Radiative effects of volcanic aerosols and clouds on terrestrial photosynthesis

The atmospheric carbon dioxide (CO2) concentration shows considerable interannual variability in its growth rate. Investigations using atmospheric CO2 inversion, O2/N2 ratios, and delta13CO2 have indicated that much of this variation has been caused by the dynamics of the terrestrial carbon cycle. Understanding the mechanisms responsible for the interannual variations in the terrestrial carbon cycle is important for accurately predicting future atmospheric CO2 concentration. In this research, we study radiative effects of aerosols, particularly volcanic aerosols, and clouds on terrestrial photosynthesis and the potential consequences on the terrestrial carbon cycle. We use the eruptions of Mt. Pinatubo in June 1991 and El Chich¢n in March 1982 as two case studies. Both eruptions injected large amounts of sulfur compounds into the stratosphere, which later developed into a global aerosol envelope. The aerosol envelope formed after the eruptions significantly increased diffuse solar radiation reaching the Earth's surface. Using radiation measurements and models of canopy photosynthesis, we demonstrate that the increased diffuse radiation increased terrestrial photosynthesis under clear sky conditions. While volcanic eruptions are rare events, clouds, which can also increase diffuse radiation, are companions of terrestrial ecosystems, especially during the growing season. We will discuss how clouds affect the terrestrial carbon cycle.