Climate change leads to increase in atmospheric moisture, water vapor itself a potent greenhouse gas

Observations and climate model results confirm that human-induced warming of the planet is having a pronounced effect on the atmosphere’s total moisture content. Water vapor itself is a potent greenhouse gas and more of it means an amplification of global warming. Those are the findings of a new study appearing in the September 19 early edition of the Proceedings of the National Academy of Sciences as an open access article. The results debunk the claims by climate-deniers that the lower atmosphere has cooled over recent decades.

Scientists from Lawrence Livermore National Laboratory’s Program for Climate Modeling and Intercomparison and eight other international research centers, found that the atmosphere’s water vapor content has increased by about 0.41 kilograms per cubic meter (kg/m²) per decade since 1988. Natural variability in climate can not explain this moisture change. The most plausible explanation is that this is due to the human-caused increase in greenhouse gases.

Positive feedback
More water vapor amplifies the warming effect of increased atmospheric levels of carbon dioxide. This is what scientists call a 'positive feedback'.

Using 22 different computer models of the climate system and measurements from the satellite-based Special Sensor Microwave Imager (SSM/I), the atmospheric scientists have shown that the recent increase in moisture content over the bulk of the world’s oceans is not due to solar forcing or gradual recovery from the 1991 eruption of Mount Pinatubo. The primary driver of this ‘atmospheric moistening’ is the increase in carbon dioxide caused by the burning of fossil fuels.

This is the first identification of a 'human fingerprint' on the amount of water vapor in the atmosphere. 'Fingerprint' studies seek to identify the causes of recent climate change and involve rigorous comparisons of modeled and observed climate change patterns. To date, most fingerprint studies have focused on temperature changes at the Earth’s surface, in the free atmosphere, or in the oceans, or have considered variables whose behavior is directly related to changes in atmospheric temperature.

The water vapor feedback mechanism works in the following way: as the atmosphere warms due to human-caused increases in carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons, water vapor increases, trapping more heat in the atmosphere, which in turn causes a further increase in water vapor:
energy :: fossil fuels :: climate change :: global warming :: greenhouse gas :: water vapor :: water cycle :: atmosphere ::

Basic theory, observations and climate model results all show that the increase in water vapor is roughly 6 percent to 7.5 percent per degree Celsius warming of the lower atmosphere.

The authors note that their findings, when taken together with similar studies of continental-scale river runoff, zonal-mean rainfall, and surface specific humidity, point toward an emerging human-caused signal in the cycling of moisture between the atmosphere, land and ocean.

This new work shows that the climate system is telling us a consistent story. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally- and physically-consistent way.

Why care?
So why should we care about a more humid atmosphere? There are at least three good reasons.

• First, water vapor is itself a potent greenhouse gas, so it is important to have a good understanding of the cause or causes of its recent increase.
• Second, atmospheric moisture content is one of the large-scale environmental conditions that influences the genesis and development of hurricanes. In the absence of countervailing changes in other factors, an increase in water vapor would favor the development of more intense hurricanes.
• Finally, the observed increase in water vapor provides independent evidence of the reality of warming of the lower atmosphere. The observed water vapor increase since 1988 is consistent with pronounced warming of the surface and lower atmosphere, but fundamentally inconsistent with claims (still made by some die-hard skeptics!) that the lower atmosphere has cooled over recent decades.

Wider implications of the study
One persistent criticism of the 'discernible human influence' findings of previous IPCC assessments is that such conclusions were largely based on 'fingerprint' studies which relied heavily on surface temperature changes. The thrust of the criticism was this: if there really is a signal of human activities lurking in the climate system, it should be manifest in many different climate variables, and not in surface temperature alone.

The new study helps to refute this criticism, and shows that we have now moved well beyond 'temperature only' fingerprint studies.


Map (click to enlarge): Estimates of the amount of atmospheric water vapor over oceans from the satellite-based Special Sensor Microwave Imager. Results are for August 28th (top panel) and August 29th, 2005 (bottom panel). Locations with high atmospheric moisture content are denoted by red and white colors. The highest water vapor values are associated with typhoons Talim and Nabi in the Pacific and with Hurricane Katrina in the Gulf of Mexico. Courtesy: Carl Mears and Frank Wentz/Remote Sensing Systems.

References:
B. D. Santer, et al., "Identification of human-induced changes in atmospheric moisture content" [open access], Proc. Natl. Acad. Sci., Published online before print September 19, 2007, DOI: 10.1073/pnas.0702872104

Lawrence Livermore National Laboratory: Increase in atmospheric moisture tied to human activities - September 18, 2007.