Stratospheric water vapor abundance affects ozone, surface climate, and stratospheric temperatures. From 30--50 km altitude, temperatures show global decreases of 3--6 K over recent decades. These may be a proxy for water vapor increases, as the GISS climate model reproduces these trends only when stratospheric water vapor is allowed to increase. Observations suggest that stratospheric water vapor is indeed increasing, though measurements are extremely limited in either spatial coverage or duration. Model results suggest that the observed changes may be part of a global, long-term trend. Furthermore, the required water vapor change cannot be accounted for by increased stratospheric production, suggesting that climate change may be altering tropospheric input. The calculated water vapor increase contributes an additional ≈24% (≈0.2 W/m2) to the global warming from well-mixed greenhouse gases over the past two decades. Observed ozone depletion is also better reproduced when destruction due to increased water vapor is included. If the trend continues, it could increase future global warming and impede stratospheric ozone recovery. ¿ 2001 American Geophysical Union