The potential for biotically mitigating global warming is receiving a great deal of policy and technical attention around the world. Elements of the political community are drawn to the notion that land‐use patterns can be modified more easily than energy consumption patterns, and some modelers suggest that the potential for storing carbon in terrestrial ecosystems is very large. Most work to date, however, uses only physical criteria in estimating how much land might be available for reforestation. Accounting for social and economic constraints is much more difficult, resulting in daunting uncertainty about what could actually be accomplished. Furthermore, our relative ignorance of the functioning of the global carbon cycle makes attempting to manipulate it for human purposes questionable at best. Nevertheless, there are many reasons besides global warming to pursue a radical restructuring of land‐use patterns around the world. Such a restructuring should be undertaken in conjunction with many other measures to slow global warming, most immediately in the energy sector.