EESS Winter Seminar Series, Dr. Joe Berry, Department of Global Ecology, Carnegie Institution for Science, Stanford
EESS Winter Seminar Series, Dr. Joe Berry, Department of Global Ecology, Carnegie Institution for Science, Stanford, "Photosynthesis of the Terrestrial Biosphere: Can we measure it?", Abstract: Photosynthesis or gross primary productivity (GPP) is the primary source of metabolic energy for the terrestrial biosphere. In addition, it is a sink for anthropogenic CO2, and it has indirect effects on the climate system through effects on the albedo of the land surface and its linkage to transpiration. Thus, many climate models (and even some weather forecast models) have a photosynthesis module that “drives” the simulation of land — atmosphere exchanges of water and energy. However, these models can have surprisingly large uncertainties in the rate of GPP, and this uncertainty may propagate to the carbon cycle and to other aspects of land-atmosphere interactions. For example, estimates of GPP for the data rich Central US vary by ±50%. The problem isn’t that the models are bad, rather, we lack observational benchmarks to constrain the models. The obvious candidate, atmospheric CO2, turns out to be a poor constraint because it “feels” both CO2 uptake by GPP and CO2 production by respiration. These co-occur, have opposing effects, and need to be modeled separately. I will describe two new approaches that appear to provide direct information on the rate of GPP (and indirectly on the rate of respiration). These are: a) carbonyl sulfide, an atmospheric trace gas that appears to “feel” GPP but not respiration, and b) a new remote sensing approach that can distinguish the intensity of chlorophyll fluorescence from the plants on the land surface. This retrieval also appears to be related to GPP. We are hopeful that these or similar approaches will lead to improved understanding of GPP at regional and continental scales.