The observed cooling of the lower stratosphere over the last two decades has been attributed, in previous studies, largely to a combination of stratospheric ozone loss and carbon dioxide increase, and as such it is meant to provide one of the best pieces of evidence for an anthropogenic cause to climate change. This study shows how increases in stratospheric water vapor, inferred from available observations, may be capable of causing as much of the observed cooling as ozone loss does; as the reasons for the stratospheric water vapor increase are neither fully understood nor well characterized, it shows that it remains uncertain whether the cooling of the lower stratosphere can yet be fully attributable to human influences. In addition, the changes in stratospheric water vapor may have contributed, since 1980, a radiative forcing which enhances that due to carbon dioxide alone by 40%. ¿ 1999 American Geophysical Union