Humans Changing Earth's Atmospheric Moisture

Humans Changing Earth's Atmospheric Moisture



2007 September -   "This is the first 
identification of a human fingerprint on the amount of water vapor in the 
atmosphere," says Benjamin Santer, describing the findings of his study on 
the effect that humans are having on the atmosphere's total moisture 
content. 
The study appears in the September 17 online edition of the "Proceedings 
of the National Academy of Sciences." 
"When you heat Earth, you increase the ability of the atmosphere to 
hold moisture," said Santer, from Lawrence Livermore National Laboratory's 
Program for Climate Modeling and Intercomparison. 
"The atmosphere's water vapor content has increased by about 0.41 
kilograms per cubic meter per decade since 1988, and natural variability 
in climate just can't explain this moisture change," he said. "The most 
plausible explanation is that it's due to the human-caused increase in 
greenhouse gases." 
More water vapor - which is itself a greenhouse gas - amplifies the 
warming effect of increased atmospheric levels of carbon dioxide in a 
cycle of positive feedback, Santer explained. 
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. 
Using 22 different computer models of the climate system and measurements 
from the satellite-based Special Sensor Microwave Imager atmospheric 
scientists from Lawrence Livermore and eight other international research 
centers 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 what they call "atmospheric moistening" is the 
increase in carbon dioxide caused by the burning of fossil fuels. 
Basic theory, observations and climate model results all show that the 
increase in water vapor is roughly six 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," Santer said. "The observed changes in temperature, moisture, and 
atmospheric circulation fit together in an internally and physically 
consistent way." 

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