Global Distribution of Photosynthetically Active Radiation as Observed from Satellites

Abstract

Concern about possible effects of a steady increase in CO2 on the earth's climate, and the fact that current estimates of sources and sinks of CO2 do not balance, generated interest to improve knowledge of rates at which carbon is cycled between the oceans, land, and atmosphere. The net primary productivity (NPP)?namely, the rate at which inorganic carbon is transformed into organic matter?is strongly controlled by the availability and intensity of photosynthetically active radiation (PAR); the distribution of photoactive pigments; the efficiency with which the light is absorbed; and the efficiency of its conversion into organic matter. In this study the feasibility to derive one of the above parameters is demonstrated?namely, PAR on a global scale. In the past, information on PAR was obtained from local ground measurements in the 0.4?0.7-µm spectral interval. In the absence of such measurements, PAR was estimated from measured total solar irradiance, using empirical ?conversion factors.? It is demonstrated that this important bigeophysical parameter can now be derived from satellite observations. The inference model is implemented with global satellite data that are available ftom the International Satellite Cloud Climatology Project (ISCCP) to produce for the fire time global fields of PAR and corresponding ?conversion factors.?