FOR prophets of global warming, forecasting remains an "iffy"
They know people are perturbing the climate system with
heat-trapping "greenhouse" gases. Yet their computer simulations
still yield only "best guesses" as to what the effects may be.
This uncertainty permeates a new attempt to project future
climate using material prepared for the United Nations Conference
on Environment and Development now under way in Rio de Janeiro. The
new study reduces a 100-year projection of global warming and sea
level rise by the Intergovernmental Panel on Climate Change (IPCC)
in 1990. Yet it warns that humanity still faces possibly serious
The first study
An IPCC team of 170 scientists from many countries based the
1990 projections on a variety of scenarios for future emissions of
carbon dioxide (CO2) and other greenhouse gases. The IPCC has
refined those scenarios for the Rio conference. Tom Wigley and
Sarah Raper of the Climate Research Unit at the University of East
Anglia in Norwich, England, have used the new scenarios to
reforecast warming and sea-level changes.
Reporting their work recently in Nature, Drs. Wigley and Raper
note that the earlier IPCC "best guess" projected a 3.3 degree C
rise in average global temperature and a 66 centimeter rise in sea
level. Their new "best guess" is for a 2.5 degree warming and 48
centimeter sea level rise.
Wigley and Raper point out that while "these are substantial
reductions ... the projected changes ... still are very large."
They emphasize that "the warming corresponds to a rate roughly five
times that observed over the past century and the sea level rise is
at a rate roughly four times that estimated for the past century."
Yet they also caution that "the uncertainty ranges for these
projections are large, and they are higher ... than in" the 1990
Thus, one of the latest and most detailed computer simulations
still fails to clear the climatologists' crystal ball.
That does not mean that climatologists have no certain
knowledge. They understand very well how the greenhouse effect
works and what kind of jolt greenhouse gas pollution is giving it.
Like a thrifty household, the climate system sticks to an energy
budget in which income balances outgo in the long run. The Sun
supplies the energy income. Clouds, bright surfaces, and aerosols
reflect about 70 percent of incoming sunshine away from the planet.
The other 30 percent - the energy income - is absorbed to drive the