Estimating terrestrial NPP: what the data say and how they may be interpreted?

The differences in the estimates of terrestrial Net Primary Production (NPP) clearly manifest the demand for a standardized data set and for a standardized way of its treatment. Starting from 1970, the estimates of terrestrial NPP range from 40 to 80 Gt C/year, with the central value of 60 Gt C/year. The latter value, which is commonly used in studies of the global carbon cycle, provides the basis for many conclusions. However, authors who calculated these estimates made no statements about reliability, and thus there is no certainty in 60 Gt C/year as well as in other estimates of the terrestrial NPP. Analyzing the database on NPP that was originally synthesized by a group of scientists from the University of Osnabrück (Germany) led by H. Lieth in the 1970s and 1980s, we conclude with 90% confidence that terrestrial NPP falls within 50–70 Gt C/year in the context of Miami model. This can hardly be the ultimate answer, as different NPP models give us different contexts for calculating terrestrial NPP. In view of the fact that global scale estimates are essentially contextual, we suggest that the uncertainty of terrestrial NPP arises from NPP models more likely than from NPP data.