Rarely does the atmosphere allow direct observation of the photochemical evolution of ozone. In most of the troposphere and lower stratosphere this slow chemistry cannot be understood without including much larger changes caused by the circulation. Yet in the tropical stratosphere, where ozone-poor air of tropospheric origin enters and rises slowly in near isolation, it can be demonstrated that O₃is created by dissociation of O₂ at a rate consistent with current theory. The parallel photolytic destruction of the unreactive source gases (for example, N₂O and CFCl₃) and the consequent evolution of chemically active odd-nitrogen (NO_y) and chlorine (Cl_y) species, however, indicate a small amount of mixing of much older, photochemically aged air from the midlatitude stratosphere into this tropical plume.

Reactive gas emissions (CO, NO_x, VOC) have indirect radiative forcing effects through their influences on tropospheric ozone and on the lifetimes of methane and hydrogenated halocarbons. These effects are quantified here for the full set of emissions scenarios developed in the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. In most of these no-climate-policy scenarios, anthropogenic reactive gas emissions increase substantially over the twenty-first century. For the implied increases in tropospheric ozone, the maximum forcing exceeds 1 W m⁻² by 2100 (range −0.14 to +1.03 W m⁻²). The changes are moderated somewhat through compensating influences from NO_x versus CO and VOC. Reactive gas forcing influences through methane and halocarbons are much smaller; 2100 ranges are −0.20 to +0.23 W m⁻² for methane and −0.04 to +0.07 W m⁻² for the halocarbons. Future climate change might be reduced through policies limiting reactive gas emissions, but the potential for explicitly climate-motivated reductions depends critically on the extent of reductions that are likely to arise through air quality considerations and on the assumed baseline scenario.

The impact of doubled atmospheric CO₂ on the climate of the middle atmosphere is investigated using the GISS global climate/middle atmosphere model. In the standard experiment, the CO₂ concentration is doubled both in the stratosphere and troposphere, and the sea surface temperatures are increased to match those of the doubled CO₂ run of the GISS 9 level climate model. Additional experiments are run to determine how the middle atmospheric effects are influenced by tropospheric changes, and to separate the dynamic and radiative influences. These include the use of the greater high latitude/low latitude surface warming ratio generated by the Geophysical Fluid Dynamics Laboratory doubled CO₂experiments, doubling the CO² only in either the troposphere or stratosphere, and allowing the middle atmosphere to react only radiatively.

As expected, doubled CO² produces warmer temperatures in the troposphere, and generally cooler temperatures in the stratosphere. The net result is a decrease of static stability for the atmosphere as a whole. In addition, the 100 mb warming maximizes in the tropics, leading to improved propagation conditions for planetary waves, and increased potential energy in the lower stratosphere. These processes generate increased eddy energy in the middle atmosphere in most seasons. With greater eddy energy comes greater eddy forcing of the mean flow and an increase in the intensity of the residual circulation from the equator to the pole, which tends to warm high latitudes. Increased gravity wave drag in some of the experiments also helps to intensify the circulation. The middle atmosphere dynamical differences are on the order of 10%–20% of the model values for the current climate, and, along with the calculated temperature differences of up to some 10°C, may have a significant impact on the chemistry of the future atmosphere including that of stratospheric ozone, the polar ozone “hole,” and basic atmospheric composition.