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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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