We can be grateful for the greenhouse effect. Without the atmosphere's natural load of heat-trapping water vapor and carbon dioxide (CO2), the average surface temperature would be minus 18 degrees C (near 0 degrees F.). Earth would be covered with ice.
What concerns delegates to the climate change conference in Kyoto, Japan, this week is that the burning of fossil fuels and other human activities add extra CO2 and other heat-trapping gases to the atmosphere. This could cause Earth to warm beyond what is desirable.
Computer simulations of what might happen are uncertain. They strongly suggest, however, that Earth's average surface temperature could rise several degrees Celsius over the next 50 to 100 years.
The consequences of such climate change also are unclear. Sea level could rise as warming ocean surface water expands and land-based ice sheets melt. Present patterns of precipitation and drought could change. The North American grain belt, for example, could dry out. Arctic tundra could melt. The slight global warming of about half a degree Celsius that has occurred during this century may be at least partly due to human activity.
Scientists have long been aware of such possible changes. Svante Arrhenius of Sweden first worked out the CO2/climate connection in 1896. It has become a political concern over the past decade because many scientists think such changes may soon be upon us. Delegates to the Kyoto conference hope to moderate any climate change by curbing man-made CO2 emissions. But rising CO2 concentration is only part of the problem.
There are other greenhouse gases to consider. They include methane, nitrous oxide from fertilizers, a variety of industrial chemicals, and the chlorofluorocarbon (CFC) refrigerants that destroy the stratospheric ozone layer. Use of CFCs is phasing out. But those already in the stratosphere will linger for decades. Molecule for molecule, they are 10,000 times more effective heat trappers than is CO2.
Taken together, these gases could double the warming effect expected from rising CO2 concentration alone. All greenhouse gases absorb some of Earth's infrared (heat) radiation. But they do so at different wavelengths. Water vapor and CO2 are transparent to infrared between 7 and 13 micrometers. This has provided an escape window for more than 70 percent of Earth's heat emissions. The other greenhouse gases dirty that window.
The increased CO2 burden the atmosphere already has, plus the increasing burden of other greenhouse gases, means that the challenge of coping with man-made climate change will be with us for many decades, even centuries, to come. This is why climate scientists warn that there are no simple solutions to global warming.