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Scientific Symposium Presentation Abstracts

September 19

Dr. Michael S. Kearney: The Potential for Significant Impacts on Chesapeake Bay

The Chesapeake Bay will face a number of significant impacts during the 21st century directly related to global warming. These impacts will affect people and ecosystems in ways that have not been witnessed since the first English settlers came to the region in 1607. How effectively future generations and their elected representatives accommodate these challenges will depend in considerable part on decisions people make today, which can lay a firm groundwork for understanding and planning for changes that in many cases are already upon us.

Sea Level Rise

Sea level rise is the major challenge facing people living next to the Bay and the coastal and upland ecosystems near its shores. If the first generation of Americans living around the Chesapeake, those who witnessed our nation’s founding, could come back to view its shores today, they would see a Bay at once familiar but obviously changed. Physical and cultural landmarks they once knew would in many cases have disappeared; large islands once supporting plantations known for generations have vanished (Kearney and Stevenson, 1991); elsewhere, shorelines have migrated 300 - 400 feet landward of former positions; and former upland forests are now marshlands. In fact, it is likely that their grandchildren living in the middle 19th century were probably the first people to have witnessed the onset of changes that reflected mainly post-glacial land subsidence, creating, in effect, a rising sea level. While the relative rate of rise may have been slow at first, the rate began to accelerate after 1850 AD as mountain glaciers retreated as the world warmed from the depths of the Little Ice Age. Since then, sea levels have continued to rise globally, with most scientists agreeing that anthropogenic greenhouse warming during the 20th century has begun to significantly increase this trend. In the Chesapeake Bay, the relative rates of sea level rise are presently the highest they have been for at least a thousand years (Kearney, 1996).

The Chesapeake Bay, like any subsiding coast, is inherently vulnerable to dramatic increases in the rate of rise of global sea level. With general land subsidence creating a “built-in” rate of rise of relative sea level of 1.6 to 2.0 mm per year (Kearney, 1996), the impacts of increases in global sea level become magnified in Chesapeake Bay. With the present rate of rise of global sea level being about 1.6 to 2.0 mm per year (Douglas et al., 2001), the rate of rise of sea level in Chesapeake Bay is presently about double the global average rate of rise.

For the future, continued melting of mountain glaciers and thermal expansion of the warming oceans are expected to further increase the rate of rise of global sea level. Recent evidence indicates that the Greenland Icecap may be melting much faster than previously thought, casting doubt on whether increasing rates of rise forecast by the Third Assessment of the Intergovernmental Panel on Climate Change (IPCC) for future sea level rise are too low. Nonetheless, these possibly conservative estimates, combined with regional subsidence, suggest that by the century’s end sea levels in the Bay could reach levels anywhere between 60 and 80 cm (23.4 - 31.2 in.) higher than today.

For many of the Bay’s very low-lying shores (particularly, the eastern shores of Maryland and Virginia) where the mainland only rises above mean tide level by around 1 foot per 2000 - 2500 feet (~half-a-mile), this could result in the drowning and huge loss of land area. Losses of coastal marshes will be severe, if not catastrophic -- many of the Bay’s largest and most ecologically-important marshes have not kept pace with sea level rise for more than half-a-century, with over 50% of those marshes that remain showing evidence of moderate to severe degradation (Kearney et al., 2001). And, of course, for the higher elevation shorelines of the Bay’s western shore, the inexorable pace of shore erosion will substantially quicken, with shoreline retreat in the tens of feet or more per year (rates of shore erosion are presently 12 - 15 feet per year in some areas of the Bay after winters with strong nor’easters).

The impact of Hurricane Isabel in 2003 -- a very modest hurricane -- demonstrated how vulnerable Bay communities along ever-more low-lying areas are to coastal flooding from storm surge. With the rate of coastal development showing no signs of slackening in the region, and houses being built often uncomfortably close to mean tide level, it takes little imagination to foresee the potential for a regional disaster if a major hurricane were to make landfall at the mouth of the Bay. As is often said for San Francisco Bay and the threat of a great earthquake, it is not if, but when. There is as yet not agreement on whether global warming has increased the number or frequency of major Atlantic hurricanes (Category 3 or above). However, it must remembered that for the Chesapeake Bay region some of the greatest damage wreaked by tropical storms in the past fifty years has been from storms which actually made landfall in the eastern Gulf of Mexico, then marched up the spine of the Appalachians to dump vast amounts of rain in the Bay region, causing widespread flooding (Stevenson and Kearney, 2005; Kearney and Stevenson, in prep.). These so-called “back door” storms were exemplified by Hurricane Agnes in 1972. Barely a hurricane (Category 1) when making landfall in the northern Panhandle of Florida, Agnes dumped so much rain in the upper Susquehanna River Basin that flooding affected the whole Bay and Piedmont surrounding it. Dramatic photographs showing downtown Norfolk flooded (see Stevenson and Kearney, 1996) reinforced the notion that large tropical storms do not have to make a direct hit on this region to produce large-scale damage.

