Factors governing long-term change in global temperature are reviewed. The magnitude and rate of change in global temperature resulting from current increases in the concentration of atmorpheric greenhouse gases are considered in relation to their impact on forests. Movement in forest zone boundaries at a rate of 2.5 km/year are possible, which is nearly ten times the rate forests have been known to move by natural reproduction. Climate models indicate that increased global temperature will affect rainfall distribution, lead to more frequent and more severe storms and icnrease climatic variability. Consequences for the world’s forests include increased frequencies of fire and blow-down, and widespread decline. Increased atmospheric CO2 concentrations may increase forest growth where the effect is not offset by reduced precipitation, but the overall effect of anticipated changes in global climate is likely to be widespread loss of forests.