It's a drought - no, flood!

It's easy to believe in global warming. The last century ended with record warmth, thinning Arctic ice, and warmer seas. That fits with predictions of computer climate models that simulate the effects of pumping carbon dioxide (CO2) and other heat-trapping gases into the air.

But beware the uncertainties. An expert panel told a recent US Senate hearing that many scientists now think they can indict humanity for climate change. But panel members added that their science still is too iffy to prove the case beyond reasonable doubt.

This leaves decision makers - including US senators - in a familiar quandary. They feel increasing public pressure to "do something" about climate change. Yet scientists can't tell them exactly what is changing, what's driving the changes, or what the future may hold. This makes it hard to justify drastic curbs on use of fossil fuels, which generates most of the extra CO2.

A new report from the International Panel on Climate Change (IPCC) expected later this year won't resolve the ambiguity. Organized by the World Meteorological Organization and the United Nations, the panel focuses the work of hundreds of scientists to try to assess what's happening to climate. It reports a new assessment every five years.

The draft of the year 2000 report, now circulating among scientists, concludes that "there has been a discernable human influence on global climate." That's stronger than the panel's 1995 assessment, which found merely that "the balance of evidence suggests a discernable human influence." Yet the panel can't say exactly what the influence is doing. Inability to deal with certain key climate factors - especially clouds - undermines computer simulations. Uncertainty over what is natural variation and what is human driven muddles interpretation of present climate trends.

Sen. John McCain (R of Arizona) says that, during his recent political campaign, younger people repeatedly asked him how he would stop climate change. Because he now "intends to become informed", he convened a hearing with five experts plus presidential science advisor Neal Lane May 17. These witnesses noted some key points on which most scientists agree.

*Human activity has been pumping up atmospheric carbon dioxide (CO2) for a century. It now appears to be at it's highest level in over 400,000 years.

*Earth's surface temperature has risen rapidly for 100 years, with the 20th century's three warmest years occurring during the past decade

*The warmest year in the past 10 centuries was 1998.

They also pointed out key issues on which there's no agreement:

*Computer simulations of how climate may vary around the earth and through time.

*How the climate's interconnected system, with feedback involving air, sea, land, and living organisms, really works.

*Whether or not, or to what extent, we are experiencing more deep droughts, more heat waves, more intense storms, and other extreme weather.

*To what extent land and ocean plants remove carbon from the air.

*How might climate change rearrange ecosystems, with "exotic" species invading new areas and tropical diseases moving north and south.

As scientists try to resolve such uncertainties, they all agree on another point. They need more data. If decision makers want more definitive knowledge from scientists, climate researchers say, then they should fund that data gathering.

Take the nagging problem of why satellite measurements of the lower atmosphere don't show the warming found by thermometers on the ground. Skeptics have used this discrepancy to challenge claims of global warming, calling the ground data flawed. A study group of the National Research Council of the (US) National Academy of Sciences took a look at this and concluded that the surface warming is "undoubtedly real." It noted that surface temperatures generally rose faster during the 1980s and 1990s than the 20th-century average rate of increase. In all, Earth's surface temperature rose 0.7 to 1.4 degrees F over the past century. But satellite data from the past two decades show little warming in the lower five miles of the atmosphere, which would be expected to warm the way the surface does.

In announcing its report last January, the council noted that a combination of human and natural causes raised surface temperatures while other human and natural causes - such as volcanic dust - may have kept the overlying air cooler. John Wallace of the University of Washington in Seattle, who chaired the study, says the bottom line is that scientists don't know what is going on. He explained that the rapid 20-year rise in surface temperature "is not necessarily representative of how the atmosphere is responding to long-term, human-induced changes." He added that "nations of the world should develop an improved climate-monitoring system to resolve uncertainties in the data...."

Sometimes relevant data are already out there, squirreled away in cubbyholes and desk drawers. Last March, Sydney Levitus and colleagues at the National Oceanographic Data Center in Silver Spring, Md. reported they had solved a major climate puzzle by rescuing such data from around the world. It's part of the United Nations-sponsored Global Oceanographic Data Archaeology and Rescue Project. Dr. Levitus and his fellow "archaeologists" dug up 2 million forgotten sea-temperature data points to add to those on file. Using 5.1 million measurements from around the world, they discovered that the world ocean has warmed significantly over the past 40 years.

This is comforting for climate modelers. Levitus explained: "One criticism of the models is that they predict more warming of the atmosphere than has been actually observed. Climate modelers have suggested that this 'missing warming' was probably to be found in the world ocean. The results of our study lend credence to this scenario."

Meanwhile, distressed coral reefs show how complex the impact of rising atmospheric CO2 can be. Corals and the algae they contain gain mutual benefits from their symbiotic way of life. But if things get too warm, corals eject the algae and turn white. This bleaching can be deadly. The 1997-98 El Nino, plus global warming, brought some of the highest sea-surface temperatures ever recorded. There was severe coral bleaching around the world. As in the past, many reefs are recovering. But not in Belize. There has been almost total mortality of several coral species in some parts of the Belize barrier reef.

Reporting last month in Nature, Richard Aronson and colleagues at Dauphin Island Sea Lab in Alabama note a "growing concern that global climate change is degrading coral reef ecosystems." They add that "our results from Belize appear to justify this concern."

Climate change is only part of the problem. In the June issue of the journal Global Biogeochemical Cycles, Christopher Langdon and colleagues from Columbia University's Lamont-Doherty Earth Observatory report that CO2 itself is bad for reefs.

Experiments at Columbia's Biosphere 2 laboratory in Oracle, Ariz. show that atmospheric CO2 levels expected to be reached later in this century change sea-surface chemistry in a way that makes it harder for corals to build their structure. The researchers project that this could reduce coral growth by as much as 40 percent from pre-industrial levels over the next 65 years.

Dr. Langdon says that "while some terrestrial ecosystems may actually benefit from elevated carbon-dioxide levels, that does not appear to be the case for shallow marine ecosystems like a coral reef." This will be an added hazard for the new preserve that President Clinton hopes will "permanently protect Hawaii's rich coral reefs."

Last fall, the American Geophysical Union summed up its understanding of the challenge in a formal position statement. It said that present knowledge of Earth science, in spite of uncertainties, "provides a compelling basis for legitimate public concern." It acknowledged that those uncertainties "will never be completely eliminated."

Nevertheless, it advised decisionmakers that "the present level of scientific uncertainty does not justify inaction in the mitigation of human-linked climate change and/or the adaptation to it."

(c) Copyright 2000. The Christian Science Publishing Society