BIOTIC FEEDBACKS IN THE WARMING OF THE EARTH

A positive correlation exists between temperature and atmospheric conc entrations of carbon dioxide and methane over the last 220,000 years o f glacial history, including two glacial and three interglacial period s. A similar correlation exists for the Little Ice Age and for contemp orary data. Although the dominant processes responsible may be differe nt over the three time periods, a warming trend, once established, app ears to be consistently reinforced through the further accumulation of heat-trapping gases in the atmosphere; a cooling trend is reinforced by a reduction in the release of heat-trapping gases. Over relatively short periods of years to decades, the correspondence between temperat ure and greenhouse gas concentrations may be due largely to changes in the metabolism of terrestrial ecosystems, whose respiration, includin g microbial respiration in soils, responds more sensitively, and with a greater total effect, to changes in temperature than does gross phot osynthesis. Despite the importance of positive feedbacks and the recen t rise in surface temperatures, terrestrial ecosystems seem to have be en accumulating carbon over the last decades. The mechanisms responsib le are thought to include increased nitrogen mobilization as a result of human activities, and two negative feedbacks: CO2 fertilization and the warming of the earth, itself, which is thought to lead to an accu mulation of carbon on land through increased mineralization of nutrien ts and, as a result, increased plant growth. The relative importance o f these mechanisms is unknown, but collectively they appear to have be en more important over the last century than a positive feedback throu gh warming-enhanced respiration. The recent rate of increase in temper ature, however, leads to concern that we are entering a new phase in c limate, one in which the enhanced greenhouse effect is emerging as the dominant influence on the temperature of the earth. Two observations support this concern. One is the negative correlation between temperat ure and global uptake of carbon by terrestrial ecosystems. The second is the positive correlation between temperature and the heat-trapping gas content of the atmosphere. While CO2 fertilization or nitrogen mob ilization (either directly or through a warming-enhanced mineralizatio n) may partially counter the effects of a warming-enhanced respiration , the effect of temperature on the metabolism of terrestrial ecosystem s suggests that these processes will not entirely compensate for emiss ions of carbon resulting directly from industrial and land-use practic es and indirectly from the warming itself. The magnitude of the positi ve feedback, releasing additional CO2, CH4, and N2O, is potentially la rge enough to affect the rate of warming significantly.