The Trouble with Ozone
O’er the earth there comes a bloom;
Sunny light for sullen gloom;
Warm perfume for vapour cold--
I smell the rose above the mould!
--Thomas Hood, Farewell, Life
Sometimes, quality trumps quantity.
Consider, for example, the stratosphere--that region of the atmosphere 10 to 50 kilometers above us. It makes up only 10 percent of the mass of the air, yet its chemistry and dynamics influence the winds and temperature in the other 90 percent of the mass of the atmosphere below. For that reason, understanding the nature of the stratosphere’s influence is crucial to any forecasts of future climate.
In the quest for better climate models, researchers are thinking that recent human-caused ozone loss in the stratosphere could alter the temperature not only in the stratosphere, but also in the rest of the atmosphere.
Could ozone loss augment changes from human-made increases in carbon dioxide concentration? Perhaps temperature records for different heights in the air might reveal the presence of those human influences.
With this in mind, P. Forster and K. Shine modeled the net temperature trends from the surface to the upper stratosphere from both human influences--ozone loss in the stratosphere and carbon dioxide enhancement in the air--in the period 1979–1997.
In the model, the atmosphere’s increasing carbon dioxide content of the last 20 years warmed the troposphere (below 10 km) by +0.25°C; it left the temperature of the lower stratosphere (10 km to 20 km up) unchanged; and it strongly cooled the upper stratosphere (20 km and above).
This last effect--dramatic cooling of the upper stratosphere--is difficult to check against reality, for two reasons. First, no reliable temperature records exist for the higher layers of air. Second, the researchers did not include in their model certain other important factors that influence stratospheric temperature, especially the varying strength of the sun’s radiation during its 11-year activity cycle and the strong winds and waves in the stratosphere’s upper reaches.
According to the model, ozone loss should mean the lower stratosphere would absorb less of the sun’s energy and produce a cooling on the order of roughly -0.4 C. In contrast, the model predicts little net temperature change in the troposphere resulting from the loss of stratospheric ozone.
Seeping stratospheric water vapor
The Forster and Shine model also includes the effects of an observed--but not readily explained--increase in the water vapor content of the lower stratosphere in the last 20 years. Assuming the observed water vapor increase is global, its modeled temperature impacts seem large--a cooling trend in the lower stratosphere that is roughly comparable to that resulting from ozone loss; and in the troposphere, a warming trend about half the magnitude of that caused by carbon dioxide increase.
What are we left with? Considering all three changes in the composition of the air over the last 20 years, the model results suggest water vapor gain and ozone loss have combined to cause a globally averaged cooling of the lower stratosphere. The troposphere, on the other hand, should have warmed as a result of the increased carbon dioxide content in the air and the water vapor enhancement of the lower stratosphere.
How do the model results compare with observed temperatures? Let’s look at the satellite temperature record during the period of study. Based on Microwave Sounder Units (MSUs) carried by satellites orbiting the globe, the record shows that indeed, the lower stratosphere has cooled over the last 20 years (Figure 2). Is the cooling a human-caused global climate change?
Perhaps not. Recall that the modeled trend of cooling in the lower stratosphere has a large component from the water vapor added to the stratosphere. Because natural processes may account for that increased water vapor, the lower stratospheric cooling trend may not be a clear fingerprint of human-made climate change after all.
And what about the predicted warming of the troposphere from both added carbon dioxide and water vapor? The MSU record fails to show this trend (Figure 3). That leaves the modeled trends of the human effects in disagreement with the observed temperature records, especially in the troposphere.
And so the quest for a climate model that can explain the observations continues.
Forster, P., and K. Shine, 1999. Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling. Geophysical Research Letters, 26, 3309-3312.