PerspectiveAtmospheric Science

Water Vapor in the Lower Stratosphere

See allHide authors and affiliations

Science  17 Aug 2012:
Vol. 337, Issue 6096, pp. 809-810
DOI: 10.1126/science.1227004

Water vapor in the stratosphere originates from the troposphere by transport of water vapor itself (1) and of methane and hydrogen, which degrade to make water (2). Water, methane, and hydrogen are transported to the stratosphere through upwelling in tropical regions (3). This upwelling followed by downwelling and horizontal transport in the extratropical stratosphere—the Brewer-Dobson circulation—is widely held to control the water vapor abundance in the stratosphere (see the figure). But could there be a more direct transport of tropospheric air containing water vapor into the stratosphere via convection outside the tropics (47)? On page 835 of this issue, Anderson et al. (8) argue that there is evidence for water vapor enhancements in the mid-latitude lower stratosphere. They further argue that increased water vapor levels could enhance ozone depletion caused by human-emitted ozone-depleting substances and thus raise ultraviolet radiation levels at Earth's surface.

Water is a powerful greenhouse gas. It accounts for 50 to 75% of the greenhouse effect today. The amount of water vapor in the tropopause (∼10 to ∼18 km above Earth's surface depending on latitude) and the lower stratosphere is particularly crucial, because it determines how much radiation escapes from the atmosphere. Clouds also play a role in Earth's radiative balance by decreasing the amount of incoming solar radiation that reaches Earth's surface and by preventing escape of infrared radiation to space. Precipitation redistributes water vapor, and both cloud formation and precipitation lead to vertical heat transport in the atmosphere.

Water thus has major consequences for the radiative balance and heat transport in the atmosphere. In addition, it is the source of OH radicals which catalytically destroy ozone in the stratosphere. Furthermore, water droplets and frozen water particles provide surfaces and liquid media for heterogeneous and multiphase reactions (9).

Earth would be a different planet without all these well-known roles of water. Yet, many of these known roles remain unquantified. The potential stratospheric ozone depletion by mid-latitude water injection is a case in point.

The role of water vapor in ozone destruction cycles and hence the maintenance of the ozone layer has been known since 1964, when Hampson suggested (10) that the hydrogen oxides OH and HO2 drive catalytic ozone destruction cycles. Indeed, the specter of human-induced ozone layer depletion was first raised in connection with the injection of water vapor into the lower stratosphere by supersonic aircraft (11). Anderson et al. now argue, based on the observed enhanced water vapor levels, that episodic extratropical convective transport of water can swell and dilute the sulfate aerosols, thereby increasing the rate of heterogeneous activation of chlorine from the benign chlorine reservoirs (ClONO2 and HCl). These enhanced active chlorine levels can then accelerate ozone destruction. However, the enhanced water abundances are localized in space and time and may not be pervasive. The extent of such events and their contribution to patches of high levels of water are not yet quantified.

What determines water vapor levels in the stratosphere?

The Brewer-Dobson circulation (light colors) transports to the stratosphere water vapor as well as methane and hydrogen, which produce water vapor [adapted from (3)]. Anderson et al. argue for an additional mechanism (red) that directly injects water at the mid-latitudes. Through this mechanism, water vapor in the lower stratosphere could be enhanced sporadically to high concentrations at some locations, until they mix with the rest of the stratosphere. Key questions remain: How pervasive is this mechanism? How will it change in the future? How does it influence the ozone layer recovery? How will it influence climate change and variability? ppmv, parts per million by volume.

The current estimate of the global and latitudinal ozone layer depletion, based on observations, must already include the influence of this mechanism. This influence could be subtle enough to have avoided detection by the observed column ozone levels or surface ultraviolet changes. However, it should be possible to estimate its contribution through retroactive analyses of the observed column ozone and local water vapor values at locations and times where convective transport is likely to have occurred.

How could the mid-latitude mechanism influence the recovery of the ozone layer? The main driver for the expected “recovery” of the ozone layer—when the ozone layer will return to 1980 levels—is the decline in the atmospheric concentrations of ozone-depleting substances, with secondary influences of climate change (cooling of the stratosphere) and changes in the composition of the troposphere. The mid-latitude mechanism could be another secondary influence on the recovery of the ozone layer, if the frequency and extent of these extratropical injection changes in the future due to climate change. If this process weakens, the recovery should occur earlier than the current estimate; if it gets stronger the recovery could be later than the current estimate.

Elevated water levels increase activation of chlorine and suppress active nitrogen oxides through heterogeneous reaction; they act together to increase ozone destruction at today's chlorine abundance. The impact of Anderson et al.'s mechanism on the recovery of the ozone layer depends on the future extent of extratropical water injection as well as the interplay between these heterogeneous reactions when levels of chlorine (and bromine) are lower. Chemistry-climate model studies that can faithfully capture the extratropical water injection would help bound the contribution of this proposed mechanism and estimate its influence on the recovery of the ozone layer.

The change in the predicted date of global ozone recovery is unlikely to be large because the extratropical water vapor injection does not appear to contribute greatly to the water abundance in the lower stratosphere on the global scale. Whether this mechanism could alter the influence of large volcanic eruptions or “geo-engineering” injections of sulfate aerosol and the subsequent activation of chlorine and suppression of nitrogen oxides remains to be shown. The mechanism is not expected to influence the Antarctic ozone hole or the large springtime Arctic ozone losses.

There are other important considerations with regard to potential enhancements in lower stratospheric water vapor. For example, the observed trend in the lower stratospheric water vapor contributes significantly to the slower than expected surface temperature rise over the past decades (12), highlighting the importance of water vapor in determining the extent of heat loss from the planet. Yet, accurate measurements and distributions of the abundances of water vapor in the lower stratosphere and understanding of the mechanism of transport are currently insufficient for accurate accounting and predictions. The suggestions of Anderson et al. adds to the list of reasons why we need more accurate measurements and fuller understanding of water vapor transport mechanisms in the lower stratosphere.

References and Notes

  1. J. Hampson, 1964, Photochemical behavior of the ozone layer, Canadian armament research and development establishment technical report, Nr. 1627, p. 280.
  2. Acknowledgment: I thank K. Rosenlof and J. Daniel for helpful discussion and NOAA's Climate Program for support of this work.

Navigate This Article