One of the less commonly realized aspects of the influence of a rising sea level on the impacts of tropical storms (and, for that matter, extra-tropical storms, such as nor’easters) in the Chesapeake Bay is that since the estuary is relatively shallow, any rise in sea level represents a disproportionately significant increase in water depth. Because water depth (apart from fetch, or the distance the wind blows across a water surface) is a major influence on the strength of waves generated by storm winds in any water body, the comparatively large increase of one foot in overall depth of the Bay that occurred in the 20th century has, for the same wind speeds, made storm waves much stronger than they were even a half-century ago. For example, the “Storm King” hurricane of August 1933, which was almost the twin of Hurricane Isabel in strength and track across the region, generated potential waves that may have been 40% weaker in power than those that likely occurred seventy years later (Kearney and Stevenson, in prep). One can readily envision how a 2 - 3 foot increase in overall water depth could influence wave power from hurricanes by the end of the century. Large waves are the real “killers” from tropical storms, but often are lumped in with “storm surge” in accounts in the popular media. Actually, storm surges often serve as the “facilitators” of the havoc wrought by huge waves by allowing them to penetrate further landward on the flood level of the surge. In September 1900, giant waves rolled across Galveston Island from the notorious hurricane that has since become known as the Galveston hurricane (Larson, 2000). The storm surge of 17 feet from the hurricane barely overtopped the island, but in the end it did not matter, as it flooded the island enough so that waves of perhaps 30 feet or more could slam into and demolish any structure that stood in their way. What this could mean for Chesapeake Bay is that piers, docks, and other shore structures may need considerable strengthening from standards currently considered acceptable, and that, in the future, houses and other residences will have to be built far away from the possible reach of large storm waves, with plans for a shifting zone landward of reasonable risk as sea level rises. This is the setback concept of coastal management, which has a long history along the open coast.

Nutrients, Increasing Salinity, and Life in the Chesapeake’s Waters

The struggle to control the pernicious effects of excess nutrients (nitrogen and phosphorus) flooding into the Chesapeake Bay has held center stage among the environmental concerns of the region for more than a quarter-century. Some successes in mitigating the effects of this problem have been achieved, most notably in the return of sea grasses to many areas of the Bay. Burgeoning population growth in the Chesapeake Bay watershed and the pollutants (like nutrients) that the development produces still remain the major threat to the Bay’s ecology. Nevertheless, future warming of the Bay waters during the summer as a result of global warming, coupled with more rainfall in the Susquehanna watershed (Fisher, 2000), could contribute to curtailing any further remediation. Warmer water has less capacity to hold dissolved oxygen, and the flushing of increasing amounts of nutrients from future development during spring floods into the Bay’s tributaries could dramatically forestall any scenario for successful management of the Bay’s living resources. At the very least, expectations for improvement in overall estuarine health may have to be revised, or even more stringent controls for nutrient influxes put into effect.

A greater unknown is the long-term impact of increasing salinity as ocean waters intrude into the system. An estuary represents a delicate balance between the mixing of freshwater from a river (in the case of the Bay, principally the Susquehanna) and saltwater from the ocean. Organisms correspondingly adjust to local variations in this balance, and even large, short-term shifts in salinity up and down the system (i.e., longitudinal changes) can exact major repercussions. The intrusion of oyster parasites like MSX into the middle and upper Bay, which are generally held in check by lower salinities, is a recent example. Salinities in the Chesapeake Bay have, of course, changed over time, as sea level rose and ocean waters have intruded further inland. But whether organisms, especially those that tolerate only a narrow range of brackish water (i.e., salinities less than those of ocean water), can adjust to a rapid, large-scale and persistent salinity change is an open question. Aspects of competition for space in an ever-narrowing zone of appropriate salinity for bottom dwellers (benthic species) have yet to be examined in detail. Nonetheless, this could be the wild card in the mix of ecological changes that sea level rise and global warming will produce, whose effects could prove more lasting than any other.