Fig. 6. Global C budgets for the 1990s (Houghton and Goodale, 2004). Based on fossil-fuel combustion and land-use change releases, total sources to the atmosphere were estimated at 11.5 Pg yr^–1. Sinks were estimated directly except for the residual terrestrial C sink that is estimated by difference of sources and sinks. Citations are for land-use change release and uptake, and residual terrestrial sink (Houghton, 2003; Houghton and Goodale, 2004; R.A. Houghton, personal communication, 2006); fossil fuel emissions and atmospheric increase (Prentice et al., 2001); and ocean uptake (Plattner et al., 2002).