Maybe we've just had a long stretch of strange weather. Or maybe it's just beginning.
In the Midwest, the "flood of the century" poured through towns on the Mississippi River twice in three years. California suffered through years of drought _ then disastrous rains that brought record flood losses and mudslides.
El Nino, the episodic warming of the Pacific, lingered for an unheard of five years, changing climates across continents. When it finally departed in 1995, the Atlantic answered with a fury. Florida endured its busiest storm season ever _ three hurricanes and one tropical storm in five months.
In the Antarctic last year, an iceberg the size of Rhode Island broke loose and drifted to sea. Half a world north, England had its hottest summer since the 1600s.
Winter brought a storm that set snowfall records in several U.S. cities and halted the federal government as effectively as a budget shutdown. Behind it came rain and floods _ in January. February brought much of the nation its coldest weather in years, then flooded the Northwest with 8 inches of rain.
What's going on?
Possibly nothing extraordinary. Scientists who study long-term climate trends say any or all of these events could be chance variations in weather.
But they also see another, more ominous, possibility: that humans are loading the climate dice. They fear the consequences ultimately will range from spreading disease epidemics to mass extinctions of plant and animal species.
When weather is measured not by week or by nation, but worldwide over decades, the evidence suggests an accumulation of fossil fuel gases is slowly warming planet Earth.
Climate scientists expect the first noticeable signs of this warming to be extreme weather _ record heat waves, huge rainstorms, cycles of drought and downpour _ from a cranked-up atmospheric system.
If so, the 1990s could be a mere warmup to the wild weather of a coming century in which global temperatures are projected to rise at a rate unmatched in human history.
In the United States, researchers at the National Climatic Data Center demonstrated recently that our nation's weather already has changed in slight but significant ways. During the 20th century, average temperatures have climbed. Autumn and winter are getting wetter, and more of our rain is falling in big storms.
"There's no smoking gun," says Thomas Karl, the center's senior scientist. But "when you're 95 percent sure, you probably have a responsibility to tell people that things look as if we're already impacting the climate. Not only the global climate, but also the regional climate where we live and work."
James Hansen, director of the NASA Goddard Institute for Space Studies, also thinks we are changing the odds of extreme weather today.
"It's always a tricky thing to discuss this," he says. "I think that global warming does affect the probabilities of extreme events. Especially the extremes of the hydrological cycles _ on the one hand droughts and forest fires, on the other hand heavy precipitation and floods.
"But that doesn't mean every drought or every flood can be blamed on global warming. It's just a matter of probability."
How much could we change the probabilities?
The prophets of our time pose this question to supercomputers that convert complex physical equations into portraits of the future.
They are getting some scary answers.
In the coming century, climatologists predict average temperatures will rise enough to melt glaciers and lift sea levels. They expect this increase to be uneven, delivering extra heat to some parts of the world and possibly cooling others if ocean currents change. They foresee rainfall and temperature shifts large enough to alter agricultural regions, spread epidemics of tropical diseases and eradicate plants and animals unable to adapt or migrate.
They also caution that the most carefully calculated picture of a climate changed by gases that trap solar heat is fraught with uncertainties.
In June 1991, one of nature's most spectacular weather influences erupted, reversing a long warming trend and cooling much of the public discussion about global climate change.
Mount Pinatubo, the largest volcano this century, injected 20 megatons of sulfur dioxide into the stratosphere, where it formed a global layer of sulfuric acid droplets that reflected sunlight back to space.
In Pinatubo's ashes, Hansen saw a chance to test scientific understanding of the mathematics of climate change. Nature's "own great climate experiment," he called it.
Using the Goddard institute's climate change model, he published a paper months after the eruption that predicted a dramatic but temporary cooling worldwide, followed by "a return to record warm levels in the later 1990s."
Today, that forecast looks remarkable. Last month the institute reported that 1995, in a tie with 1990, was the world's warmest year on record.
Hansen doubts the record will last long.
Barring another Pinatubo, he predicts record global temperatures again in the late 1990s. He ventures that "most people will notice within five years" that the climate is changing.
In the coming decades he foresees more frequent heat waves, droughts and intense storms, and ultimately, "climate changes far beyond the range that man has ever experienced."
A century from now, he says, "it's certainly going to be warmer than it has been in millions of years."
Hansen expects average global temperatures to rise 2 to 3 degrees Celsius _ or 3.6 to 5.4 degrees Fahrenheit _ during the 21st century.
That's at the high end of climate change forecasts from the world's leading atmospheric scientists, but just barely.
Within the scientific community, a broad consensus has been reached that people probably have begun to affect weather on a global scale already.
It came in the form of a report written and reviewed by 2,500 scientists and accepted in December by the United Nations-sponsored Intergovernmental Panel on Climate Change.
The scientists reported that average global temperatures have risen close to 1 degree Fahrenheit since the late 19th century, and sea levels have risen between 4 and 10 inches. "The balance of evidence," they concluded, "suggests a discernible human influence on global climate."
