Large quantities of freshwater are locked in what are referred to as permanent ice. These bodies of ice include glaciers, ice sheets, ice shelves, and ice caps. While commonly referred to as permanent ice reservoirs, their size varies in response to global environmental changes and usually expand or contract over geologic time periods of thousands or tens of thousands of years. Since the changes occur over periods greater than that of human-scale perception of time, the ice reservoirs seem to humans to be permanent. Glaciers are bodies of ice located over land that slowly, on a geologic time scale, move over the land below. Ice sheets are large bodies of ice located over land. These include large glaciers as well as large expanses of ice that do not migrate across the Earth’s crust. Ice sheets can be found on continental land and under the ocean at the edge of a continent. Presently, ice sheets are located in Antarctica and Greenland. Ice shelves extend from the edges of continental glaciers out over the open ocean. They are supported by ocean water below, not land. When they melt, no land is exposed, only water. In the Western Hemisphere, ice shelves are located in Antarctica, Greenland and Canada. Ice caps are small reservoirs of ice located on the tops of high mountains. One result of recent increases in global atmospheric warming is the exponential increase in the rate of melting of these bodies of ice. The type of ice reservoir, glacier, ice sheet, ice shelve, or ice cap, differs in its contribution to present sea level rise. All of these ice reservoirs contribute to the albedo effect. The white surfaces refract the sun’s radiation away from Earth’s surface, resulting in a cooling effect. Ice sheets located below the surface of the ocean contribute less to this phenomenon then the other reservoirs because the radiation is captured by the open ocean water above. The larger the ice formation, the greater the contribution to albedo. Because they are located over ocean water rather than land, the melting of ice shelves contributes the most to directly increasing thermal ocean expansion as the ocean water previously protected from solar radiation absorbs direct infrared radiation. The proportion of thermal ocean expansion attributed to ice melt is not independently measured. All sources of expansion are combined and measured by recording changes in ocean temperatures.
In Maryland, the melting of glaciers is by far the greatest contributor to present sea level rise among the types of ice reservoirs (MCCC 2013). Glacial melt is anticipated to increase in its contribution to local sea level rise going forward during the 21 st century (NRC 2015). The Chesapeake Bay formed as a glacier retreated north at the close of the last ice age, about 12,000 years ago. In coastal Maryland, the most important glacial process is glacial isostatic adjustment (MCCC 2013). The melting of glaciers following the last ice age resulted in a readjustment of Earth’s crust. The crust is compressed under the weight of the glacier. As it melts, the crust beneath rebounds. The land furthest from the polar region under the influence of a glacier adjusts downward in a see-saw effect. Maryland is located at the southern edge of the region influenced by the remnants of the glacier that retreated to Greenland. As the glacier in Greenland melts, Maryland’s coast moves vertically downward. The resulting lower elevation makes the coastal areas more susceptible to sea level rise. Estimates from one model are available for tide gauge sites around the world and indicate the net glacial isostatic adjustment effect on relative sea level to range from 0.76 to 1.02 mm/year for Maryland tide gauge sites (PSMSL 2018).
The Intergovernmental Panel on Climate Change (IPCC) states with high confidence that since the early 1970s, ocean thermal expansion and glacier mass loss from warming together explain about 75% of the observed global mean sea level rise (IPCC 2014b). According to projections in the Climate Change 2014: Impacts, Adaptation and Vulnerability, The Intergovernmental Panel on Climate Change (IPCC) Working Group II Contribution to the IPCC 5th Assessment Report, thermal expansion accounts for 30-55 % of 21st century global mean sea level rise and glacier melt accounts for 15-35 % (IPCC 2014a). However, as time goes on, the proportional contribution by the loss of mass of the Greenland and Antarctic ice sheets is expected to increase (NRC 2015). Steps taken over the next 30 years to control greenhouse gas emissions and stabilize global temperatures during this century will largely determine how great the sea-level rise challenge is for coastal residents in subsequent centuries. After that, there is not much that can be done to slow sea-level rise because of the inertia of ocean warming and polar ice sheet loss (MCCC 2013).
The surface of the world’s oceans is not level, but varies regionally due to spatial variations in temperature, gravity, and the dynamic motions of ocean currents, among other effects. As the world warms and more water is added to the oceans, the rise in sea level will also not be uniform. Melting of polar ice sheets will reduce the polar land mass and thus the gravitational attraction of ocean water, counter-intuitively resulting in sea-level decline in nearby polar regions and sea-level increase in temperate and tropical regions (MCCC 2013). As a result, the U.S. East Coast, including Maryland, will experience sea level rise as the ice mass in Greenland and Antarctica melt. Sea level will increase less close to the ice masses because the gravitational attraction of ocean water is diminished (MCCC 2013).
As Greenland’s glacial ice and adjacent Artic ice sheets melt, cold, freshwater is released and diffuses into the Atlantic Ocean. In the North Atlantic near Iceland, this cold freshwater mixes with the warm, dense saltwater of the Gulf Stream. This decreases the velocity of the Gulf Stream. Prior to this accelerated ice melt, the Gulf Stream moved warm, dense saltw ater swiftlyfrom the tropics to the edge of the Artic near Iceland. The warm, salt-dense water abruptly met the cold, less-dense freshwater. The Gulf Stream was quickly cooled, resulting in sinking of the now cold, dense saltwater at a high rate of speed, transporting approximately four billion cubic feet of water per second (NOAA 2018). This drives the circulation of the deep ocean conveyor. The recent accelerated Greenland and Artic ice melt is delivering large quantities of cool, freshwater into the North Atlantic where it disperses over a wide area, semi-cooling the North Atlantic waters. This large expanse of cool freshwater mixes with the Gulf Stream as it travels toward Iceland. The salt in the Gulf Stream is diluted by this increased volume of freshwater, making it less-dense and mildly cooler. Consequently, when the Gulf Stream arrives at the point near Iceland where it usually meets the Arctic cold, it drops vertically to the bottom of the ocean more slowly. The slowing of the Gulf Stream effects sea level along the Mid-Atlantic coast. As the Gulf Stream flows from the coast at Cape Hatteras and turns north-eastward, the Coriolis force, resulting from the rotation of the earth, acts to force water offshore. To balance this effect, ocean water is drawn off the shelf in the Middle Atlantic Bight and the sea surface along the coast is typically about one meter lower than in the open ocean on the far side of the Gulf Stream (MCCC 2013). If the flow of this massive current declines, the height gradient is diminished, with the sea surface falling in the open ocean, but rising along the coast. Recent research suggests higher rates of sea-level rise along the Mid-Atlantic coast during the past decade or two (Ezer & Corlett. 2012, Ezer & Corlett. 2012, Moss et al. 2010) and links this trend with the decline in strength of the Gulf Stream (Ezer et al. 2013). Sea-level projections for Maryland should take such regional ocean dynamics into consideration (MCCC 2013). Sea level at Chesapeake Bay tidal gauges varied over several years in relation to variations in Gulf Stream flow. Beginning around 2004, however, the flow of the Gulf Stream went into steady decline and, by 2007, sea level at the tide gauges in the Middle Atlantic Bight was showing a steady increase (MCCC 2013).