The challenge of global warming to the Chesapeake

Rapid sea level rise will be the most important agent of global warming in changing the physical characteristics and ecological functioning of the Chesapeake Bay. It will also enhance very significantly the hazards of building and living around the Bay's shores. Nonetheless, though the changes may be unavoidable to a large degree; how the region and the nation — the Chesapeake Bay is certainly a national resource — respond, could allay the disquiet that surrounds what the future holds.

Environmental hazards posed by future sea level rise from a scientific and engineering standpoint may be the easiest to address. The science has converged on figures for global sea level rise that between the Third and Fourth Assessments changed comparatively little compared to the range of trends predicted in the 1980s. These figures ultimately may prove woefully too conservative if the Greenland Icecap and WAIS collapse, but at present this is an imponderable that must await further research. In the mean time, with better data on coastal elevations in low-lying areas from technologies such as LIDAR, better storm surge models more able to address the high spatial resolution required for a complex coast, better estimates of shoreline erosion rates, and better storm wave models, uncertainties can be addressed about flood risks and damage. Such work is already being undertaken in the Chesapeake Bay, and in the immediate future could prove a boon to coastal landowners concerned about whether large private insurers will continue to underwrite their homes and property.

What will be more intractable is a formulation of a coast-wide policy to minimize threats from sea level rise. Principles of English Common Law and political and economic realities inevitably wend their way through such discussions, and consensus can be hard to achieve. There is nevertheless ample precedent on strategies such as setback regulations, economic incentives and disincentives, land acquisition programs, and the like (see Klee, 1998) to provide a basis for decision making. It is certain that no one approach can apply to all of the Chesapeake Bay, as the risks from sea level rise in the Tidewater Virginia region — a low-lying, sprawling mix of port, major recreational and government land uses exposed to the full force of hurricanes making landfall in the Bay — are considerably different from those of Baltimore, where a more compact port facility is juxtaposed with more traditional city development. The differing impacts of Hurricane Isabel exemplified these differences.

The question of planning for ecological impacts poses greater scientific challenges because the science is less developed. For example, we have only in the last couple of decades begun to tease out the details of eutrophication in the Chesapeake Bay, its relations to land use, interannual and intraannual variations in nutrient inputs relative to rainfall, and aspects of bioprocessing (organisms modifying nutrient inputs) within the system. Although we expect global warming to yield generally warmer Bay waters and possibly greater rainfall in the Chesapeake's watershed, the landscapes and streams that mediate the sources of nutrients into the estuary will also be affected by such changes, and our grasp of how these integral components of the Bay system operate in the present context needs significant refinement if these considerations are to be taken into account in addressing a changing climate.

Similarly, the impacts of increasing salinity in the Chesapeake Bay are also difficult to gauge, especially apart from general impacts on tidal freshwater marshes, or sessile (i.e., attached to the sea bottom) organisms like oysters. What is particularly worrying is how the lower trophic levels (i.e., the lower levels of the food chain) might be impacted, especially planktonic species. The broad outlines are known of where oligohaline (essentially freshwater to very slightly brackish), mesohaline (brackish) and polyhaline (higher salinity brackish to open ocean) species are organized longitudinally up the Bay's waters, however, the historical baseline is slim. As a result, it is not yet clear what would happen if irreversible systemic changes occur; for example, it is unclear what would happen if salinities increased dramatically and permanently rather than temporarily shifted as occurs as a result of droughts or periods of high rainfall. Ultimately, such changes could strike at the heart of the Bay's ecology — as would also be the case in other estuaries — and such irreversible changes might cause the most damaging impacts of all.

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