Simulations of preindustrial (PI) to present-day (PD) changes showtropospheric ozone burden increases of 11% while the stratospheric burdendecreases by 18%. The resulting tropopause RF values are −0.06 W/m2from stratospheric ozone and 0.40 W/m2 from tropospheric ozone. Globalmean mass-weighted OH decreases by 16% from the PI to the PD. STE ofozone also decreased substantially during this time, by 14%. Comparisonof the PD with a simulation using 1979 pre-ozone hole conditions for thestratosphere shows a much larger downward flux of ozone into the tropospherein 1979, resulting in a substantially greater tropospheric ozone burden thanthat seen in the PD run. This implies that reduced STE due to stratosphericozone depletion may have offset as much as 2/3 of the tropospheric ozoneburden increase from PI to PD. However, the model overestimates the downwardflux of ozone at high Southern latitudes, so this estimate is likely anupper limit.
In the future, the tropospheric ozone burden increases by 101% in 2100 forthe A2 scenario including both emissions and climate changes. The primaryreason is enhanced STE, which increases by 124% (168% in the SHextratropics, and 114% in the NH extratropics). Climate plays a minimalrole in the SH increases, but contributes 38% in the NH.Chemistry and dry deposition both change so as to reduce tropospheric ozone,partially in compensation for the enhanced STE, but the increased ozone influxdominates the burden changes. The net RF due toprojected ozone changes is 0.8 W/m2 for A2.The influence of climate change alone is −0.2 W/m2, making it a substantialcontributor to the net RF. The troposphericoxidation capacity increases seven percent in the full A2 simulation, and 36%due to A2 climate change alone.
