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The relationship between CO2 emissions and global warming is more than theoretical. Two weeks ago, a Soviet-French research team announced impressive evidence that CO2 levels and worldwide average temperatures are intimately related. By looking at cores of Antarctic ice, the researchers showed that over the past 160,000 years, ice ages have coincided with reduced CO2 levels and warmer interglacial periods have been marked by increases in production of the gas.
Although the region-by-region effects of rapid atmospheric warming are far from clear, scientists are confident of the overall trend. In the next half- century, they fear dramatically altered weather patterns, major shifts of deserts and fertile regions, intensification of tropical storms and a rise in sea level, caused mainly by the expansion of sea water as it warms up.
The arena in which such projected climatic warming will first be played out is the atmosphere, the ocean of gases that blankets the earth. It is a remarkably thin membrane: if the earth were the size of an orange, the atmosphere would be only as thick as its peel. The bottom layer of the peel, the troposphere, is essentially where all global weather takes place; it extends from the earth's surface to a height of ten miles. Because air warmed by the earth's surface rises and colder air rushes down to replace it, the troposphere is constantly churning. A permanent air flow streams from the poles to the equator at low altitudes, and from the equator to the poles at higher levels. These swirling air masses, distorted by the rotation of the earth, generate prevailing winds that drive weather across the hemispheres and aid the spread of pollutants into the troposphere. Above this turmoil, the stratosphere extends upward to about 30 miles. In the lower stratosphere, however, rising air that has been growing colder at higher and higher altitudes begins to turn warmer. The reason, in a word: ozone.
Ozone (O3) is a form of oxygen that rarely occurs naturally in the cool reaches of the troposphere. It is created when ordinary oxygen molecules (O2) are bombarded with solar ultraviolet rays, usually in the stratosphere. This radiation shatters the oxygen molecules, and some of the free oxygen atoms recombine with O2 to form O3. The configuration gives it a property that two- atom oxygen does not have: it can efficiently absorb ultraviolet light. In doing so, ozone protects oxygen at lower altitudes from being broken up and keeps most of these harmful rays from penetrating to the earth's surface. The energy of the absorbed radiation heats up the ozone, creating warm layers high in the stratosphere that act as a cap on the turbulent troposphere below.
Ozone molecules are constantly being made. But they can be destroyed by any of a number of chemical processes, most of them natural. For example, the stratosphere receives regular injections of nitrogen-bearing compounds, such as nitrous oxide. Produced by microbes and fossil-fuel combustion, the gas rides the rising air currents to the top of the troposphere. Forced higher still by the tremendous upward push of tropical storms, it finally enters and percolates slowly into the stratosphere.