There is no such thing as normal weather
There is no such thing as normal weather. the average daytime high temperature for New York City this week should be 14°C, but on any given day the mercury will almost certainly fall short of that mark or overshoot it, perhaps by a lot. Manhattan thermometers can reach 18° in January every so often and plunge to 10° in July. And seasons are rarely normal. Winter snowfall and summer heat waves beat the average some years and fail to reach it in others. It's tough to pick out over-
all changes in climate in the face of these natural fluctuations. An unusually warm year, for example, or even three in a row don't necessarily signal a general trend.
Yet the earth's climate does change. Ice ages have frosted the planet for tens of thousands of years at a stretch, and periods of warmth have pushed the tropics well into what is now the temperate zone. But given the normal year-to-year variations, the only reliable signal that such changes may be in the works is a long-term shift in worldwide temperature.
And that is precisely what's happening. A decade ago, the idea that the planet was warming up as a result of human activity was largely theoretical. We knew that since the Industrial Revolution began in the 18th century, factories and power plants and automobiles and farms have been loading the atmosphere with heat-trapping gases, including carbon dioxide and methane. But evidence that the climate was actually getting hotter was still murky.
Not anymore. As an authoritative report issued a few weeks ago by the United Nations-sponsored Intergovernmental Panel on Climate Change makes plain, the trend toward a warmer world has unquestionably begun. Worldwide temperatures have climbed more than .5°C over the past century, and the 1990s were the hottest decade on record. After analyzing data going back at least two decades on everything from air and ocean temperatures to the spread and retreat of wildlife, the ipcc asserts that this slow but steady warming has had an impact on no fewer than 420 physical processes and animal and plant species on all continents.
Glaciers, including the legendary snows of Kilimanjaro, are disappearing from mountaintops around the globe. Coral reefs are dying off as the seas get too warm for comfort. Drought is the norm in parts of Asia and Africa. El Niño events, which trigger devastating weather in the eastern Pacific, are more frequent. The Arctic permafrost is starting to melt. Lakes and rivers in colder climates are freezing later and thawing earlier each year. Plants and animals are shifting their ranges poleward and to higher altitudes, and migration patterns for animals as diverse as polar bears, butterflies and beluga whales are being disrupted.
Faced with these hard facts, scientists no longer doubt that global warming is happening, and almost nobody questions the fact that humans are at least partly responsible. Nor are the changes over. Already, humans have increased the concentration of carbon dioxide, the most abundant heat-trapping gas in the atmosphere, to 30% above pre-industrial levels-and each year the rate of increase gets faster. The obvious conclusion: temperatures will keep going up.
Unfortunately, they may be rising faster and heading higher than anyone expected. By 2100, says the ipcc, average temperatures will increase between 1.4°C and 5.8°C-more than 50% higher than predictions of just a half-decade ago. That may not seem like much, but consider that it took only a 5°C shift to end the last ice age. Even at the low end, the changes could be problematic enough, with storms getting more frequent and intense, droughts more pronounced, coastal areas ever more severely eroded by rising seas, rainfall scarcer on agricultural land and ecosystems thrown out of balance.
But if the rise is significantly larger, the result could be disastrous. With seas rising as much as 1 m, enormous areas of densely populated land-coastal Florida, much of Louisiana, the Nile Delta, the Maldives, Bangladesh-would become uninhabitable. Entire climatic zones might shift dramatically, making central Canada look more like central Illinois, Georgia more like Guatemala. Agriculture would be thrown into turmoil. Hundreds of millions of people would have to migrate out of unlivable regions.
Public health could suffer. Rising seas would contaminate water supplies with salt. Higher levels of urban ozone, the result of stronger sunlight and warmer temperatures, could worsen respiratory illnesses. More frequent hot spells could lead to a rise in heat-related deaths. Warmer temperatures could widen the range of disease-carrying rodents and bugs, such as mosquitoes and ticks, increasing the incidence of dengue fever, malaria, encephalitis, Lyme disease and other afflictions. Worst of all, this increase in temperatures is happening at a pace that outstrips anything the earth has seen in the past 100 million years. Humans will have a hard enough time adjusting, especially in poorer countries, but for wildlife, the changes could be devastating.
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Like any other area of science, the case for human-induced global warming has uncertainties-and like many pro-business lobbyists, President Bush has proclaimed those uncertainties a reason to study the problem further rather than act. But while the evidence is circumstantial, it is powerful, thanks to the ipcc's painstaking research. The U.N.-sponsored group was organized in the late 1980s. Its mission: to sift through climate-related studies from a dozen different fields and integrate them into a coherent picture. "It isn't just the work of a few green people," says Sir John Houghton, one of the early leaders who at the time ran the British Meteorological Office. "The ipcc scientists come from a wide range of backgrounds and countries."
