Over the last 200 years the terrestrial carbon cycle has undergone profound direct and indirect changes. Direct changes have been driven by the interaction between an increasing global population and agrarian as well as industrial changes in land use. Indirect changes have been consequences of these economic and social changes. In particular, the rise in Ca from 280 to ~380 ppm has been caused by the addition of ~400 Pg C to the atmosphere, primarily through the burning of fossil fuels and land-use change (Sabine et al., 2004). The rate of input of nitrogen into the terrestrial nitrogen cycle has doubled due to humans (Vitousek et al., 1997a), with extreme inputs of up to 60 kg/ha/year recorded in some European ecosystems (MacDonald et al., 2002). Our understanding of the relative roles that direct land-use change and indirect climate changes have played in changing terrestrial productivity in the recent past has important implications for our ability to predict the future, and is the focus of this section.
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