Climatologists have puzzled over why global average temperatures have stayed roughly flat in the past decade, despite a long-term warming trend. New research suggests that lower levels of water vapor in the stratosphere may partly explain the anomaly.

The study, appearing in the journal Science, points out that the concentration of water vapor in the stratosphere has dropped about 10% in the past decade, triggered by unexplained cooler temperatures at certain high altitudes above the tropics. The study concludes that in the...