This influence is expected to grow as greenhouse gases build up in the atmosphere. In the IPCC report, scientists estimate Earth will get 2 to 6 degrees Fahrenheit warmer by 2100 and sea levels will rise 6 inches to 3 feet, potentially doubling the number of people vulnerable to flooding from storm surges.
In Florida, one of the world's flattest places, a 1-foot rise in sea level could erode 100 to 1,000 feet of sand from its beaches.
Even at the low end of this predicted climate change, the IPCC report notes, "the average rate of warming would probably be greater than any seen in the last 10,000 years . . ."
What does this mean?
A temperature increase of a few degrees in a century may not sound like much, considering that a cold front can lower the temperature in Chicago by 50 degrees in an afternoon.
But over time, on a worldwide scale, small and sustained temperature shifts can change climate radically.
Global temperatures during the last Ice Age, which buried Canada and parts of the United States under ice, averaged about 50 degrees Fahrenheit. That's about 9 degrees cooler than today _ a difference not far beyond the range of temperature changes projected in the next century. And the dinosaur age, when tropical plants grew in what is now Alaska? About 80 degrees, 20 degrees warmer than today's average world temperature.
Much smaller climate shifts have been associated with historic disasters and remarkable achievements: the 19th century Irish potato famine, the great cathedral construction and plagues of the Middle Ages, the establishment of Norse colonies in Iceland and Greenland 500 years before Columbus _ colonies that ultimately starved and perished in a cooling world.
An average temperature difference of a few degrees can shift the ideal place to grow corn by hundreds of miles. It can change which trees grow in a forest and which animals live there. It can extend the range of deadly infectious diseases.
Last month, public health scientists warned in a Journal of the American Medical Association report that global warming could cause as many as 1-million more deaths each year from malaria alone.
Other diseases considered most likely to spread: St. Louis encephalitis and dengue fever, both carried by temperature-sensitive mosquitoes, and schistosomiasis, a fever transmitted by snails.
Jonathan Patz, the primary researcher, sees Florida as well-protected from these diseases, but not invulnerable.
Our risk is minimized by mosquito control programs, treated drinking water, the air-conditioned safety of indoor living and a public health system that carefully tracks any sign of a new epidemic.
But "if there's a dropoff in support for disease prevention, I would be quite concerned in Florida," Patz says.
Rita Colwell, president of the American Association for the Advancement of Science, sees a risk that global warming also could promote two diseases associated with certain species of plankton: cholera, and the shellfish toxin commonly called red tide.
Given warming oceans, "I suspect _ and this is just hypothesis _ that we will see increasing episodes like the Latin American experience in 1991-92 when several thousand people died" from cholera, she said.
On the positive side, warmer winters can reduce the risks of respiratory diseases.
More carbon dioxide
The gases that can change Earth's climate amount to less than 1 percent of its atmosphere. What makes them special is the way they react to radiant energy. They let light from the sun pass through relatively freely, but they absorb most of the infrared heat rays emitted from Earth _ just like a greenhouse.
The most important of these greenhouse gases is water vapor. The main greenhouse gas released by human industry is carbon dioxide, a byproduct of every fire, smokestack and tailpipe. The main culprits: coal, oil and deforestation.
These gases make life possible. Without them, average temperatures on Earth would hover around zero.
The risk is that people are putting too much of a good thing in the air too fast. The history of ancient climates, deduced partly from the composition of air frozen in Antarctic ice, indicates a close correlation between CO and global temperatures. And carbon dioxide levels are exceeding anything seen in 200,000 years.
Already, the atmosphere holds one-third more carbon dioxide than the air our ancestors breathed, largely because of the burning of fossil fuels and the loss of forests that consume carbon dioxide. During the 21st century, the natural level of carbon dioxide is expected to double _ and trap more solar heat.
Solar radiation is the engine that drives the weather, from its cycle of evaporation and rain to the temperature conflicts between poles and equator that keep air and ocean currents in perpetual motion.
Tinker with the air filter to that engine, and the physical results range from enormously complicated to unpredictable.
Double the levels of carbon dioxide and temperatures rise, melting ice and exposing new surfaces that absorb radiation and intensify the greenhouse effect. More water becomes vapor, a fellow greenhouse gas. Vapor could become low clouds, which offset the greenhouse effect by shading sunlight, or high clouds that trap heat and intensify the effect.
The variables multiply. Oceans warm more slowly than continents, delaying the greenhouse effect. Coal plants emit carbon dioxide and clouds of smog _ miniature Pinatubos that can diminish or even reverse a warming trend locally.
As temperature conflicts increase between land and sea, or between industrial and non-industrial regions, so could prospects for violent storms.
Raise the heat, and soil dries. At the same time, more water evaporates and must fall somewhere. One predicted outcome: drought in some places, deluges in others.
To peer into the future, scientists calculate the combined physical effects of human weather influences on supercomputers that produce climate models, known as general circulation models.
To give these models a reality check, they also are asked to "predict" what is known about present and past climates.
The newest models used by scientists at the Intergovernmental Panel on Climate Change lowered their previous forecast of global warming by about one-third, primarily by accounting for the local cooling effects of smog.
