GLOBAL WARMING: WHAT WE KNOW Time bomb or teapot tempest? Scientists still think the earth is heating up, though they're less sure how much or how fast. Some precautions make sense in any case.
By Peter Nulty REPORTER ASSOCIATE Alicia Hills Moore

(FORTUNE Magazine) – IT IS THE YEAR 2000-something. The earth is warming up because gases like carbon dioxide (CO2), created by burning fossil fuels, are accumulating in the atmosphere. Pilot T. J. ''Red'' Barren is a veteran of the global campaign to reverse the buildup. He has flown over the world's oceans, dropping fertilizers to promote growth of microorganisms that consume CO2. He has seeded the atmosphere with aerosols, tiny particles that cause water vapor to condense into clouds that reflect the sun's rays away from the earth. From his home overlooking Denver, Barren sometimes tries to picture the city as it must have been decades ago, back in the 1990s. Then Denver was smoggy by day and blazed with light at night. Now it's dark at night and ablaze by day. Stiff taxes on carbon dioxide pollution have made coal-generated electricity too expensive to waste on outdoor lighting. And in the sun, the roofs of Denver are an eye-squinting white, required by law in order to reflect the sun's rays. A dark green forest of drought-resistant loblolly pines begins at the city's outskirts and stretches eastward almost unbroken across the once Great Plains to the Mississippi. When ranchers started going bust in the heat, the whole country went nuts planting trees to soak up the excess CO2.

Could this really happen? Two years ago, when the Midwest was parched by drought and Yellowstone Park was in flames, the scientists answered with a resounding yes. Some even warned that a global warming has begun that will bring catastrophe in the 21st century, destroying agriculture and flooding coastal lowlands like Florida and Bangladesh. Since then, however, further research strongly suggests that natural phenomena like clouds and ocean currents may mitigate the greenhouse effect. So while most experts still believe in their guts that the globe will warm up, they are now less certain that disaster will result. Nearly everyone is worried, but not as worried as two years ago. The critical questions are how much the earth will warm, and how fast. Right now forecasts for global warming are little more than hunches based on skimpy evidence from a very young branch of science. Computer models estimate that the mean global temperature will rise between 1.8 degrees and 10 degrees Fahrenheit sometime in the next 50 to 100 years -- a very broad range of possibilities. A 2 degrees increase in 100 years might be manageable, while 9 degrees over 50 years could raise sea levels and burn out croplands at a disastrous clip. Resolving the debate among climatologists will take a while. Yet if the uncertainties are great, so are the consequences of misjudgment. Reducing the gases that promote the greenhouse effect could cost trillions -- wasted money if the globe isn't warming much after all. But if nobody does anything and the world heats up rapidly, the damage could be incalculable and irreversible. Fortunately, business, government, and citizens can do much to help, including conserving energy and planting a billion trees a year as President Bush has proposed. Most of the ideas make sense whether global warming gets worse or not. Here's what scientists know, what they don't know, and what can be done until they solve the riddle: The facts about global warming are sparse but compelling. Certain gases in the atmosphere, principally water vapor and CO2, trap heat radiating from the earth's surface. If they did not, the earth's average temperature would be roughly 0 degrees instead of just over 59 degrees, and everything would be frozen solid. Human activity creates greenhouse gases that include CO2 (mainly from combustion), methane (from crops and livestock), and chlorofluorocarbons, or CFCs (from aerosol spray cans, air conditioners, and refrigerators). Unquestionably the greenhouse gases mankind is spewing forth are accumulating in the atmosphere. Regular measurements of CO2 taken since the 1950s, for instance, show that concentrations have increased at the rate of about 0.5% per year. James Hansen, a leading expert with NASA's Goddard Institute for Space Studies, estimates that the CO2 added to the atmosphere since the Industrial Revolution got rolling about 1800 has the heating power of roughly one watt -- equal to a single tiny Christmas tree bulb -- per square meter of the earth's surface. That may not sound like much, but it doesn't take a lot to alter the world as we know it. The global mean temperature at the height of the last ice age 18,000 years ago was 51 degrees, just 8 degrees cooler than today. Add to these facts three pieces of circumstantial evidence and it becomes clear why alarms are sounding: Laboratory analysis of glacial ice as much as 160,000 years old indicates that global temperature and CO2 levels in the atmosphere do in fact rise and fall together. Temperature readings that are ^ not necessarily reliable suggest that the globe has warmed about 1 degrees in the past 100 years (see chart). And the decade of the 1980s was the warmest in this century.

