GLOBAL C, N, P BIOGEOCHEMICAL CYCLES: HUMAN MODIFICATIONS, FEEDBACKS, AND CLIMATE CHANGE

In this talk we present model calculations that underscore the role of human-induced perturbations in changing Earth’s climate and the fate of anthropogenic carbon, phosphorus, and nitrogen in the surface environment of Earth since the year 1840 with projections to the year 2040. Anthropogenic carbon emissions have accumulated in the atmosphere, terrestrial biosphere, oceans, and the sediments of the coastal zone during historical time. At the same time, there has been a net loss of organic matter from terrestrial reservoirs to the coastal oceanic realm. Nitrogen and phosphorus emissions in the form of nitrogenous fertilizers and combustion products and phosphate fertilizers have been sequestered in the terrestrial living phytomass and groundwater since about 1940 and are continuously leached into aquatic environments where they have led to increases in new production. Because of land use activities during most of this 100-year time span, the soil humus reservoir has lost and will continue to lose organic carbon and both nutrient nitrogen and phosphorus.

The enhanced discharge of nitrogen and phosphorus to coastal waters via rivers and the atmosphere has led to increased new production and burial of organic carbon in marine sediments, a small sink of anthropogenic CO2, and enhanced denitrification. It is only recently that coastal waters have become a sink of atmospheric CO2. With increasing concentrations of atmospheric CO2, these waters will continue to absorb CO2, lowering the supersaturation state of the waters with respect to carbonate minerals. This might lead to slowing of the rates of carbonate formation and storage and hence a shift in the balance toward greater storage of organic carbon than carbonate carbon. Finally we show that anticipated changes in the thermohaline circulation of the ocean owing to global warming might lead to a decrease in the upwelling rates of nutrient nitrogen and phosphorus to the coastal zone and new production, and a further strengthening of coastal waters as a sink for anthropogenic CO2.