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 U.N.-sponsored Intergovernmental Panel on Climate Change makes plain, the trend toward a warmer world has unquestionably begun. Worldwide temperatures have climbed more than 1[degree]F 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 Nino 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 2.5[degrees]F and 10.4[degrees]F--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 9[degrees]F 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 3 ft., 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.