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models consistent with ocean warming observations
Climate models consistent with ocean
warming observations
LIVERMORE,
Calif. –
Climate models are reliable tools that help researchers better
understand the observed record of ocean warming and variability.
That’s
the finding of a group of Livermore scientists, who in collaboration
with colleagues at Scripps Institution of Oceanography,
had earlier established that climate models can replicate
the ocean warming observed during the latter
half of the 20th century, and that most of this recent warming
is caused by human activities.
The
observational record also shows substantial variability in
ocean heat content on interannual-to-decadal time scales.
The new research by Livermore scientists demonstrates that
climate models represent this variability much more realistically
than previously believed.
This is an image of global sea
surface temperatures taken from Japan National Space Development
Agency's (NASDA) AMSR-E instrument aboard NASA's Aqua spacecraft
on August 27, 2003. The colors in this false-color map represent
temperatures of the ocean's surface waters, ranging from a
low of -2?C (28?F) in the darkest green areas to a high of
35?C (95?F) in the brightest yellow-white regions. Sea ice
is shown as white and land is dark gray.
Using
13 numerical climate models, the researchers found that the
apparent discrepancies between modeled and observed variability
can be explained by accounting for changes in observational
coverage and instrumentation and by including the effects
of volcanic eruptions.
The
research, which will appear in the June 18 early online edition
of the journal Proceedings of the National Academy
of Sciences, casts doubt on recent findings that
the top 700-meters of the global ocean cooled markedly from
2003-2005.
“Our
analysis shows that the 2003-2005 ‘cooling’ is
largely an artifact of a systematic change in the observing
system,” said Krishna AchutaRao, previously of Livermore’s
Program for Climate Model Diagnosis and Intercomparison
(PCMDI), now at the Indian Institute of Technology
Delhi and the lead author of the paper. “The
previous research was based on looking at the combined ocean
temperature observations from several different instrument
types, which collectively appear to have a cooling effect.
But if you look at the observational instruments individually,
there is no cooling.”
Three-dimensional view of projected
surface air temperature and ocean warming due to greenhouse
gases as calculated by a low-resolution GFDL coupled ocean-atmosphere
climate model.
Observational
estimates of ocean heat content change in
the 2005 World Ocean Atlas are based on millions of individual
temperature measurements; however, they are unevenly distributed
in space and time. In fact, until recently, many portions
of the global ocean were very poorly sampled. To get a complete
four-dimensional picture of global ocean temperatures,
most researchers use statistical methods to “infill”
missing observational data.
Climate
models provide spatially complete ocean temperature
data, so unlike the incomplete observations, infilling
is not required. By sampling the models only where there are
observations, the Livermore team found that infilling had
a pronounced effect on observed estimates of ocean variability,
and brought model results closer in line with observations.
Models agreed even more closely with observations when the
cooling effects of intermittent volcanic eruptions were included.
The
research team also looked at the impacts of changes in ocean
observing systems. A warm bias in the older instruments was
recently discovered by researchers in Germany. With the introduction
of new instruments called Argo floats, more
complete and more reliable ocean temperature
measurements have become possible. The first Argo floats were
deployed in the Atlantic in 2000 and their network has rapidly
ramped up to several thousand floats with near-global coverage
of the world’s oceans.
“This
transition from a measuring system biased warm to a more realistic
one appears as a cooling. Obviously, models can’t account
for spurious variability caused by instrument changes,”
AchutaRao said.
Other
Livermore authors include Benjamin Santer, Peter Gleckler
and Karl Taylor. The Japan Agency for Marine-Earth
Science and Technology, the National Oceanic
and Atmospheric Administration and the National
Center for Atmospheric Research also contributed
to this report.
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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