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Plants and animals will always be found regardless of climatic changes in the ranges discussed here. The threat to the natural communities of plants and animals, called ecosystems, from greenhouse warming also comes from its projected rate of change as much as its magnitude. If the climate changes as rapidly as some computer models project, the present natural ecosystems may become fragmented and break up. New communities may replace them with different mixes of species. Long-lived plants like trees, for example, might persist. If ill adapted to the new conditions, however, they would fail to compete and reproduce. Species better fitted to the new climate would immigrate, sometimes hastened by disturbances of various kinds. Species well suited to the changing conditions may become more dominant, or pioneer species that could fill a particular niche may thrive in the new conditions. Certain ecosystems might vanish if the climate that currently sustains them disappears or changes its location faster than the key species are able to migrate.

The Role of Innovation

Much human adaptation involves the invention and diffusion of technological "hardware" or "software." Examples of technological hardware include air conditioners that make hot days comfortable and tractors that cultivate large tracts of land in a few days if spring is late. Software includes information, rules, and procedures like weather forecasts or insurance restrictions. Knowledge and new procedures are generally indispensable for adopting new hardware. Major breakthroughs like irrigation usually consist of innovations in social organization and financing as well as new machinery.

Many past innovations in hardware and software have helped people adapt themselves and their activities to climate and variable weather. Food preservation in warm weather, refrigeration and air conditioning, antifreeze for all-weather automobile travel, and weather satellites to aid prediction all help. Such innovations can occur rapidly in comparison to the 40 to 50 years envisioned for the equivalent doubling of atmospheric CO2. For example, in 1900 California had little crop production; in 1985 it produced twice as many dollars of crops as second-place Iowa. Penicillin was discovered in 1928; by 1945 it was saving thousands of lives.

The question frequently asked is how rapidly inventions can replace existing equipment and how fast other technology can be supplanted. About two-thirds of capital stock in most industrialized countries is in machinery, and one-third is in buildings and other structures. This capital stock turns over more rapidly than might be expected. Most current office space, for example, is in buildings built in the last 20 years. In Japan, the average period for virtually complete replacement of machinery and equipment ranges