Introduction

In the past few years, application of improved measurements and models suggests a robust partitioning of CO, emissions from fossil fuel consumption and land use: about one-third remains in the atmosphere, one-third is reassimilated by land surfaces, and one-third is absorbed by the oceans (Keeling el al., 1996). The terrestrial component of the sink has special political interest, because it is that part of the global carbon which can most directly be managed. If we were able to change the large fluxes of assimilation and respiration, as they were summarized by Schimel

(1996), a tiny bit towards assimilation, we would be able, in principle, to compensate for fossil fuel emissions. The Kyoto Protocol

(1997) is based on this assumption, and mirrors the attempt of mankind to actually manage a major global biogeochemical cycle (Schellnhuber, 1999). The commercial idea to market carbon sinks has initiated a major discussion about where on earth the largest sink capacity exists. Ciais et al. (1995) had proposed that the sink exists in the Northern hemisphere with its center in the Eurasian region. This was countered by Fan et al. (1998) who propose, based on analysis of gradients of C02 in the atmosphere, that continental USA was the major carbon sink in the Northern hemisphere. Schimel et al. (2000) argue against Fan's result based on models and in situ measurements. Lloyd (1999), on the other hand, predicts that the main terrestrial carbon sink is in the tropics. It is surprising that despite the intensive research going on in the field of production biology at global scale since the International Biological Programme in the sixties, there is so little consensus about the ability of terrestrial surfaces to absorb C02.

In the following sections, we will discuss the problems in locating a sink, and we will emphasize the spatial, temporal, and biological variability of processes on local as well as continental scale.

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