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December 14, 2018
Maryland, with 3,100 miles of tidal shoreline from the Chesapeake Bay and its tributaries to the Atlantic Ocean and coastal bays, is highly vulnerable to sea-level rise. The level of Chesapeake Bay water with respect to the land is now rising about three times as fast as it was during Colonial times, threatening more densely built communities and infrastructure.
Acceleration of sea-level rise—resulting from expansion of the ocean due to its warming, melting of glaciers and polar ice sheets, and dynamic ocean variability—is impacting coastal sea level and exacerbating tidal flooding of low-lying areas. The effects are already apparent, including shoreline erosion, deterioration of tidal wetlands, and saline contamination of low-lying farm fields. “Nuisance” tidal flooding (also referred to as high tide flooding) that occurred just a very few days per year in Annapolis in the 1950s now occurs 40 or more days per year. Storm surges from tropical storms or Nor’easters also spread farther and higher, riding on the higher sea level.
Scientific understanding indicates that the rate of sea-level rise will continue to accelerate into the foreseeable future. Reliable projections of sea level along Maryland's coasts are critical for planning the state's future and insuring its resilience in the face of changes ahead.
In fulfillment of requirements of the Maryland Commission on Climate Change Act of 2015, Sea-Level Rise Projections for Maryland 2018 provides updated projections of the amount of sea-level rise relative to Maryland coastal lands that is expected into the next century.
These projections represent the consensus of an expert group drawn from the Mid-Atlantic region and can be used in planning and regulation, infrastructure siting and design, estimation of changes in tidal range and storm surge, developing inundation mapping tools, and adaptation strategies for high-tide flooding and saltwater intrusion.
The framework for these projections is tied to the projections of global sea-level rise included in the Intergovernmental Panel on Climate Change Fifth Assessment (2014) and incorporates regional factors such as subsidence, distance from melting glaciers and polar ice sheets, and ocean currents. Comparable projections using these methods have been recently adopted in several other states and metropolitan areas.
"Later this century, rates of sea-level rise increasingly depend on the future pathway of global emissions of greenhouse gases during the next sixty years."
Chair of Sea-Level Rise Expert Group
The likely range (66% probability) of the relative rise of mean sea level expected in Maryland between 2000 and 2050 is 0.8 to 1.6 feet, with about a 1-in-20 chance it could exceed 2.0 feet and about a one-in-one hundred chance it could exceed 2.3 feet. Thus, the "rule of thumb" Maryland has used to plan for 2.1 feet in sea-level rise in that time frame remains prudent.
After 2050, rates of sea-level rise depend increasingly on the future pathway of global emissions of greenhouse gases during the next sixty years. If emissions continue to grow well into the second half of the 21st century, the likely range of sea-level rise experienced in Maryland is 2.0 to 4.2 feet toward the end of this century, two to four times the relative sea-level rise experienced during the 20th century. Moreover, there is a 1-in-20 chance that it could be over 5.2 feet.
On the other hand, if global society were able to bring net greenhouse gas emissions to zero—sufficient to meet the goals of the Paris Climate Agreement to limit the increase in global mean temperature to less than 2°Celsius over pre-industrial levels—the likely range for 2100 is 1.2 to 3.0 feet, with a 5% chance that it would exceed 3.7 feet.
The difference in sea-level rise between these contrasting scenarios would diverge even more during the next century, with the failure to reduce emissions in the near term resulting in much greater sea-level rise 100 years from now. Moreover, recent research suggests that, without imminent and substantial reductions in greenhouse gas emissions, the loss of polar ice sheets—and thus the rate of sea-level rise—may be more rapid than assumed in these projections, particularly if emissions continue to grow. If this happens, sea-level in Maryland could rise over 6 feet by 2100 and over 10 feet by the middle of the next century.
"Later this century, rates of sea-level rise increasingly depend on the future pathway of global emissions of greenhouse gases during the next sixty years," said University of Maryland Center for Environmental Science Professor Don Boesch, who chaired the sea-level rise Expert Group that assembled the report.
The sea-level rise driven by global warming will increase both high and low tide levels and storm surges by a generally similar amount as the increase in mean sea level. Tidal heights will depend on the degree to which shorelines are protected by bulkheads, rip-rap and other armoring. If low-lying lands are allowed to become permanently inundated by higher sea levels, the tidal range might actually decline by 2 to 3 inches. If, on the other hand, shorelines are extensively armored, the tidal amplitude may increase by as much as 0.3 feet.
Maryland is vulnerable from storm surges generated by tropical storms and hurricanes. For storms moving northeastward just off the coast, such as Hurricanes Irene (2011) and Floyd (1999), northeast to northerly winds can cause dangerous storm surges along the Atlantic coast, but sea level may actually drop in the upper Chesapeake Bay. For storms making landfall and moving inland, such as Hurricane Isabel (2003) and Sandy (2012), southeasterly to easterly winds drive water into the Chesapeake Bay, creating significant tidal flooding in Washington D.C., Baltimore, Annapolis, and Maryland’s Eastern Shore
More severe tropical storms are likely to reach the Mid-Atlantic region as Earth continues to warm. A Category 2 storm following a path of Hurricane Isabel (2003) late this century would result in water levels at Baltimore about 10.6 feet above the present mean sea level compared to 7.3 feet during Isabel. The wetlands and rural landscapes of Dorchester County on Maryland’s Eastern Shore will also see an increase in the extent and depth of storm surge inundation.
Nuisance flooding is inundation during high tides that causes public inconvenience. Such flooding events that recently occurring less than 10 days per year are likely to occur 30 days per year by 2040 and nearly 100 days per year by 2050. Later this century, whether greenhouse gas emissions are curbed or not, high tides exceeding the nuisance level would become virtually a daily occurrence.
Avoiding the unmanageable and managing the unavoidable
The consequences of taking global action to reduce, then eliminate, greenhouse gas emissions for Maryland sea level are starkly apparent beyond the 21th century with median projections of 6.8 feet for the Growing Emissions pathway by 2200, compared to 3.8 feet under the Paris Agreement pathway.
Even if global mean temperature is stabilized later this century, sea level will continue to rise for centuries to come. The oceans will continue to absorb heat and expand, and the melting of ice on land that has already been initiated and will continue to increase even if global mean temperature is stabilized. However, actions taken over the next 60 years to limit global warming will affect the rate of polar ice-sheet melting and sea-level rise.
On the other hand, if the rate of emissions of greenhouse gases continues to grow until late this century, Earth’s surface temperature will warm well above 2.0°C and increasingly rapid loss of polar ice sheets will be assured well into the future. Future generations will be powerless in stopping the accelerated rise by eliminating emissions or other actions.