Technical Abstract: 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 toxic 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 tress representative of northern hemisphere forests. Current ambient [O3] (40 ppb on average) significantly reduced the total biomass of trees by 7% compared to trees grown in charcoal-filtered (CF) controls, which approximate pre-industrial [O3]. Above- and below-ground productivity were equally affected by ambient [O3] in these studies. Elevated [O3] of 64 ppb, which is similar to levels anticipated for ~2050, reduced total biomass by 11% compared to trees grown at ambient [O3] while elevated [O3] of 97 ppb, similar to levels anticipated for ~2100, reduced total biomass of trees by 17% compared to 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. 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 the future if O3 rises to elevated levels as projected. 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.