SOME OF the unanswered questions are most intriguing. Global climate is the product of interactions among many elements. The largest single factor is the oceans, which have 1,000 times more capacity to store heat than the atmosphere. But climate is also affected by land masses, the biosphere (living things), the atmosphere, clouds, glaciers, the sun, the tilt of the earth, and more. The computer programs that predict global warming are mainly simulations of the atmosphere called general circulation models. So far they take little account of how the atmosphere is cooled or warmed by the oceans, clouds, and other factors. Then consider the Case of the Missing CO2. The finite quantity of carbon on earth is recycled through the atmosphere, water, and living things. Plants, for instance, take carbon dioxide from the atmosphere, break it apart, give off the oxygen, and use the carbon to build new cells and grow. When plants die and decay, CO2 is formed and passes into the air or water. Fossil fuels like coal and oil constitute a huge store of carbon that was taken out of the cycle millions of years ago when the vegetation that created them became trapped in the earth. Man is putting that carbon back into circulation by burning fossil fuels. Using a rough estimate of how much fossil fuel the modern world has consumed, scientists have calculated how much CO2 has been released into the atmosphere by the process. But when they analyze the atmosphere, they find only half the predicted amount. At a recent hearing held by the National Academy of Sciences' policy committee, James Hansen was asked where the carbon went. ''We're not really sure,'' he replied. ''There must be a carbon sink somewhere. Maybe it's the northern forest.'' Translation: Something big is absorbing carbon -- maybe all those trees in Canada and Scandinavia and the U.S.S.R. But for forests to soak up half the CO2 produced, the trees would have to be either bigger or more numerous than they were before the Industrial Revolution. A spokesman for the American Forestry Association says no evidence indicates that has happened. Maybe the mysterious sink is something in the oceans, which contain 55 times as much carbon as the atmosphere and 20 times more than plants. Nor were the experts able to tell the NAS hearing what causes CO2 levels to rise and fall with temperature through time, as evidenced by those samples of glacial ice. Clearly some natural force or forces can move CO2 levels up and down independently of man. No one knows what they are -- or what they are up to right now. ANOTHER MYSTERY: the so-called feedbacks, reactions to global warming that will either speed it up or slow it down. An atmosphere richer in CO2, a natural fertilizer, could make plants grow larger. Will we have basketball- size tomatoes, or just weeds as big as telephone poles? How much carbon will super plants soak up? Or take clouds, a potentially bigger factor. In a warmer world, more water will evaporate and the weather could become cloudier. More clouds could reflect more sunlight. But they will also trap more heat from the earth's surface because they are made of water vapor, another greenhouse gas. Will more clouds have a net warming or a net cooling effect? One fascinating aspect of the cloud question: Scientists have discovered that sulfur dioxide, a pollutant from smokestacks that is blamed for acid rain, also causes clouds to form. That might explain why many industrial regions of Earth have not warmed up as much in the past century as the computer climate models say they should have. And it raises the possibility that if the U.S. scrubs sulfur from smokestack emissions, the air will be not only cleaner but hotter as well. Says Ned Ostenso, an assistant administrator of the National Oceanic and Atmospheric Administration: ''Our knowledge of the global system is still pretty rudimentary, but we're learning that it's made up of wheels within wheels within wheels.'' No one knows how these feedbacks will add up, and that has led to some sharp exchanges between the alarmists and the skeptics among climatologists. One prominent skeptic, Richard Lindzen, a professor of meteorology at MIT, recently suggested that in certain climes global warming might decrease the amount of water vapor in the upper atmosphere, which would have a cooling effect. Alarmists scoff at Lindzen's suggestion, arguing that water vapor will increase in the lower atmosphere -- with