Feeling the heat

The average temperature of the earth's surface has risen by 0.74 degrees C since the late 1800s. It is expected to increase by another 1.8° C to 4° C by the year 2100 - a rapid and profound change - should the necessary action not be taken. Even if the minimum predicted increase takes place, it will be larger than any century-long trend in the last 10,000 years.

The principal reason for the mounting thermometer is a century and a half of industrialization: the burning of ever-greater quantities of oil, gasoline, and coal, the cutting of forests, and the practice of certain farming methods.

These activities have increased the amount of "greenhouse gases" in the atmosphere, especially carbon dioxide, methane, and nitrous oxide. Such gases occur naturally - they are critical for life on earth, they keep some of the sun's warmth from reflecting back into space, and without them the world would be a cold and barren place. But in augmented and increasing quantities, they are pushing the global temperature to artificially high levels and altering the climate. Eleven of the last 12 years are the warmest on record, and 1998 was the warmest year.

Climate change can be difficult - you could ask the dinosaurs, if they weren't extinct. The prevailing theory is that they didn't survive when a giant asteroid struck the earth 65 million years ago, spewing so much dust into the air that sunlight was greatly reduced, temperatures plummeted, many plants didn't grow and the food chain collapsed.

What happened to the dinosaurs is a rare example of climate change more rapid than humans are now inflicting on themselves. . . but not the only one. Research on ice cores and lake sediments shows that the climate system has suffered other abrupt fluctuations in the distant past. The climate appears to have "tipping points" that can send it into sharp lurches and rebounds. Although scientists are still analyzing what happened during those earlier events, it's clear that an overstressed world with 6.3 billion people is a risky place to be carrying out uncontrolled experiments with the climate.

The current warming trend is expected to cause extinctions. Numerous plant and animal species, already weakened by pollution and loss of habitat, are not expected to survive the next 100 years. Human beings, while not threatened in this way, are likely to face mounting difficulties. Recent severe storms, floods and droughts, for example, appear to show that computer models predicting more frequent "extreme weather events" are on target.

The average sea level rose by 10 to 20 cm during the 20th century, and an additional increase of 18 to 59 cm is expected by the year 2100. (Higher temperatures cause ocean volume to expand, and melting glaciers and ice caps add more water.) If the higher end of that scale is reached, the sea could overflow the heavily populated coastlines of such countries as Bangladesh, cause the disappearance of some nations entirely (such as the island state of the Maldives), foul freshwater supplies for billions of people, and spur mass migrations. Agricultural yields are expected to drop in most tropical and sub-tropical regions - and in temperate regions too - if the temperature increase is more than a few degrees C. Drying of continental interiors, such as central Asia, the African Sahel, and the Great Plains of the United States, is also forecast. These changes could cause, at a minimum, disruptions in land use and food supply. And the range of diseases such as malaria may expand.

Global warming is a "modern" problem - complicated, involving the entire world, tangled up with difficult issues such as poverty, economic development and population growth. Dealing with it will not be easy. Ignoring it will be worse.