Principle of the mechanism leading to a cooling of the stratosphere

The physics of radiation transfer in the atmosphere is relatively well known because it obeys classic laws for absorption and emission of radiation by molecules. Therefore, it has been possible to measure the greenhouse effect due to CO₂ of anthropogenic origin, with a good degree of confidence, to be about 1.5 W/m2 of radiative energy redirected toward the earth's surface. This effect is the result of the budget of radiation, emitted and absorbed by CO₂, integrated over the height of the atmosphere. However, the study of the radiative budget for the different layers of the atmosphere shows that, in the case of the stratosphere, more energy is emitted toward space than absorbed by the GHGs present at that altitude. This means that this region of the atmosphere will undergo cooling with the increase of CO₂ concentration.

Implication for the stratospheric ozone depletion

The key variable controlling ozone depletion is the stratospheric chlorine loading. It has been shown recently (Anderson et al. 2000) that, as a result of the Montreal Protocol and its amendments, the stratospheric chlorine loading is now decreasing. In the absence of any other change, this normally should start the ozone recovery process but the situation is more complex and depends on the location. To clearly understand this effect one must also keep in mind the difference between polar regions and the mid latitude area.

1- Mid latitude: between 65°N and 65°S (non polar regions)
In this area which covers the major part of the earth, observations are now showing a stabilization of the average concentration of ozone in the layer. Although it is too early to demonstrate on a statistical basis that ozone recovery has started, it is at least in agreement with observed stratospheric chlorine decrease after allowing for the effect of the Pinatubo eruption. This is illustrated by satellite and ground based observations reported in WMO 1998, showing a stabilization of the ozone column since 1994 (See figure below).

2- In the Antarctic
The Antarctic is the polar region of the southern hemisphere where the ozone hole develops every year from about mid August to the end of the year, with a maximum depletion reached generally in mid-October. Air masses are isolated within a stratospheric vortex caused by circulating winds that are driven by large temperature differences between outside and inside the Antarctic area. Temperatures within the stratospheric Antarctic vortex are well under the threshold of formation of PSCs, which means that further cooling is not likely to change the frequency of their formation to a large extend. Therefore no further deepening of the ozone hole might be expected from that effect. The observations of the last four years rather show similar ozone values and surface coverage within the expected variability of that phenomenon.

3- In the Arctic
The Arctic has different topography from the Antarctic, consequently temperatures are higher there and air masses travel in a more complex way, leading to the formation of an unstable Arctic vortex. In those conditions, no ozone hole develops in the Arctic but temporary ozone changes may develop, generally from January to March, if the vortex persists then.

In the case of the Artic, the cooling of the stratosphere could actually favours ozone depletion for a longer period than one would expect from the stratospheric chlorine decrease. This effect has been studied using 3 dimensional models (WMO 1998) and appears to be extremely complex. Delay of ozone recovery may be expected from the stratospheric cooling due to CO₂ increases. However the classic greenhouse effect, which warms the lower troposphere, may break the vortex before the ozone depletion develops, so having the reverse effect. More work and consensus is still needed among 3-D models to draw a final conclusion on those effects.

60 North-60 South Average satellite and ground based ozone column observation from WMO 1998

(April 2003)