the opposite result. With uncertainties like these, it is perhaps not surprising that sometimes the alarmists can be hard to tell from the skeptics. Lindzen says his hunch is that the globe will warm between 1 degrees and 2 degrees in the next century. One of his chief antagonists, Stephen Schneider of the National Center for Atmospheric Research in Boulder, Colorado, says he's 90% sure of at least that much warming. NASA's Hansen, who stunned Congress in 1988 by suggesting that global warming had begun, says he is more optimistic today that the worst-case scenario won't come to pass. Still, he adds, ''We could be building a time bomb for ourselves.'' What then should be done? The Congressional Budget Office is studying ''carbon charges,'' excise taxes that would penalize the use of carbon-rich fuels like coal. The less carbon in the fuel, the lighter the tax. The CBO estimates that the charges needed just to keep the CO2 emission rate where it is would come to $17 for a ton of coal, which now costs $30, and 8.6 cents per gallon of gasoline. A study by the Electric Power Research Institute, an industry R&D organization, estimates that after discounting to present value, the cost to the U.S. economy of cutting CO2 emissions 20% would be $800 billion to $3.6 trillion over the next century. The lower estimate assumes growing use of clean energy at a price competitive with coal. Such a technology exists: nuclear power. With so much at stake, the Bush Administration is wisely seeking international participation in any plan to roll back CO2 emissions. The expense of unilateral reductions would put the U.S. at a disadvantage in world markets while its industrialized competitors and the Third World increase emissions in pursuit of economic development. Besides, one country acting alone wouldn't have much effect. The U.S. is participating in an intergovernmental panel on climate control that will issue a scientific report in the fall on the dangers of global warming. Members, including Western European powers, Japan, and the Soviet Union, will try to get together on solutions. The model is the Montreal protocol of 1988, in which 30 nations agreed to phase out CFCs. Roger Sedjo of Resources for the Future, a research center in Washington, D.C., estimates that planting 1.1 billion acres of new forest, roughly equivalent to the area of the contiguous states west of the Mississippi, would soak up all 2.9 billion tons of carbon that gets added to the atmosphere each year. Says Sedjo: ''We are talking big numbers, increasing the world's forests by some 16% at a cost of maybe $500 billion. But if this is an emergency and it's paid for out of a global checkbook, it can be done.'' BUSINESS CAN DO a number of things that make sense in their own right and also help limit global warming. Pacific Gas & Electric is building a $10 million research center to look for ways to save energy in lighting. Du Pont, among others, is at work on substitutes for CFCs, which deplete the ozone layer. Weyerhaeuser is developing loblolly pines that get by with little water. Trees also help conserve energy: Lawrence Berkeley Laboratories and the American Forestry Association estimate that the shade produced by 100 million trees planted in empty spaces in suburban and commercial neighborhoods around the country could save $4 billion a year in air conditioning bills. One creative new product comes from Ben & Jerry's Homemade Inc. The Vermont gourmet ice cream maker is mixing a new flavor called Rain Forest Crunch that contains Brazil nuts harvested from the wilds of the Amazon jungle. Since carbon dioxide entering the atmosphere as a result of deforestation accounts for roughly 20% of the worldwide annual buildup of CO2, Ben & Jerry's figures that creating demand for nuts might save the trees from the settler's ax -- and help curb global warming. Think of the salve to your conscience next time you happen on some Rain Forest Crunch. Eat all you want: Save a tree.

CHART: NOT AVAILABLE CREDIT: NO CREDIT CAPTION: 100 YEARS OF CO2 Some scientists fear that the century-long rise in global temperatures, shown here as a five-year running average, results partly from carbon dioxide produced by burning fossil fuels like coal and oil. Skeptics point out, however, that most of the temperature rise came before CO2 levels started moving up sharply and that the average temperature worldwide actually fell between 1940 and the mid-1960s.