Water vapor trends over Boulder, Colorado

Abstract

Water vapor in the atmosphere is responsible for a significant portion of the greenhouse effect, and even small changes in the upper troposphere or lower stratosphere can have a large effect on climate. A new analysis of water vapor measurements by balloon‐borne frost point hygrometers over Boulder, Colo., shows that stratospheric water vapor has increased over the past 30 years. Hurst et al. broke the long measurement record into four discrete time periods and determined the water vapor trends in each period for five 2‐kilometer‐thick stratospheric layers (16–26 kilometers in altitude). The scientists found that, on average, stratospheric water vapor increased by about 1 part per million by volume (27%) over the past 30 years, though there were many shorter‐term variations in the record. Water vapor levels increased during 1980–1989 and 1990–2000, decreased from 2001 to 2005, and then increased again after 2005. The authors found that, at most, 30% of the observed water vapor increases can be attributed to greater amounts of methane oxidation in the stratosphere. The 2001–2005 decrease in midlatitude water vapor has been linked to observations of anomalously low tropopause temperatures in the tropics, but, to date, no connection between the observed water vapor increases and tropical tropopause temperatures has been found despite ongoing efforts. (Journal of Geophysical Research‐Atmospheres, doi:10.1029/2010JD015065, 2011)