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
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
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."