Fire is one of several natural and man-controlled agents that can alter the mass balance of terrestrial ecosystems. Frequent long-term burning generally decreases biomass and detritus, which together constitute a global carbon reservoir about three times that of the atmosphere. A quantitative assessment of variation in fire frequency over the Last several millennia is not possible; instead, we have estimated why, through changes in fire frequency and other agents, biomass and thus carbon storage have probably increased or decreased in North America in particular and in the world as a whole. We estimate that the net carbon reservoir of terrestrial ecosystems has definitely decreased in the last few thousand years, probably more from land clearing and subsequent burning than from a change in fire frequency itself. Special attention is directed to the global importance of the growing demand for wood as fuel.