We present simulations of present-day tropospheric O3 and of changes in its concentrations and the associated radiative forcing since the industrial revolution. Numerical experiments were conducted using a global tropospheric climate-chemistry model (GCCM) developed by incorporating the University of Oslo (UiO) photochemical module (a reduced tracer scheme that lumps nonmethane volatile organic compounds (NMVOCs) to simulate the CO-NOx-HOx-O3 system) into State University of New York at Albany--Community Climate Model 3 (SUNYA CCM3). The GCCM was run with emissions of pollutants corresponding to two periods, the early 1990s and the preindustrial period, and the simulated tropospheric O3 is used for off-line calculations of radiative forcing. The study consists of two parts: an evaluation of the GCCM-simulated present-day distributions of tropospheric chemical species with sensitivity experiments to test the effects of NO production by lightning and stratospheric O3 influx on the simulated results, and the radiative forcing due to O3 changes since the industrial revolution. The model can reproduce the temporal and spatial variations of O3 precursors, including CO and NOx (NO + NO2), in the Northern Hemisphere (NH), although there are biases such as the larger CO concentrations in the Southern Hemisphere (SH) and the smaller NOx concentrations in the tropical middle-upper troposphere. The simulated OH concentrations are sensitive to the NO production by lightning. The simulated concentrations of tropospheric OH have a global and annual average of about 9.8 ¿ 105 molecules/cm3 (corresponding to a tropospheric CH4 lifetime due to OH of 8.6 years), when the annual production of NO by lightning is about 6 TgN/yr. Compared with O3 sonde climatology, the model can simulate the seasonal variations of lower to middle tropospheric O3 within the observed interannual variations when the stratospheric O3 influx is about 600 Tg/yr, although detailed biases exist in the middle troposphere during fall to winter and in the continental boundary layer during summer. The present-day tropospheric O3 burden is calculated to be about 376 Tg (or 34 DU) with a net tropospheric chemical production of about 513 Tg/yr. Using realistic stratospheric O3 concentrations instead of a prescribed O3 influx as the upper boundary condition enhances the model biases in the extratropical upper-middle tropospheric O3 concentrations. The model calculates a globally and annually averaged increase in tropospheric O3 burden of about 13 DU since the preindustrial period, if the production of NO by lightning is kept the same for both the present-day and preindustrial periods. The annually and globally averaged instantaneous radiative forcing due to the O3 changes is about 0.54 W m-2, with forcing larger than 1 W m-2 occurring over most subtropical to middle-latitude areas in the NH during summer. An assumed 50% reduction in NO production by lightning in the preindustrial period results in an increase of tropospheric O3 change by 10%, with the associated radiative forcing enhanced by 11%.