Humidities drop while moisture climbs at upper levels

The atmosphere is moistening 6 to 9 miles (10–15 km) above sea level even as relative humidities decrease, according to a new climate simulation paired with data from NASA's Upper Atmosphere Research Satellite (UARS). The findings hint that the overall positive feedback between water vapor and air temperature may be a stronger influence on global climate than a drying effect in high tropical altitudes.

Andrew Dessler (University of Maryland's Earth System Science Interdisciplinary Center and NASA's Goddard Space Flight Center) and Ken Minschwaner (New Mexico Institute of Mining and Technology) describe their NASA- supported work in the 15 March issue of the Journal of Climate.

As increases in water vapor and other greenhouse gases lead to atmospheric warming, evaporation from the oceans increases, thus putting more water vapor into the atmosphere—a process known as positive water vapor feedback. Global climate models that include water vapor feedback and that hold relative humidity constant produce nearly twice as much surface warming over the next century as do models without such feedback. "Projections of surface warmings as large as 5.8°C [10.4°F] over the next 100 years are based largely on the expectation of a positive water vapor feedback in the atmosphere," write the authors.

However, some theoretical and observational studies have pointed toward factors that could cause drying in the upper troposphere. For instance, showers and thunderstorms cover only about 10% of the tropics at any point in time. These storms might reach greater heights in a warmer climate, which could strengthen the compensatory sinking and drying of air across those regions that are free of storms.

Dessler and Minschwaner studied these regions of sinking, drying air using a radiative-convective model and tested their work using UARS data from 1992 to 1999. Their model shows that, while water vapor does increase with temperature in the upper reaches of the storm-free tropical troposphere, relative humidity drops by a few percent.

The results support the notion of a positive water vapor feedback, according to the authors, as both temperatures and absolute amounts of moisture may increase in the upper troposphere as global climate warms. Even under the most extreme assumptions, they add, "we find that the model is unable to produce a negative feedback simulation."