Increase In Atmospheric Moisture Tied To Human Activities
- These study results are consistent with your findings on rising dewpoints
in the Midwest.
Increase In Atmospheric Moisture Tied To Human Activities
September 19, 2007
Livermore CA (SPX) - 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. Those are the findings of a new
study appearing in the Sept. 17 online edition of the Proceedings of the
National Academy of Sciences. "When you heat the planet, you increase the
ability of the atmosphere to hold moisture," said Benjamin Santer, lead
author 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 (kg/m�) per decade
since 1988, and natural variability in climate just can't explain this
moisture change. 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. 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),
atmospheric scientists from LLNL 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
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," Santer said.
"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.
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," Santer said. "The observed changes in temperature, moisture, and
atmospheric circulation fit together in an internally- and
Q1: What is the scientific focus of our PNAS paper"
Our paper looks at the causes of changes in the moisture content of
Earth's atmosphere. Since the start of routine satellite-based
measurements of atmospheric water vapor in September 1987, atmospheric
moisture content has increased markedly. This increase had been noted in
previous scientific papers, and was prominently mentioned in the IPCC's
Fourth Assessment Report. But the causes of the increase were not known.
Our investigation was the first to use rigorous statistical "fingerprint"
methods to tackle the question of why water vapor has increased.
Q2: What is climate "fingerprinting""
Basically, "fingerprinting" involves searching for a computer
model-predicted pattern of climate change (the "fingerprint") in observed
climate records. Fingerprint techniques allow researchers to examine a
change in some property of the climate system and make rigorous
statistical tests of the different possible explanations for that change.
Q3: What were the primary findings of our study?
Our key findings were as follows:
Despite the relatively short length (19 years) of the observed water
vapor data, we were able to identify a "fingerprint" of human activities
in this observational record.
Unlike most previous "fingerprint" work, our study used results from
virtually all of the world's major climate models. We showed that our
identification of a human "fingerprint" in satellite-based water vapor
records was robust to current uncertainties in climate models.
The model results enabled us to "disentangle" the contributions of
different factors to the overall increase in water vapor. We found that
in climate models, this increase in water vapor was primarily due to
human-caused increases in greenhouse gases.
Bottom line: our results suggest that there is an emerging signal of
human activities in the moisture content of Earth's atmosphere. 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.
Q4: Does our work have any larger implications"
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".
Our study helps to refute this criticism, and shows that we have now
moved well beyond "temperature only" fingerprint studies.
Q5: Why should we care about the behavior of water vapor"
There are at least three reasons why we should pay attention to water
vapor. 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.
The Livermore authors included Karl Taylor, Peter Gleckler, Jim Boyle and
Stephen Klein. Other scientists contributing to this research were Carl
Mears and Frank Wentz at Remote Sensing Systems in Santa Rosa, Calif.;
Tom Wigley, Jerry Meehl, and Warren Washington at the National Center for
Atmospheric Research in Boulder; Tim Barnett and Dave Pierce at Scripps
Institution of Oceanography in La Jolla; Wolfgang Bruggemann at the
University of Hamburg in Germany; Nathan Gillett at the University of
East Anglia and Peter Stott at the Hadley Centre for Climate Prediction
and Research (both in the U.K.); Toru Nozawa at the National Institute
for Environmental Studies in Japan; and Mike Wehner at Lawrence Berkeley