THE CONTRIBUTION OF TERRESTRIAL SOURCES AND SINKS TO TRENDS IN THE SEASONAL CYCLE OF ATMOSPHERIC CARBON-DIOXIDE

We characterized decadal changes in the amplitude and shape of the sea sonal cycle of atmospheric CO2 with three kinds of analysis. First, we calculated the trends in the seasonal cycle of measured atmospheric C O2 at observation stations in the National Oceanic and Atmospheric Adm inistration Climate Monitoring and Diagnostic Laboratory network. Seco nd, we assessed the impact of terrestrial ecosystems in various locali ties on the mean seasonal cycle of CO2 at observation stations using t he Carnegie-Ames-Stanford Approach terrestrial biosphere model and the Goddard Institute for Space Studies (GISS) atmospheric tracer transpo rt model, Third, we used the GISS tracer model to quantify the contrib ution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric CO2 for the period 1961-1990, specifically examining t he effects of biomass burning, emissions from fossil fuel combustion, and regional increases in net primary production (NPP). Our analysis s upports results from previous studies that indicate a significant posi tive increase in the amplitude of the seasonal cycle of CO2 at Arctic and subarctic observation stations. For stations north of 55 degrees N the amplitude increased at a mean rate of 0.66% yr(-1) from 1981 to 1 995. From the analysis of ecosystem impacts on the mean seasonal cycle we find that tundra, boreal forest, and other northern ecosystems are responsible for most of the seasonal variation in CO2 at stations nor th of 55 degrees N. The effects of tropical biomass burning on trends in the seasonal cycle are minimal at these stations, probably because of strong vertical convection in equatorial regions. From 1981 to 1990 , fossil fuel emissions contributed a trend of 0.20% yr(-1) to the sea sonal cycle amplitude at Mauna Loa and less than 0.10% yr(-1) at stati ons north of 55 degrees N. To match the observed amplitude increases a t Arctic and subarctic stations with NPP increases, we find that north of 30 degrees N a 1.7 Pg C yr(-1) terrestrial sink would be required. In contrast, over regions south of 30 degrees N, even large NPP incre ases and accompanying terrestrial sinks would be insufficient to accou nt for the increase in high-latitude amplitudes.