They also come much closer to tracking real climate trends of the last 30 years. What emerges is "an embarrassment of correlation" that points to a human fingerprint on weather patterns, says Stephen Schneider, a prominent climatologist who participated in the IPCC conference.
He recalls asking a roomful of scientists confronted with this evidence "how many people believe more strongly" in the reality of global warming. "Everybody in the room put their hand up."
In the political world, the weight of scientific opinion remains to be tested.
Given uncertainties about the timing and severity of global warming, coal and oil interests have argued against changing energy consumption in ways that could disrupt the global economy.
Within the United States, these interests support the work of scientists who publicly dispute the seriousness of the greenhouse effect.
In World Climate Report, an industry-financed publication, University of Virginia climatology professor Patrick Michaels derides the "climate apocalysts" of the scientific establishment as people who manage to find evidence of global warming in a blizzard.
Yet even Michaels believes in the greenhouse effect. He just expects the effect to be small, and possibly not at all unpleasant. He notes that the largest global warming observed to date has occurred in a very cold place.
"Who would have predicted _ who has mentioned _ such a remarkably salubrious result as a warming of the Siberian winter?" he asks.
Our energy needs
As global atmospheric problems go, ozone depletion was relatively simple. A limited number of manufacturers produced CFCs, and chemical substitutes were available.
Global warming is a much more difficult problem.
Fossil fuels are wedded to the wealth of nations, to the power and prosperity of the West, to the ambitions of a developing world.
The major causes _ coal, oil and deforestation _ are subsidized by U.S. tax policies that provide depletion allowances, mining reclamation funds and cheap timber sales.
"We have urged developed countries, quite sanely, to rethink their energy policies," said Irving Mintzer, an author of the IPCC report. "That advice is so sane that we might want to take it ourselves."
In today's world, everyone produces greenhouse gases. Worldwide, the average person produces about a ton every year; in the United States, about 6 tons a year. Carbon, the basic ingredient, is stored in every fossil fuel and every tree. Combined with oxygen in combustion, it stays in the air a very long time.
Robert Watson, who headed the U.S. delegation to the IPCC conference, considers global warming inevitable. "I do not believe any policies, enacted locally or globally, will be enough to avoid some climate change," he says.
He thinks much can be done to limit the scope of that change, but only if wealthy nations help populous nations such as China and India develop with alternative fuels and energy-efficient technologies.
"Without concerted action in developing countries, we will fail," he says. "This is going to have to be a global effort, no question about it."
So far, efforts to halt the growth of carbon dioxide emissions have failed even in the United States, the world's top emitter of greenhouse gases.
Four years ago, the United States joined other developed nations in a non-binding commitment to stabilize emissions at 1990 levels by the year 2000.
The Clinton administration opted to honor that commitment with voluntary energy efficiency programs, many of which were cut by Congress. The nation's economy grew faster than expected. Energy prices stayed low. To the auto industry's delight, millions of Americans switched from small to larger cars, and from cars to vans, trucks and sport vehicles.
Slowly and steadily, U.S. greenhouse gas emissions grew.
At the White House Office of Science and Technology Policy, Watson has been watching the emissions data carefully. With four years to go in the decade, he concedes the United States will not keep its global warming promise.
The size of the gap between commitment and reality is still being measured. But "we're clearly going to fall short," he says.
Just a few degrees:
A global temperature increase of a few degrees may not sound like much, considering that local temperatures can vary greatly from one day to the next. But when average temperatures worldwide change for long periods of time, dramatic climate changes result.
80 degrees F: Dinosaurs
150-million years ago the dinosaurs lived in a much warmer climate than we do today. The average global temperature was around 80 degrees.
62-65 degrees F 2090
Scientists estimate that in the next 100 years the average global temperature could rise as much as 2-6 degrees, compared with only one degree over the past 100 years.
59 degrees F 1990
58 degrees F 1890
Beginning of the Industrial Revolution. Burning of fossil fuels to power growing factories increases dramatically.
50 degrees F Ice age
Scientists fear that global warming may have these effects:
Sea levels rise
Agricultural regions shift
Ocean currents shift
Rain, snow patterns change
Severe weather increases
What is the greenhouse effect?
Increased carbon dioxide emissions are contributing to a rise in global temperatures _ the so-called "greenhouse effect", which could adversely affect climate.
Earth in balance
Sun's energy penetrates atmosphere
Heat radiated from Earth's surface escapes through the atmosphere.
Some carbon dioxide escapes into atmosphere; most absorbed by trees, plants.
Imbalance caused by too much carbon dioxide
Carbon dioxide acts like greenhouse windows, trapping the sun's energy.
Trapped heat causes average temperatures to rise worldwide.
Burning of fossil fuel (coal, gasoline, other petroleum products) adds more carbon dioxide to the atmosphere than can be absorbed naturally.
What this means to us:
Global temperatures are expected to rise during the 21st century at a rate unmatched in human history, so the effects are inherently unpredictable. But scientists expect tropical diseases will spread, agricultural belts will move toward the poles, shorelines will erode, some forests may die, and severe weather _ record heat waves, large storms, regional droughts _ is likely to increase.
Sources: Knight-Ridder Tribune, National Geographic Research & Exploration, World Book.