^^Abstract ^^
Gross primary productivity (GPP) is an key process in modern models of the land surface; the carbon cycle, and in some cases the hydrologic cycle. However, GPP is not well constrained by measurements at relevant geophysical scales. The fundamental problem is that gross CO2 uptake associated with GPP is difficult to disentangle from CO2 release associated with respiration and fossil fuel consumption. At regional and continental scales sources and sinks for CO2 are approximately equal. An alternative approach is to estimate GPP from measurements of the light-use-efficiency (LUE) from fluorescence or spectral indices and the flux of absorbed light. We are also exploring the possibility of using atmospheric measurements of the trace gas carbonyl sulfide as a proxy for GPP. This trace gas is taken up in parallel with CO2 by leaves, but there is no significant source of COS from terrestrial ecosystems. I will review the status of these approaches for solving the "GPP problem."