Recent model calculations of the global mean radiative forcing from tropospheric ozone since preindustrial times fall in a relatively narrow range, from 0.3 to 0.5 W m-2. These calculations use preindustrial ozone fields that overestimate observations available from the turn of the nineteenth century. Although there may be calibration problems with the observations, uncertainties in model estimates of preindustrial natural emissions must also be considered. We show that a global three-dimensional model of tropospheric chemistry with reduced NOx emissions from lightning (1--2 Tg N yr-1) and soils (2 Tg N yr-1) and increased emissions of biogenic hydrocarbons can better reproduce the nineteenth century observations. The resulting global mean radiative forcing from tropospheric ozone since preindustrial times is 0.72--0.80 W m-2, amounting to about half of the estimated CO2 forcing. Reduction in the preindustrial lightning source accounts for two thirds of the increase in the ozone forcing. Because there is near-total titration of OH by isoprene in the continental boundary layer of the preindustrial atmosphere, isoprene and other biogenic hydrocarbons represent significant ozone sinks. The weak or absent spring maximum in the nineteenth century observations of ozone is difficult to explain within our understanding of the natural ozone budget. Our results indicate that the uncertainty in computing radiative forcing from tropospheric ozone since preindustrial times is larger than is usually acknowledged. ¿ 2001 American Geophysical Union