January 5, 2004
Warming is likely to drive big changes in California's
coastal waters through effects on upwelling
By Tim Stephens
Global warming could have profound effects on the wind-driven upwelling
of deep ocean water along the California coast, according to recent
studies by UCSC researchers.
The researchers used a high-resolution regional climate model
to look at the effects of rising concentrations of carbon dioxide
in the atmosphere.
The studies showed changes in both the intensity and the seasonal timing
of the upwelling, which brings cold, nutrient-rich water into coastal
This seasonal upwelling supports California's diverse marine life and
productive fisheries, but how changes in the upwelling will affect these
and other aspects of coastal ecosystems remains uncertain.
The researchers, led by professor of Earth sciences Lisa Sloan, used
computer simulations of the regional climate to show that wind-driven
upwelling along the California coast will likely intensify over the
next 50 years as a result of increased concentrations of carbon dioxide
in the atmosphere. In addition, the models showed the upwelling season
extending later into the fall.
The first set of experiments was published in the August 14 issue of
Geophysical Research Letters (GRL), and a second paper with additional
findings was published online in December by the Proceedings of the
National Academy of Sciences (PNAS).
Previous studies by other researchers found that the intensity of upwelling
along the California coast has been increasing over the past 30 years,
leading some to speculate that the trend is a result of global warming,
said Mark Snyder, lead author of the GRL paper. Snyder earned his Ph.D.
in December working with Sloan.
"Some people think we may already be seeing the effects of climate
change on the upwelling regime, so we thought we would use our climate
models to see how increases in greenhouse gases would affect the winds
that drive the upwelling," he said.
These winds are the result of differences in atmospheric pressure over
the land and the ocean that develop because the land surface heats up
faster than the ocean surface. Similarly, global warming could be expected
to raise temperatures more over land than over the ocean, making the
differences in atmospheric pressure even greater and creating stronger
winds. Sloan's group set out to test this hypothesis using powerful
computers to run complex models of the climate system.
The researchers used a high-resolution regional climate model, centered
over California and driven by inputs from a global climate model with
coarser resolution, to look at the effects of rising concentrations
of carbon dioxide in the atmosphere. Concentrations of carbon dioxide,
a greenhouse gas that traps heat in the atmosphere, are increasing due
to emissions from the burning of fossil fuels.
The first set of experiments, published in the GRL paper, showed that
with increasing carbon dioxide in the atmosphere, the intensity of upwelling
is decreased in the early season (April to May) and is dramatically
increased during the peak season (typically July to August). The results
also showed the duration of the upwelling season extends further into
"The increase we saw in the upwelling intensity in the peak season
supports some of the observational work showing a trend toward increased
upwelling intensity in the present day," Snyder said.
The new results just published by PNAS reinforce these findings. In
these experiments, the researchers included the effects on the climate
system of changes in vegetation that are likely to occur with increased
atmospheric carbon dioxide. The results showed the same general effects
on upwelling as in the previous studies, only more pronounced, said
postdoctoral researcher Noah Diffenbaugh, lead author of the PNAS paper.
"The interactions between vegetation and climate that are set
in motion by increasing carbon dioxide concentrations enhance the effects
on the upwelling regime," Diffenbaugh said.
The projected changes in vegetation result from warmer and dryer conditions
created by global warming and lead to changes in the overall energy
balance of the land surface, he said.
Diffenbaugh cautioned, however, that despite the complexity of the
climate models, they still represent a simplified view of an extraordinarily
complicated system of feedbacks and interactions. Nevertheless, as human
activities continue to pour more carbon dioxide into the atmosphere,
these studies show just how wide-ranging its effects may be.
"The regional climate model is certainly the best tool we have
at the moment, and it shows us that elevated carbon dioxide is important
not only for its direct effects on the climate, but also because it
induces other changes that feed back into the climate system, and the
magnitudes of those feedbacks appear to be substantial," Diffenbaugh
Researchers at UCSC and elsewhere are currently working to understand
how changes in upwelling conditions will affect fisheries and the dynamics
of coastal ecosystems. The new climate studies highlight the importance
of those efforts, Snyder said.
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