In this sensitivity study, we have applied the Terrestrial Ecosystem Model ((TEM) version 4.1) to examine the responses of terrestrial ecosystems to transient changes in atmospheric CO2 concentration and climate in the 21st century at the scales of the globe, biomes, latitudinal gradient, and economic regions. Three predictions of transient change in climate and atmospheric CO2 concentration in the 21st century from the Integrated Global System Model developed at Massachusetts Institute of Technology were used. The TEM estimates a global annual net ecosystem production (NEP) of about 0.8 Pg C yr-1 in 1990. Global annual NEP in 2100 increases by about 2.6 Pg C yr-1 for the HHL (higher CO2 emissions and temperature increases), 1.8 Pg C yr-1 for the RRR (reference CO2 emissions and temperature increases), and 0.5 Pg C yr-1 for the LLH (lower CO2 emissions and temperature increases) climate change predictions. The boreal and tropical evergreen forests account for a large portion of the increased global annual NEP. Latitudinal distribution of total annual NEP along 0.5¿-resolution latitudinal bands shifts significantly from the tropics to the northern middle and high latitudes over time. The potential CO2 uptake over the period of 1990--2100 differs substantially among the 12 economic regions of the world. As we used potential mature natural vegetation in the global extrapolation of TEM, these NEP estimates represent the potential CO2 uptake or the upper bound for long-term carbon sequestration by the terrestrial biosphere. This sensitivity study shows that the temporal dynamics and spatial distribution of carbon, nitrogen, and water fluxes of terrestrial ecosystems are very sensitive to the magnitudes and paths of transient changes in atmospheric CO2 concentration and climate in the 21st century. ¿ 1998 American Geophysical Union