Tropospheric O3 is an important greenhouse gas. Lightning is a major source of NOx, and thus of tropospheric O3. It has recently been suggested that due to an apparent strong correlation between lightning strike rates and surface temperatures, tropospheric O3 may significantly increase if the climate warms, resulting in a substantial positive climate forcing. This paper attempts to quantify the extent of this forcing and the associated positive climate feedback. Simulations incorporating a tropospheric O3-surface temperature parameterization are performed of the last glacial maximum and of a climate in which greenhouse gas concentrations have been doubled. The O3 parameterization is based on results from a two-dimensional chemical model. The simulations are obtained using a one-dimensional radiative-convective model, in which CO2, CH4, tropospheric O3, and in the case of the paleoclimate simulation the surface albedo, are varied both independently and in combination. In the paleoclimate case, the tropospheric O3 feedback has roughly two thirds of the effect on surface temperatures as the change in tropospheric O3 due to industrialization alone. During climate warming, the effect on surface temperatures is about 60% of that due to a doubling of CH4. The results indicate that a surface temperature-lightning-O3 feedback, currently absent in general circulation models, could significantly affect anthropogenic climate change. Improved modeling and observations are required to confirm this.¿ 1997 American Geophysical Union