Measuring the warming that has already taken place is relatively simple; the trick is unraveling the causes and projecting what will happen over the next century. To do that, ipcc scientists fed a wide range of scenarios involving varying estimates of population and economic growth, changes in technology and other factors into computers. That process gave them about 35 estimates, ranging from 6 billion to 35 billion tons, of how much excess carbon dioxide will enter the atmosphere.
Then they loaded those estimates into the even larger, more powerful computer programs that attempt to model the planet's climate. Because no one climate model is considered definitive, they used seven different versions, which yielded 235 independent predictions of global temperature increase. That's where the range of 1.4°C to 5.8°C (2.5°F to 10.4°F ) comes from.
The computer models were criticized in the past largely because the climate is so complex that the limited hardware and software of even a half-decade ago couldn't do an adequate simulation. Today's climate models, however, are able to take into account the heat-trapping effects not just of CO2 but also of other greenhouse gases, including methane. They can also factor in natural variations in the sun's energy and the effect of substances like dust from volcanic eruptions and particulate matter spewed from smokestacks.
That is one reason the latest ipcc predictions for temperature increase are higher than they were five years ago. Back in the mid-1990s, climate models didn't include the effects of the El Chichon and Mount Pinatubo volcanic eruptions, which threw enough dust into the air to block out some sunlight and slow down the rate of warming. That effect has dissipated, and the heating should start to accelerate. Moreover, the ipcc noted, many countries have begun to reduce their emissions of sulfur dioxide in order to fight acid rain. But sulfur dioxide particles, too, reflect sunlight; without this shield, temperatures should go up even faster.
The models still aren't perfect. One major flaw, agree critics and champions alike, is that they don't adequately account for clouds. In a warmer world, more water will evaporate from the oceans and presumably form more clouds. If they are billowy cumulus clouds, they will tend to shade the planet and slow down warming; if they are high, feathery cirrus clouds, they will trap even more heat.
Research by M.I.T. atmospheric scientist Richard Lindzen suggests that warming will tend to make cirrus clouds go away. Another critic, John Christy of the University of Alabama in Huntsville, says that while the models reproduce the current climate in a general way, they fail to get right the amount of warming at different levels in the atmosphere. Neither Lindzen nor Christy (both ipcc authors) doubts, however, that humans are influencing the climate. But they question how much-and how high temperatures will go. Both scientists are distressed that only the most extreme scenarios, based on huge population growth and the maximum use of dirty fuels like coal, have made headlines.
It won't take the greatest extremes of warming to make life uncomfortable for large numbers of people. Even slightly higher temperatures in regions that are already drought- or flood-prone would exacerbate those conditions. In temperate zones, warmth and increased CO2 would make some crops flourish-at first. But beyond 1.5° of warming, says Bill Easterling, a professor of geography and agronomy at Penn State and a lead author of the ipcc report, "there would be a dramatic turning point. U.S. crop yields would start to decline rapidly." In the tropics, where crops are already at the limit of their temperature range, the decrease would start right away.
Even if temperatures rise only moderately, some scientists fear, the climate would reach a "tipping point"-a point at which even a tiny additional increase would throw the system into violent change. If peat bogs and Arctic permafrost warm enough to start releasing the methane stored within them, for example, that potent greenhouse gas would suddenly accelerate the heat-trapping process.
By contrast, if melting ice caps dilute the salt content of the sea, major ocean currents like the Gulf Stream could slow or even stop, and so would their warming effects on northern regions. More snowfall reflecting more sunlight back into space could actually cause a net cooling. Global warming could, paradoxically, throw the planet into another ice age.
Even if such a tipping point doesn't materialize, the more drastic effects of global warming might be only postponed rather than avoided. The ipcc's calculations end with the year 2100, but the warming won't. World Bank chief scientist, Robert Watson, currently serving as ipcc chair, points out that the CO2 entering the atmosphere today will be there for a century. Says Watson: "If we stabilize [CO2 emissions] now, the concentration will continue to go up for hundreds of years. Temperatures will rise over that time."
That could be truly catastrophic. The ongoing disruption of ecosystems and weather patterns would be bad enough. But if temperatures reach the ipcc's worst-case levels and stay there for as long as 1,000 years, says Michael Oppenheimer, chief scientist at Environmental Defense, vast ice sheets in Greenland and Antarctica could melt, raising sea level more than 9 m. Florida would be history, and every city on the U.S. Eastern seaboard would be inundated.
In the short run, there's not much chance of halting global warming, not even if every nation in the world ratifies the Kyoto Protocol tomorrow. The treaty doesn't require reductions in carbon dioxide emissions until 2008. By that time, a great deal of damage will already have been done. But we can slow things down. If action today can keep the climate from eventually reaching an unstable tipping point or can finally begin to reverse the warming trend a century from now, the effort would hardly be futile. Humanity embarked unknowingly on the dangerous experiment of tinkering with the climate of our planet. Now that we know what we're doing, it would be utterly foolish to continue.
-Reported by David Bjerklie, Robert H. Boyle and Andrea Dorfman/New York and Dick Thompson/Washington
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