Humankind’s influence on the atmosphere and the world’s climate will gain further attention worldwide this century. The seasonal Antarctic ozone hole may have reached a natural limit, but the Arctic region—where ozone thinning occurs but to a lesser extent than over the southern polar region—causes a greater concern because it has a far larger population potentially susceptible to the harmful effects of the Sun’s ultraviolet radiation. New evidence suggests that 10- to 20micrometer particles of frozen water, nitric acid, and sulfuric acid form in the stratosphere in winter water over the Arctic. These particles—known as polar stratospheric cloud (PSC) particles—have perhaps 3000 times the mass of previously known PSC particles and provide a surface on which chlorine and bromine convert from inactive to active molecules that destroy ozone. This discovery illustrates the many unknown effects that gases and submicrometer particles released into the atmosphere—and their interactions—can have. Probing these questions will require not just more measurements and more sensitive sampling and analytical techniques but also new, testable theories that apply atmospheric chemistry, fluid mechanics, and thermodynamics to the problem. Aerosol particles pose a particular challenge because unlike gases, which can remain in the atmosphere for decades, particles in the micrometer range typically last but a few days.
The vast majority of scientists who have assessed the issue now regard global warming, and the human contribution to it through the burning of fossil fuels, as real. This view was reinforced in a recent report by the National Academy of Sciences/National Research Council to President Bush.1 Climate simulations reported last year, for example, compared two sets of data: natural contributions to global warming, such as volcanic activity and variations in solar radiation alone, versus natural contributions plus those from human activities. The results indicated that had natural factors alone prevailed, atmospheric warming, in the 20th century would have stopped about 1960 and a cooling trend would have followed. Combining human and natural activities, however, produced a continued warming pattern up to the present.
Aside from reducing the burning of fossil fuels, several other approaches are under investigation to limit the release of carbon dioxide into the atmosphere. One of these would inject carbon dioxide produced by large combustion plants underground into salt-brine-containing rock to sequester it. Another possibility, but one that requires far more study, is to dissolve carbon dioxide in the ocean