Download Effects of climate change belong on Chesapeake Bay agenda

Bay Journal Vol 8 - Number 10
January - February 1999
Effects of climate change belong on Chesapeake Bay
agenda
By Donald F. Boesch & L. Donelson Wright
As the leaders of the two largest marine science institutions in the Bay region, we have
observed that, for well over a decade, climate change has been the major focus of
oceanographic research. Major international programs such as the World Ocean
Circulation Experiment, Joint Global Ocean Flux Study, the Ocean Drilling Program and
Global Ecosystems Dynamics have massively addressed, respectively: how the
circulatory system of the world's oceans regulate climate and is affected by climate
changes; the ocean's role in removing CO2 from the atmosphere; the Earth's climatic
history; and the effect of climate variability and change on oceanic food chains. Some of
our own faculty members have been key participants in these important research
programs. As the steady stream of information regarding warmer temperatures and more
extreme weather events during the late 20th century seeps into the public consciousness,
it alerts us to the real prospect of a significantly changed climate within our lifetimes. In
contrast to the extensive attention directed to the interactions of climate and the open
ocean, the effects of climate changes on coastal environments-where most people livehave been little studied. We believe it's about time that the consequences of climate
change become a serious part of the Chesapeake Bay science and management agenda.
Before briefly reviewing how climate change might influence the Chesapeake Bay, it is
important to understand that there is far less controversy regarding the certainty of future
climate changes than is reflected in the popular press. There is, in fact, widespread
agreement among experts on the geophysical processes governing the planet's climate
that over the next century the Earth as a whole will be warmer, wetter and subject to more
extreme variations. Furthermore, the present rate and momentum of human-caused
increases of CO2 and other greenhouse gases assures us that such changes will take place
no matter what steps we take in the coming decades to curtail these emissions.
Like the devil, the differences in climate predictions are in the details. Different
assumptions regarding such things as the removal of CO2 by growing forests or changes
in cloudiness affect the relative amount and rate of climate change predicted by
increasingly sophisticated models. Furthermore, these models are not fully adequate to
predict climate changes on regional or local scales, which may be more or less dramatic
than, or even contrary to, global trends. For example, some regions may become cooler or
drier. Such regional scale patterns - obviously important as we consider the Chesapeake
Bay and its watershed - are being addressed under the Congressionally mandated
National Assessment of Climate Variability and Change. Our institutions and others in
the Bay area, including Penn State and Johns Hopkins, are participating in this
assessment.
Global scale models do not predict water temperatures in coastal regions such as the
Chesapeake Bay or even changes in the seasonal distribution of local air temperature.
What the global models do offer, however, are widely accepted estimates of the latitudedependent temperature distributions that cause altered pressure gradients and force
corresponding changes in winds, weather and precipitation. It is these higher-order
effects that will probably have the greatest effect on the Chesapeake Bay and its
contiguous coastal waters. Finer-grid models being used in the National Assessment, and
regional models nested within global models, such as those developed for the MidAtlantic region by Penn State climatologists, are beginning to provide greater insight to
the changes that may be in store for the Bay.
We may reasonably expect that regional warming will narrow the annual temperature
range experienced by the Bay. Winters and transitional seasons are likely to be warmer
whereas summer temperatures will probably not change appreciably. Because the
Chesapeake Bay is rather delicately poised in a transitional biogeographic region, such an
altered temperature regime could affect the species that occur in the Bay. For example,
species that are near their southern limits, such as the soft clam (Mya arenaria), may no
longer survive or be prolific in the Bay, whereas warm temperate species found in
estuaries in the Carolinas (e.g., commercially important penaeid shrimp or, possibly, the
toxic dinoflagellate Pfiesteria piscicida) could become more common. Temperature and
the timing of seasonal changes in temperature affect other important physical, chemical
and biological processes in ways that are complex and difficult to forecast. Seasonal
warming and cooling and the temperature differences between surface and bottom waters
affect circulation, stratification, plankton production, seasonal oxygen depletion and the
survival and growth of larvae.
Most of the attention paid to the effects of climate change on coastal environments has
focused on sea-level rise. Over the last century, global sea level has been rising at an
average rate of about 1.8 millimeters per year for a total rise of 18 centimeters (7 inches).
