CLIMATIC FEEDBACKS IN THE GLOBAL CARBON-CYCLE

Increasing atmospheric CO2 is likely to produce chronic changes in global climate, as it may have done in the geologic past. Future CO2-induced changes in temperature and precipitation distribution changes could equal or exceed the changes which have occurred over the past 160,000 years and have affected the global carbon cycle. We consider ocean and terrestrial processes that could involve large changes in carbon fluxes (> 2 Pg C.yr-1) or changes in storage in large carbon pools (> 200 Pg C) resulting from CO2-induced climate changes. These include (1) air-sea exchange of CO2 in response to changes in temperature and salinity; (2) climate-induced changes in ocean circulation; (3) changes in oceanic new production and regeneration of organic debris caused directly by climate change; (4) altered oceanic nutrient supply needed to support new production due to climate-induced alteration of ocean circulation and river discharge; (5) CaCO3 compensation in sea water; (6) altered river nutrient flux and effects on coastal organic matter production and sediment accumulation; (7) seasonal balance between GPP and decomposition-respiration in terrestrial ecosystems in response to changes in temperature and precipitation; (8) successional processes in terrestrial ecosystems and formation of new plant associations in response to climatic change; (9) effects on soil nutrient availability, which amplifies ecosystem responses to climate change; (10) and responses of northern forests, tundra, and peatlands which have, until recently, been a sink for CO2. The potential effect of these processes on the rate of atmospheric CO2 concentration changes are estimated where possible, but not much quantitative information at a global scale is known, so uncertainty in these estimates is high. Each of these secondary feedbacks, however, has the potential of changing atmospheric CO2 concentration in magnitude similar to the effects of the direct human processes (fossil fuel burning and land clearing) responsible for the concern about global warming in the first place. It is therefore urgent that these uncertainties be resolved. Lines of research to accomplish this are suggested.