Sea-level change
Sea-level change refers to the variations in sea levels caused by factors such as the shape and size of ocean basins, total mass of seawater, and fluctuations in water temperature and salinity. Since 1920, global sea levels have risen by approximately 6 to 8 inches, with predictions indicating a further rise of 10 to 12 inches by 2050, and up to 2 feet by 2100. The dynamics behind these changes include thermal expansion of water due to warming temperatures, as well as contributions from melting glaciers and ice sheets, particularly in Antarctica and Greenland.
The consequences of rising sea levels are significant, especially for coastal regions, as they lead to beach erosion, flooding of low-lying areas, and increased vulnerability to storm impacts. Developing countries in Asia and Africa, along with island nations like the Maldives and Kiribati, are particularly at risk, with potential mass displacement of populations. Additionally, rising sea levels threaten freshwater supplies through saline intrusion into aquifers, exacerbating water shortages.
Recent studies have shown that the melting of ice sheets is occurring at an accelerated rate, which could have profound long-term effects on global sea levels. With the majority of the world’s population residing in coastal areas, understanding and addressing the implications of sea-level change has become increasingly urgent.
Sea-level change
Rising sea levels associated with global warming pose a threat to beaches and coastlines, coastal communities, and aquifers, directly or indirectly affecting the lives of hundreds of millions of people.
Background
Sea level can change, both globally and locally, as a result of changes in the shape and size of the ocean basins, changes in the total mass of seawater, and variations in water temperature and salinity. It is estimated that the global sea level rose by 6 to 8 inches from 1920 to 2020, and forecasts made in 2022 by the National Oceanic and Atmospheric Administration (NOAA)predicted an increase of 10 to 12 inches between 2020 and 2050. Also, in 2022, government agencies predicted by 2100, that number will likely increase to 2 feet.
Dynamics of Sea-Level Change and Rise
The forecasts made by the Intergovernmental Panel on Climate Change (IPCC) in 2007 took some account of runoff from melting mountain glaciers and melting ice sheets in Antarctica and Greenland. For the most part, however, they were estimates of thermosteric sea-level change. Those made by NOAA and the National Aeronautics and Space Administration (NASA) in 2022 follow similar methodology. Water increases in volume as it grows warmer, although it is difficult to establish a timescale for such expansion, delayed as it is by the thermal inertia of oceans (especially at their greatest depths). Thus, at least some of the observed changes in sea level in the recent past are undoubtedly due to earlier changes in air and sea temperature, while sea levels will probably rise over a period of several hundred years even if temperatures are brought under control. Regional and local sea levels are and will continue to be affected by such factors as salinity, atmospheric pressure, river runoff, currents, storms, tides, and wave patterns.
Consequences of Sea-Level Rise
Although the dynamics of sea-level change are complex and not completely understood, there is less uncertainty about the consequences of sea-level rise, which poses severe dangers to coastlines by eroding beaches and flooding adjacent low-lying areas. There is no indication that rising sea levels (as opposed to warmer seas) contribute to the intensity of storms, but they do leave coastal areas more vulnerable to their effects by overwhelming estuaries and covering barrier islands. In particular, rising sea levels threaten the developing countries of Asia and Africa, where major population centers are often situated in or near low-lying deltas. Island nations such as Maldives and Kiribati will in time disappear below the sea, necessitating the mass relocation of their entire populations. China is the country that will potentially have the largest percent of its population affected by sea level rise, with 43 million Chinese living in threatened coastal locations. Rising sea levels also pose the danger of saline pollution to aquifers, which in turn will lead to shortages of freshwater needed for agriculture and human consumption. Because of growing populations, such shortages are already a serious problem in many parts of the world.
Increased Danger
The IPCC acknowledged that limited understanding of the processes involved in melting ice sheets meant that their effects were underestimated in its 2007 report. Soon afterward, however, more thorough studies of the Greenland and Antarctic ice sheets revealed that they were melting more rapidly than expected. This melting was due not only to increases in temperature but also to disintegration caused by water seepage in crevasses. Coupled with other findings, this evidence suggested that sea levels would increase by 12 inches in the first half of the twenty-first century and double that number by 2100.
