BOULDER, Colo., July 19 (UPI) --A new study reports that there is a 90 percent chance that global warming by 2100 will increase by between 1.7 and 4.9 degrees Celsius (3.1 to 8.8 degrees Fahrenheit) as compared with 1990. The study also predicts a sharp rise in temperature by 2030.

The work, which appears tomorrow in the journal Science, is by climate scientists Tom Wigley and Sara Raper. Both have contributed to the oft-quoted global warming reports of the Intergovernmental Panel and Climate Change. Raper did much of the computer work on the latest IPCC report.

Whereas the IPCC report predicted a global mean temperature increase of between 2.5 and 10.4 degrees Fahrenheit (1.4 and 5.8 degrees Celsius) by 2100, compared with 1990, the IPCC report did not say that any one temperature increase was more likely to happen than another. Wigley and Raper's new work addresses this issue.

Both the IPCC reports and Wigley and Raper make their projections based on the assumption that there are no policy steps taken this century to control greenhouse gas emissions. The global warming that has been measured over the last 100 years, of about 1.1 degrees Fahrenheit (0.6 degrees Celsius), is generally believed to be caused by the cumulative buildup of greenhouse gases in the planet's atmosphere.

An increase of 5.4 degrees Fahrenheit (3 degrees Celsius) is considered to be about the critical point where widespread damage to crops could occur, although there will be increasing declines in agricultural production in many parts of the tropics related to heat stress for lesser increases as well. Many of the other expected changes, such as rising oceans, changing storm intensities, distribution of disease patterns, human migration due to crop failures would increase in intensity the more that temperature increased.

The Wigley and Raper Science paper, "Interpretation of high projections for global-mean warming,"also predicts that by 2030 there is a 50 percent chance that the world will warm up by 1.1 degrees Fahrenheit (0.6 degrees Celsius) compared with today. This 1.1 degrees Fahrenheit (0.6 degrees Celsius) increase would be equal to all the global warming of the last 100 years. In fact, over the next hundred years Wigley and Raper predict that the average rate of global warming will be five times as great as the last 100 years.

In an attempt to narrow the range of possibilities further, the authors state that there is a 50 percent chance that global warming by 2100 will increase by between 4.3 and 6.8 degrees Fahrenheit (2.4 and 3.8 degrees Celsius).

Many factors influence global warming. The global amount of CO2 emissions, how sensitive the climate is to the build-up of concentrations of greenhouse gases, how the global carbon cycle works, the amount of heat uptake by the oceans, and how many tiny particles (aerosols) are in the air are just some of the factors.

The method Wigley and Raper used to conclude that some temperature increases were more likely than others was to make certain assumptions about the information on these influential factors that they entered into their computers to begin with.

One assumption they made was that increasing CO2 concentrations in the atmosphere were more likely to cause temperature increases of a specific amount. The IPCC reports had simply assumed that a doubling of CO2 would cause an increase somewhere between 3.1 and 7.6 degrees Fahrenheit (1.7 and 4.2 degrees Celsius), without saying any one increase was more likely than another. Wigley and Raper assumed, based on a lot of study and generally accepted scientific and mathematical principles, that a doubling of CO2 was more likely to cause a temperature increase around the middle of the 3.1 and 7.6 degrees Fahrenheit (1.7 to 4.2 degree Celsius) range. Other increases were considered possible, but less likely.

This general approach, of considering a specific amount of temperature increase more likely than other temperature increases, for most of the five climate-influencing factors listed above, is the main way in which the Wigley and Raper work differs from the IPCC work.

Once their assumptions about different parts of their climate equation were made, the data was entered in their simplified computer models and the program was run. "Our results are only as realistic as the assumptions upon which they are based," the article says. Wigley is at the National Center for Atmospheric Research in Boulder and Raper is at the University of East Anglia, in England.

Wigley told United Press International, "In all of these cases you have to pick what you think is the best estimate value, then a range of values, and then you draw sort of one of these curves that characterizes those possibilities, a range of possibilities, and the central value in some mathematical form."

One expert on uncertainty and policy analysis spoke favorably of Wigley and Raper’s work and offered some analysis. "Their (temperature) range corresponds to the same range that IPCC produces, but what you find is that the bulk of the probability, that is most of the betting, lies across a rather narrower range; which I don’t think is a big surprise. . . . It’s not that the range of 1.4 to 5.8 (Celsius, 2.5 and 10.4 degrees Fahrenheit) is wrong, it's just that this shows that the probability at the upper and lower ranges are not very likely,"Granger Morgan told UPI.

"It takes some subjective judgement to reach this conclusion and other people might reach slightly different conclusions. It's pretty unlikely, though, that other people could make plausible assumptions that would arrive at dramatically different conclusions. . . . I think it's a very fine and exciting piece of work that pushes forward the state of understanding of the uncertainties involved in these estimates,"Morgan said.

Morgan is head of engineering and public policy at Carnegie Mellon University and has written extensively on uncertainty and policy analysis.

But not everyone thinks that the paper is good science. According to Paul Georgia an environmental policy analyst at the Competitive Enterprise Institute, "The main problem with the paper is that it’s based on a lot of assumptions. . . . Assumptions about sulfate aerosols are simply an assumption. There's been no measurement of the cooling effect of the sulfate aerosols. There have been several estimates and those are all over the place."

Georgia told UPI, "Trying to calculate, when we don’t even know what the climate sensitivity is for certain, and trying to calculate a probability over the next 100 years seems like a fairly fruitless exercise." The paper is based on certain assumptions in the IPCC report that Georgia described as "flimsy."

One criticism of both the IPCC report and Wigley and Raper's work that Georgia pointed out is that it assumes that certain chemicals in the air, namely sulfates such as sulphur dioxide from burning coal, will decease over the next 100 years. Sulphur dioxide in the air creates a cooling effect, so the more sulphur dioxide you have, the more cooling effect will be exerted. As sulphur dioxide levels drop, therefore, you get more global warming, the opposite of how CO2 works, for example.

There is controversy about how much sulphur dioxide will be reduced in this century. Sulphur dioxide causes acid rain and many countries are making very serious efforts to reduce sulphur dioxide, which comes mainly from burning coal, a very widely used fossil fuel.

Almost all of the 4.1 degree Fahrenheit (2.3 degree Celsius) increase added on to the third IPCC report, as compared with the second IPCC report, came from making new assumptions about how much sulphur dioxide levels will be reduced in the next 100 years.

Copyright 2001 by United Press International.