Because of the combined effects of global sea-level rise and regional land subsidence, the
relative rate of rise throughout many parts of the Chesapeake Bay has been about 3.3
millimeters per year over the past 60 years. This has caused shoreline erosion and the
inundation of low-lying islands and salt marshes in the Bay. By simple extrapolation of
past trends and ignoring any influences of global warming, we would infer an additional
rise of 33 centimeters (about 1 foot) in the Bay by the end of the coming century.
However, the Intergovernmental Panel on Climate Change's medium-sensitivity forecast
predicts an increase in the rate of global sea-level rise to 5 millimeters per year by the end
of the century based on the thermal expansion of the ocean alone. These effects must be
added to ongoing local trends in the Bay, suggesting that throughout much of the Bay, the
relative rise will be at least doubled or rise more than 2 feet by 2100.
These relative rises in sea level will cause the inundation of tidal wetlands (the landward
retreat of which will be restricted by steps taken by land owners to prevent the loss of
fastlands), shoreline erosion and the further loss of islands and other tidewater lands. The
depth and volume of the Bay would also be expected to increase, although those effects
would be partially offset by sedimentation from increased shoreline erosion and changes
in the erosion potential of the Bay bottom. Increased depth and volume could result in
intrusions of higher salinity up the Bay and its tributaries, with concomitant biological
changes and the increased potential for salinization of ground water. The salinity of the
future Bay will also be affected by changes in the freshwater runoff as discussed later.
Some scientists have predicted that global warming will increase the frequency and
severity of hurricanes and tropical storms. But the recent consensus of climate modelers
is that such projections remain shrouded in uncertainty. On the other hand, some predict
that, because of the compression of the latitudinal gradients in ocean temperatures,
extratropical storms could increase in frequency and intensity. Notably, it is such storms,
specifically the "northeasters" that involve the west-to-east tracking of large, lowpressure systems across the coast, that have the largest and most destructive impact in the
Mid-Atlantic region. These storms, most common in autumn and winter, often bring
strong and prolonged onshore (northeasterly) winds combined with high precipitation and
waves to the region. Among the most notorious of such storms was the "Halloween
storm" of October 1991, which had major effects on the Delmarva coast and the Bay, but
also resulted in the human tragedies in the North Atlantic vividly chronicled in Sebastian
Junger's best seller, "The Perfect Storm."
Potential increases in precipitation in the watershed and runoff of freshwater into the Bay
constitute a "sleeper" issue that has gotten little attention but could have profound
consequences for our efforts to restore and manage the Bay ecosystem. National Oceanic
and Atmospheric Administration studies have shown that average annual precipitation
has increased by more than 20 percent in the Susquehanna River basin over the past 100
years. Seasonal high flow records were set in 1996 and 1998. Furthermore, as reported in
the lead article in this issue of Bay Journal, regional climate models indicate that the
Bay's watershed should experience increased winter-spring precipitation as global
warming proceeds.
An increase in winter-spring precipitation that increased freshwater inflow by 30 percent,
for example, would raise the average flow to the levels seen previously only in extreme
high-flow years. This would deliver more nutrients to the Bay, making the goal of
reducing controllable nutrient inputs by 40 percent (below 1985 levels) more difficult and
more of a moving target. On the other hand, increased evapotransporation during the
summer may reduce soil moisture, increasing demand on water resources for irrigation
and growing populations, and reducing summer-fall flows. Does this sound like 1998?
The amount and timing of freshwater flow into the Bay greatly influence salinity,
stratification, circulation and sediment and nutrient inputs. Higher spring flows into the
Bay result in the increased delivery of nutrients and the growth of algae and greater
depletion of dissolved oxygen in the deep parts of the Bay.
We don't want to be alarmist. Some of the potential consequences to the Bay that we
discussed may not occur. However, we are convinced that the Bay of the 21st century
will be different from the Bay of the 20th century in important ways as a result of climate
change. We feel that a concerted scientific effort is needed to understand and predict the
complex consequences of climate change on the Chesapeake Bay and other coastal
ecosystems. Fortunately, the Chesapeake Bay Program has capable monitoring and
modeling programs that provide retrospective information on climate variability and
ecosystem responses that allow us to ask "what if" questions. Moreover, even with the
current level of understanding, we believe that we should now be taking climate change
into account in Chesapeake Bay management strategies, ranging from tidal wetland
protection and nutrient reduction to fisheries management.
Donald F. Boesch is president of the University of Maryland Center for Environmental
Science and L. Donelson Wright is director of the Virginia Institute of Marine Science,
College of William and Mary.
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