Water Vapor in the Lower Stratosphere
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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 (4–7)? 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.
