Anthropogenic emissions of short-lived, chemically reactive gases, such as NOx and CO, are known to influence climate by altering the chemistry of the global troposphere and thereby the abundance of the greenhouse gases O3, CH4 and the HFCs. This study uses the characteristics of the natural modes of the tropospheric chemical system to decompose the greenhouse effect of NOx and CO emissions into (i) short-lived modes involving predominantly tropospheric O3 and (ii) the long-lived mode involving a global coupled CH4-CO-O3 perturbation. Combining these two classes of greenhouse perturbations-large, short-lived, regional O3 increases and smaller, long-lived, global decreases in CH4 and O3-we find that most types of anthropogenic NOx emissions lead to a negative radiative forcing and an overall cooling of the earth. ¿ 2001 American Geophysical Union