Concerns about rising sea levels heightened in April 2009, with the collapse of an ice bridge linking the Wilkins Ice Shelf, which once covered some 16,000 square kilometers of the Antarctic Ocean, to Charcot Island. It was one of nearly a dozen such formations in the region to have collapsed in the preceding fifty years. In 2022, the Conger Ice Shelf, a Hong Kong-sized piece of ice in East Antarctica, collapsed after temperatures in the region soared above normal. Because ice shelves float, their disintegration does not raise water levels, but in their absence, the glacier ice accumulated behind them enters the sea more readily—a process that does raise water levels.
Forecasts of sea-level rise created by melting ice sheets are complicated by two other factors. Post-glacial rebound would decrease the size of the ocean basins, while small but significant changes in the Earth’s rotation are also likely to follow the disappearance of the enormously heavy ice sheets in Antarctica, resulting in higher sea levels in the Indian Ocean.
Context
Fragmentary evidence suggests that sea level began to rise in the middle of the nineteenth century, while more compelling evidence shows that this rate began to increase toward the end of the twentieth century. A number of factors enter into this increase, but because so many people live in coastal areas around the world, the immediate consequences of what appears to be an increasingly rapid rate of sea-level rise are grave.
The long-term consequences of melting ice sheets are even more profound. For instance, the West Antarctic Ice Sheet, a formation that scientists believe is particularly vulnerable to disintegration as world temperatures increase and say the entire ice sheet would release enough meltwater to raise sea levels by 3 meters. The Earth’s glaciers, ice caps, and ice sheets combined contain an extraordinary sea-level equivalent of 68.5 meters. While no one predicts that the ice sheets are in immediate danger of wholesale melting, the momentum of such a change, should it begin, would be virtually impossible to counter by any technology currently available to humankind.
Key Concepts
aquifer: a layer of sand, gravel, or permeable rock containing waterglacier: a flowing body of land iceice cap: a domed body of ice covering a highland; smaller than an ice sheetice sheet: a deep body of ice covering and dominating a land massice shelf: a floating body of ice extending from a coast and often attached to an ice sheetpost-glacial rebound: a rise in elevation of a land mass after a glacier on top of it meltssea level: the theoretical level of the surface of the ocean, halfway between high and low tidesea-level equivalent (SLE): the change in global average sea level that would occur if a given amount of water or ice were added to or removed from the oceansthermosteric sea-level change: variability in sea level due to changes in water temperaturewater table: the upper level of groundwater
Bibliography
"2022 Sea Level Rise Technical Report." NOAA's National Ocean Service, oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html. Accessed 19 Jan 2023.
Douglas, Bruce C., Michael S. Kearney, and Stephen P. Leatherman, eds. Sea Level Rise: History and Consequences. San Diego: Academic Press, 2001.
Duncombe, Jenessa. "A Clue to Antarctic Ice Shelf Collapse." Eos.org, 22 Apr. 2022, eos.org/articles/a-new-clue-to-antarctic-ice-shelf-collapse. Accessed 19 Jan. 2023.
Johansen, Bruce E. “Sea-Level Rise.” In Melting Ice and Warming Seas. Vol. 2 in Global Warming in the Twenty-first Century.
King, Michael D., ed. Our Changing Planet: The View from Space. New York: Cambridge University Press, 2007.
MacCracken, Michael C., Frances Moore, and John C. Topping, Jr., eds. Sudden and Disruptive Climate Change: Exploring the Real Risks and How We Can Avoid Them. Sterling, Va.: Earthscan, 2008.
Phelan, Joe. "What Countries and Cities Will Disappear Due to Rising Sea Levels?" Live Science, 27 Mar. 2022, www.livescience.com/what-places-disappear-rising-sea-levels. Accessed 19 Jan. 2023.