INFOTERRA: gw: Scientists discover global warming linked to increase in tropopause height over
http://www.eurekalert.org/pub_releases/2003-01/dlnl-sdg010303.php
Contact: Anne Stark
stark8@llnl.gov
925-422-9799
DOE/Lawrence Livermore National Laboratory
Scientists discover global warming linked to increase in tropopause
height over past two decades
LIVERMORE, Calif. -- Researchers at the Lawrence Livermore National
Laboratory have discovered another fingerprint of human effects on
global climate.
Recent research has shown that increases in the height of the tropopause
over the past two decades are directly linked to ozone depletion and
increased greenhouse gases.
The tropopause is the transition zone between the lowest layer of the
atmosphere -- the turbulently-mixed troposphere -- and the more stable
stratosphere. The tropopause lies roughly 10 miles above the Earth's
surface at the equator and five miles above the poles. To date, no
scientist has examined whether observed changes in tropopause height are
in accord with projections from climate model greenhouse warming
experiments.
The comparison was made by Livermore scientists Benjamin Santer, James
Boyle, Krishna AchutaRao, Charles Doutriaux and Karl Taylor, along with
researchers from the National Center for Atmospheric Research, NASA
Goddard Institute for Space Studies, the Max-Planck Institute for
Meteorology and the Institut für Physik der Atmosphäre in Germany. Their
findings are reported in the today's (Jan. 3) online edition of the
Journal of Geophysical Research-Atmospheres.
This research undercuts claims by greenhouse skeptics that no warming
has occurred during the last two decades. Such claims are based on
satellite measurements of temperatures in the troposphere, which show
little or no warming since the beginning of the satellite record in
1979.
"Weather balloons and weather forecast models show that there's been a
pronounced increase in the height of the global tropopause over the last
two decades," Santer said. "Our best understanding is that this increase
is due to two factors: warming of troposphere, which is caused by
increasing greenhouse gases, and cooling of the stratosphere, which is
mainly caused by depletion of stratospheric ozone. Tropopause height
changes give us independent evidence of the reality of recent warming of
the troposphere."
The Livermore research supports the bottom-line conclusion of the 2001
Intergovernmental Panel on Climate Change (IPCC), which states that,
"most of the observed warming over the last 50 years is likely to have
been due to the increase in greenhouse gas concentrations."
Earlier research showed that changes in the Earth's surface temperature,
ocean heat content, and Northern Hemisphere sea ice cover are other
indicators of human effects on climate change.
"The climate system is telling us a consistent story -- that humans have
had a significant effect on it," Santer said. "We're seeing detailed
correspondence between computer climate models and observations, and
this correspondence is in a number of different climate variables.
Tropopause height is the latest piece of the climate-change puzzle."
To support the research, Livermore scientists examined tropopause height
changes in climate-change experiments using two different computer
climate models. Both models showed similar decadal-scale increases in
the tropopause height in response to changes in human-caused climate
forcings. The patterns of tropopause height change were similar in
models and so-called 'reanalysis' products (a combination of actual
observations and results from a weather forecast model).
The model experiments focused on both manmade climate forcings, such as
changes in well-mixed greenhouse gases, stratospheric and tropospheric
ozone, and on natural forcings, such as changes in volcanic aerosols.
The forces have varying effects on atmospheric temperature, that in turn
affect tropopause height, the report concludes.
###
Founded in 1952, Lawrence Livermore National Laboratory is a national
security laboratory, with a mission to ensure national security and
apply science and technology to the important issues of our time.
Lawrence Livermore National Laboratory is managed by the University of
California for the U.S. Department of Energy's National Nuclear Security
Administration.
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