We present results using a coupled chemistry/climate model to study changes in tropospheric ozone between 2000 and 2100. We assess changes first due to the increased emissions of NOx and VOCs (using the IPCC SRES scenario A2) and then due to both emission changes and the anticipated climate change with doubled CO2. In 2100, with the scenarios used, there is a substantial calculated increase in tropospheric O3, but in contrast to earlier studies, the increase is larger for doubled CO2. The increases are most pronounced in the extratropical middle and upper troposphere; changes in circulation and a chemically induced increase in lower stratospheric O3, mainly due to reduced temperatures there, both enhance stratosphere/troposphere exchange. These changes in the lower stratosphere are crucial to our results.