FOR prophets of global warming, forecasting remains an "iffy" business.
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 climate change. 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. Warnings remain
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 forecasts.
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 weather. An equal amount - the energy outgo - is sent back into space as infrared (thermal) radiation, completing the income-outgo balance.
Earth would freeze if all that infrared radiation came from (or left) its surface. It would cool to a frigid minus 18 degrees C (about zero F). However, naturally occurring carbon dioxide, ozone, and water vapor and clouds trap some of the outgoing radiation and reradiate it. This is the greenhouse effect. The climate system adjusts to this reradiation by raising the average surface temperature to a comfortable 15 degrees C (59 F) at present.
As measured by satellites, the absorbed solar energy - Earth's energy income - now runs to about 237 watts per square meter when averaged over the entire planet and over a full year. The planet sends 237 watts per square meter back to space as infrared radiation, mainly from the colder upper atmosphere.
However, the surface emits 390 watts per square meter. The difference of 153 watts per square meter is trapped in the atmosphere.
Climatologists generally accept that pollution by greenhouse gases such as CO2 from burning fossil fuels or methane from agriculture has added about 2.5 watts per square meter to this natural greenhouse energy trapping.
They also know how much more heat will be trapped as such pollution continues. A doubling of atmospheric CO2, for example, will trap around an additional 2 watts per square meter. Unsure of moisture
Thus there is no doubt that humans are perturbing the climate system. But computer simulations can't yet tell exactly how that system will adjust. They can't deal well with clouds and moisture or with the effect of the oceans.
David Randall of Colorado State University at Fort Collins led a study by 31 scientists in eight countries using the 19 best computer climate models. This showed the models give differing results partly because they all handle processes involving moisture differently.
Computer models also fall short because scientists don't know the present state of the climate very well.
Reviewing computer forecasts earlier this year in Technology Review magazine, meteorologist Peter Stone at the Massachusetts Institute of Technology said that "the key ingredient" climatologists need to improve forecasting "is more comprehensive observations of the climate system." If they don't know what the climate is doing today, they can't project clearly what it will do tomorrow.