The northern hemisphere temperate and boreal forests currently provide an important carbon sink; however, current tropospheric ozone concentrations ([O3]) and [O3] projected for later this century are damaging to trees and have the potential to reduce the carbon sink strength of these forests. This meta-analysis estimated the magnitude of the impacts of current [O3] and future [O3] on the biomass, growth, physiology and biochemistry of trees representative of northern hemisphere forests. Current ambient [O3] (40 ppb on average) significantly reduced the total biomass of trees by 7% compared with trees grown in charcoal-filtered (CF) controls, which approximate preindustrial [O3]. Above- and belowground productivity were equally affected by ambient [O3] in these studies. Elevated [O3] of 64 ppb reduced total biomass by 11% compared with trees grown at ambient [O3] while elevated [O3] of 97 ppb reduced total biomass of trees by 17% compared with CF controls. The root-to-shoot ratio was significantly reduced by elevated [O3] indicating greater sensitivity of root biomass to [O3]. At elevated [O3], trees had significant reductions in leaf area, Rubisco content and chlorophyll content which may underlie significant reductions in photosynthetic capacity. Trees also had lower transpiration rates, and were shorter in height and had reduced diameter when grown at elevated [O3]. Further, at elevated [O3], gymnosperms were significantly less sensitive than angiosperms. There were too few observations of the interaction of [O3] with elevated [CO2] and drought to conclusively project how these climate change factors will alter tree responses to [O3]. Taken together, these results demonstrate that the carbon-sink strength of northern hemisphere forests is likely reduced by current [O3] and will be further reduced in future if [O3] rises. This implies that a key carbon sink currently offsetting a significant portion of global fossil fuel CO2 emissions could be diminished or lost in the future.