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 standar dized way of its treatment. Starting from 1970, the estimates of terrestria l NPP range from 40 to 80 ct C/year, with the central value of 60 ct 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 calcul ated these estimates made no statements about reliability, and thus there i s no certainty in 60 Gt C/year as well as in other estimates of the terrest rial NPP. Analyzing the database on NPP that was originally synthesized by a group of scientists from the University of Osnabruck (Germany) led by H. Lieth in the 1970s and 1980s, we conclude with 90% confidence that terrestr ial NPP falls within 50-70 Gt C/year in the context of Miami model. This ca n hardly be the ultimate answer, as different NPP models give us different contexts for calculating terrestrial NPP. In view of the fact that global s cale estimates are essentially contextual, we suggest that the uncertainty of terrestrial NPP arises from NPP models more likely than from NPP data. ( C) 1999 Elsevier Science B.V. All rights reserved.