, to completely negate the
warming effects of the US鈥檚 emissions of carbon dioxide
(This Week, 24 October 1998, p 5).
The consortium used climate models to work out how atmospheric
CO₂ should be distributed across the globe, given known carbon sources
and sinks, and then compared these values with measurements of atmospheric
CO₂concentrations made between 1988 and 1992 by a network of monitoring
stations around the world.

When this suggested that huge quantities of carbon were being mopped up by
North America, the researchers argued that the continent鈥檚 terrestrial
ecosystems might be storing more than their share of carbon as trees grew on
abandoned farmland and previously logged forests, perhaps boosted by higher
levels of CO₂.

But now a new study reported in Science(vol 285, p 574) indicates
that during the 1980s, ecosystems in the US stashed away no more than a fifth of
the carbon calculated by the Climate Modeling Consortium. Richard Houghton and
his colleagues at the Woods Hole Research Center in Massachusetts used a
book-keeping approach to total up the amount of carbon stored or released
because of changes in land use between 1700 and 1990. 鈥淲hat we did was to
reconstruct land use changes,鈥� says Houghton.鈥漌e brought in every method I could
think of to estimate where emissions might have gone in the US.鈥�

Some of the difference between the two studies may be due to their very
different methods. Christopher Field, an ecologist at the Carnegie Institution
of Washington, contrasts the consortium鈥檚 atmosphere-based, 鈥渢op down鈥� approach
with Houghton鈥檚 ground-based, 鈥渂ottom-up鈥� approach. But while Houghton鈥檚 team
may have missed some carbon-storage mechanisms, he suspects that the true answer
lies nearer to their estimate than to the Carbon Modelling Consortium鈥檚 because
other researchers are arriving at similar figures. 鈥淔orests are not going to
save us,鈥� Field concludes.