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Increase In Atmospheric Moisture Tied To Human Activities

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  • mtneuman@juno.com
    These study results are consistent with your findings on rising dewpoints in the Midwest. ... Increase In Atmospheric Moisture Tied To Human Activities
    Message 1 of 1 , Sep 18 9:24 PM
      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
      physically-consistent way."

      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
      National Laboratory.
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