Tropospheric ozone is an important greenhouse gas. In the upper troposphere one of the major sources of ozone are the nitrogen oxides produced by lightning [IPCC, 1995>. Recently it has been shown that the number of lightning flashes may be very sensitive to changes in the surface temperature [Williams, 1992>. Here we use a global two-dimensional atmospheric model and find that for a warmer surface and constant lightning ozone decreases globally except for in the polluted Northern Hemisphere. However, a warming with increased lightning can more than offset this decrease. We find that for a 20% increase of lightning the global mean radiative forcing by tropospheric ozone is about +0.1 Wm-2. There is therefore a possibility of a positive climate feedback mechanism. The implied sensitivity of ozone to lightning and temperature suggests a major uncertainty in quantifying anthropogenic perturbations of upper tropospheric ozone. ¿ American Geophysical Union 1996