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 live-have 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 latitude-dependent 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 Mid-Atlantic 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, low-pressure 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.