Reaction of Ozone and Climate to Increasing Stratospheric Water Vapor
The abundance of water vapor in the stratosphere affects ozone, surface climate, and stratospheric temperatures. Increases in stratospheric moisture that have been observed for the past several decades may therefore have important consequences. We've used the NASA Goddard Institute for Space Studies (GISS) climate model to examine the impact of a wetter stratosphere on climate and on ozone over populated areas (this study did not look at polar ozone).
Upper atmospheric temperature and water changes have been observed for several decades, but at only a very few locations. High up in the stratosphere, from 30 to 50 km above the ground, the measurements show increasing water vapor and a very large, global cooling trend of 3° to 6°C (5° to 11°F) over recent decades.
The climate model reproduces the temperature trends only when stratospheric water vapor also increases. The satellite observations of temperature and water vapor are reasonably consistent with the model results (Figure 1), making us more confident that we can calculate their trends correctly. Water vapor breaks down in the stratosphere, releasing reactive hydrogen oxide molecules that destroy ozone. These molecules also react with chlorine containing gases, converting them into forms that destroy ozone as well. So a wetter stratosphere will have less ozone.
Observations of ozone show a thinning of the Earth's protective stratospheric ozone layer by about 3 to 8% overall since the 1970s. In the upper stratosphere, ozone depletion has been from 15 to 20%. Again, the model is better able to reproduce these values when increased water vapor is included. This is especially true in the upper stratosphere, where ozone is most sensitive to water. The model indicates that increased water vapor accounts for about 40% of the ozone loss in the upper stratosphere, and about 20% of the overall loss to date.
There are two driving forces behind the change in stratospheric moisture. Increasing emissions of methane are transformed into water in the stratosphere by chemical reactions. This can account for about a third of the observed increase in moisture there.
In addition, there is a greater transport of water from the lower atmosphere, which happens for several reasons. First of all, more water may be available in the lower atmosphere to be carried up. Warmer air holds more water vapor than colder air, so global warming will make the lower atmosphere wetter. Another possibility is that air is carried up more rapidly into the stratosphere. Climate models indicate that greenhouse gases such as carbon dioxide and methane may enhance the transport of air from the lower atmosphere up into the stratosphere. Additionally, the coldest temperature through which the air passes could change, which would alter the amount of water that freezes out along the way.
Though not fully understood, the increased transport of water vapor to the stratosphere seems to have been caused at least partially by human activities. Because rising greenhouse gas emissions account for all or part of the water vapor increase, it is likely to continue for many decades. This will have an effect on the recovery of ozone from depletion caused by chlorofluorocarbons (CFCs). Since international agreements have limited CFC emissions, their impact on ozone is expected to decrease over time. The year 1980 is often used as a benchmark for normal ozone levels, as large-scale depletion due to CFCs set in shortly thereafter. The amount of ozone-depleting chemicals from CFCs is expected to reach 1980 levels again in about 2055, so that ozone would return to its 1980 values in that year if there were no other changes to the atmosphere.
Increasing greenhouse gases cool the stratosphere, though, slowing down the rates of chemical reactions that destroy ozone. This leads to more ozone overall (though the opposite effect is seen in the chemistry that controls the polar ozone holes, which proceeds faster as temperatures decrease). We find, however, that the increasing water trend exerts an even stronger influence, leading to a net loss of ozone due to the interactions with greenhouse gases and water vapor (Figure 2). A wetter stratosphere also affects surface climate, because both water vapor and ozone are greenhouse gases, which trap heat leaving the Earth. Increased water vapor in the stratosphere makes it warmer on the ground by trapping heat, while the ozone loss makes it colder on the ground. However, water vapor has a much larger effect, so that overall the changes increase global warming by about 10-15%. (Feedbacks in the climate system could make this value quite uncertain, however). Thus, the influence of water vapor on climate is expected to continue and grow for many decades.
Shindell, D.T. 2001. Climate and ozone response to increased stratospheric water vapor. Geophys. Res. Lett. 28, 1551-1554.