Roles of volcanic eruptions, aerosols and clouds in global carbon cycle

The cataclysmic eruption of Mt. Pinatubo in 1991 released about 20 megatons of sulfur dioxide into the stratosphere. It brought markedly changes to the surface radiation environment over the globe in the next two years with significant enhancement in diffuse radiation (+50% in 1992) but only a small reduction in global solar radiation (<5%). Immediately after the eruption, the rate of atmospheric carbon dioxide rise dropped. Results from d13C measurements and atmospheric inverse modeling studies indicate that this rate drop was due to enhanced carbon uptake by the terrestrial biosphere during the two years following the volcanic eruption. The surface cooling scenario suggests that the enhanced terrestrial carbon uptake is caused by reduced soil and plant respiration in response to the volcanic cooling. This explanation is questionable for two reasons. First, estimates put the volcanic cooling on the order of 0.5K. This is small and well within the natural range of temperature variability. Second, carbon cycle model estimates of terrestrial responses based on the surface cooling scenario led to direct contradictions with atmospheric inverse models in terms of regional patterns of carbon uptake after the Pinatubo eruption and with atmospheric oxygen measurements. Here we suggest that the increased diffuse radiation is the primary factor for the enhancement in terrestrial biosphere carbon uptake and therefore the dip in the rate of atmospheric carbon dioxide rise during the two-year period following the Pinatubo eruption in 1991. Using tower flux measurements from a variety of vegetation sites, we will demonstrate that plant canopies have higher quantum use efficiencies for diffuse radiation than for direct beam radiation. The superiority of diffuse radiation over direct beam radiation for canopy photosynthesis increases with radiation level. We will show that under a turbid atmospheric environment where part of the reduction in direct beam radiation is converted into diffuse radiation, terrestrial ecosystem productivity can be enhanced. In addition, we will discuss how clouds can increase terrestrial carbon sequestration.