Jun 20, 2001 (United Press International via COMTEX) -- Since June 6, when the National Academy of Sciences gave their stamp of approval to the work of the United Nations Intergovernmental Panel on Climate Change, U.S. officials have had a tough time dismissing the conclusions of the IPCC report.

Yet uncertainties big and small about global warming do remain. Indeed, uncertainty itself is a field of scientific study -- one almost tailor-made for climate research. Predicting potential climate futures involves hundreds, if not thousands, of variables.

Climate modeling, as scientists call it, can be boiled down to two basic steps, however. First, construct scenarios that predict how much carbon dioxide and other greenhouse gases will be emitted. Second, predict how the greenhouse gases will affect global temperature and overall climate.

So the recent 25-page NAS climate science report, which references the IPCC report 47 times, opens by saying human activity has increased levels of greenhouse gases in Earth's atmosphere and that global temperatures are rising. Then in the third sentence comes the uncertainty: "The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes are (sic) also a reflection of natural variability."

There's virtually no longer a debate whether Earth has warmed up in the last several decades. It has. Indeed, as computers grow more powerful and researchers nail down more climate factors, the scientific debate whether humans have contributed to the warm-up has quieted down.

"The chance that this would happen randomly is less than one in 100. The correlation of these temperatures do not look random," Nobel-winning chemist F. Sherwood Rowland told United Press International. "That's probably what most of us think." Rowland, a University of California, Irvine, professor and one of 11 authors of the NAS report, shared a Nobel prize in 1995 for work on atmospheric ozone.

That leaves the question of how much, that is, how much is a natural swing in temperature -- after all, 50 years is less than a blink of geological time -- and how much comes from humans driving cars, burning forests, converting fossil fuels to power and ultimately products. No one is yet willing to venture numbers on the split, but the IPCC report is a start.

Increased levels of atmospheric carbon dioxide, often abbreviated CO2, are generally believed to have led to a rise in global temperature of about 0.7 degree centigrade (1.3 degrees Fahrenheit) in the last 140 years, with about a 0.3 degree centigrade ( 0.5 degree Fahrenheit) increase in about the last 20 years. The discussion about global warming centers around how much CO2 will be emitted during the next 100 years from fossil fuels such as gasoline, coal, natural gas and fuel oil.

The most CO2-intense IPCC future emission scenario is a world in which population peaks mid-century at about 9 billion and declines to about 7 billion by 2100. It's a world of rapid economic growth, rapid introduction of more efficient technologies, and a heavy reliance on fossil fuels.

Under this scenario, with the significant assumption that there is no international policy about reducing emissions, CO2 levels rise to about four times current levels. With it the IPCC projects a global temperature rise of 3.0 to 5.4 degrees Celsius (5.4 to 9.7 degrees Fahrenheit) for this scenario.

At the other extreme of six basic IPCC scenarios: The world population also peaks at about 9 billion and then declines to 7 billion by 2100, but there is a big shift away from industrialism toward a service and information economy. Clean and energy efficient technologies are introduced. And there is a strong shift away from fossil fuels.

In this hypothetical future, CO2 emissions drop a bit and atmospheric concentrations stabilize at about 500 ppm -- a rise of 125 ppm over today's levels -- with a temperature increase of 1.2 to 2.4 degrees centigrade (2.2 to 4.3 degrees Fahrenheit). This scenario presumes an increased environmental awareness but not adoption of any formal international climate treaty.

The IPCC studiously avoids saying that any one scenario is more probable than another.

"On the low side you would see twice the current energy demand and on the high side at least five times the current demand," said the convening lead author of the IPCC emissions scenarios report, Nebojsa Nakicenovic. "The low-demand scenarios are usually associated with quite a strong shift to what would be considered sustainable energy development, in particular in developing countries. ... The very high scenarios assume limited oil and gas and so economies turn back to coal."

Nakicenovic spoke with UPI by telephone from the International Institute for Applied Systems Analysis in Austria, where he leads a project on transitions to new technologies.

The second step of the prediction process, translating the projected CO2 concentrations into climate change, is at least equally uncertain. In fact, one reason scientists end up with such wide ranges for any particular scenario is the uncertainty in what is referred to as climate sensitivity.

Climate sensitivity is a measure of how much the global temperature will change in response to a change in atmospheric CO2. The straightforward physics calculation of what happens when atmospheric CO2 doubles says that an increase of approximately 1.2 degrees will ultimately occur. But other factors are also involved.

Warming from the CO2 will produce increased water vapor, which is in itself a greenhouse gas. Increased water vapor will lead to increased clouds. Additionally, warming will cause changes in the amount of ice and snow, which will reduce the amount of incoming sunlight reflected, resulting in increased warming. These are the major feedback systems and each has associated uncertainties. For example, will increased cloud cover ultimately correct temperature raises by blocking sunlight, or reinforce them by insulating Earth?

The IPCC, with some measure of consensus, has lumped all the uncertainties together to produce a range of climate sensitivities, in which a doubling of CO2 ultimately produces a global mean temperature increase of from 1.7 to 4.2 degrees centigrade (3.1 to 7.6 degrees Fahrenheit). In other words, the uncertainty range built into the calculation is 2.5 degrees centigrade (4.5 degrees Fahrenheit) for a doubling of CO2.

When IPCC researchers run 35 different future world energy-use/fuel-mix scenarios with seven different computer climate models, they generate the range of 1.2 to 5.6 degrees centigrade ( 2.2 to 10.1 degrees Fahrenheit) increase by 2100.

Interestingly, however, running the same 35 energy-use/energy-mix scenarios, but now using a computer model that averages the seven computer climate models, yields a much smaller range of projected temperature rise, from 1.8 to 4.4 degrees centigrade (3.2 to 7.9 degrees Fahrenheit).

Although there is an even greater range of projections possible, the IPCC tried to go with the mainstream of accepted climatological thought. Sarah Raper, who is a lead author of the IPCC chapter on the global climate model projections, told UPI, "This is not the extreme range of possibilities, this is just the particular seven models were that were considered."

"There have been some (computer) models that gave a bigger climate sensitivity than this and there have been some that gave a smaller one," Raper continued. "These are the well-established general circulation models that ... were close to the commonly accepted range for climate sensitivity." Raper is a climate researcher at the University of East Anglia, in England.

Whatever the ultimate portion of responsibility that humans share in global warming, however, policy makers can't wait for the hard-and-fast numbers.

"Some of these uncertainties are just never going to be eliminated on the time-scale of the next century or two," said Granger Morgan, head of an engineering and public policy program at Carnegie Mellon University and author of "Uncertainty," a book that examines policy and risk analysis in light of uncertainty. "The geophysical research community has an obligation to start trying to tell decision makers the limits to how much it's going to be able to reduce uncertainties."

Copyright 2001 by United Press International.