In a partial factorial model experiment, we used the Terrestrial Ecosystem Model (TEM, version 4.0) to assess the relative roles of changes in CO2, temperature, precipitation and cloudiness in equilibrium responses of primary production and carbon storage. In the experiment, we used two levels of atmospheric CO2 concentration (315 and 522 ppmv CO2), contemporary climate and changes in temperature, precipitation and cloudiness as estimated by a 3-dimensional atmospheric general circulation model (Geophysical Fluid Dynamic Laboratory-GFDL) and a 2-dimensional climate model (Land-Ocean climate model at Massachusetts Institute of Technology) for doubled CO2. The results show that elevated CO2 and projected increases in temperature account for most of the overall equilibrium responses of NPP and carbon storage to changes in climate and CO2, while the projected changes in precipitation and cloudiness contribute least. This is partly attributable to the magnitudes of changes in CO2 and climate variables as projected by the climate models. The results also show that the interactions among changes in CO2 and climate variables play a significant role in the equilibrium responses of NPP and carbon storage to changes in CO2 and climate. Of all the interaction terms, the interaction between a change in CO2 and a change in temperature is the most significant.Includes bibliographical references (p. 21-25).Abstract in HTML and technical report in HTML and PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).