OCEANS & SEA LEVEL RISE
Consequences of Climate Change on the Oceans
Melting of Glaciers and Ice Sheets
One of the most pronounced effects of climate change has been melting of masses of ice around the world. Glaciers and ice sheets are large, slow-moving assemblages of ice that cover about 10% of the world’s land area and exist on every continent except Australia. They are the world’s largest reservoir of fresh water, holding approximately 75% (1).
Over the past century, most of the world’s mountain glaciers and the ice sheets in both Greenland and Antarctica have lost mass. Retreat of this ice occurs when the mass balance (the difference between accumulation of ice in the winter versus ablation or melting in the summer) is negative such that more ice melts each year than is replaced (2). By affecting the temperature and precipitation of a particular area, both of which are key factors in the ability of a glacier to replenish its volume of ice, climate change affects the mass balance of glaciers and ice sheets. When the temperature exceeds a particular level or warm temperatures last for a long enough period, and/or there is insufficient precipitation, glaciers and ice sheets will lose mass.
One of the best-documented examples of glacial retreat has been on Mount Kilimanjaro in Africa. It is the tallest peak on the continent, and so, despite being located in the tropics, it is high enough so that glacial ice has been present for at least many centuries. However, over the past century, the volume of Mount Kilimanjaro’s glacial ice has decreased by about 80% (3). If this rate of loss continues, its glaciers will likely disappear within the next decade (4). Similar glacial meltbacks are occurring in Alaska, the Himalayas, and the Andes.
Image from global-greenhouse-warming.com
When researching glacial melting, scientists must consider not only how much ice is being lost, but also how quickly. Recent studies show that the movement of ice towards the ocean from both of the major ice sheets has increased significantly. As the speed increases, the ice streams flow more rapidly into the ocean, too quickly to be replenished by snowfall near their heads. The speed of movement of some of the ice streams draining the Greenland Ice Sheet, for example, has doubled in just a few years (5). Using various methods to estimate how much ice is being lost (such as creating a ‘before and after’ image of the ice sheet to estimate the change in shape and therefore volume, or using satellites to ‘weigh’ the ice sheet by computing its gravitational pull), scientists have discovered that the mass balance of the Greenland Ice Sheet has become negative in the past few years. Estimates put the net loss of ice at anywhere between 82 and 224 cubic kilometers per year (5).
Image from UNEP
In Antarctica, recent estimates show a sharp contrast between what is occurring in the East and West Antarctic Ice Sheets. The acceleration of ice loss from the West Antarctic Ice Sheet has doubled in recent years, which is similar to what has happened in Greenland. In West Antarctica, as well as in Greenland, the main reason for this increase is the quickening pace at which glacial streams are flowing into the ocean. Scientists estimate the loss of ice from the West Antarctic ice sheet to be from 47 to 148 cubic kilometers per year. On the other hand, recent measurements indicate that the East Antarctic ice sheet (which is much larger than the West) is gaining mass because of increased precipitation. However, it must be noted that this gain in mass by the East Antarctic ice sheet is nowhere near equal to the loss from the West Antarctic ice sheet (5). Therefore, the mass balance of the entire Antarctic Ice Sheet is negative.
Impacts
The melting back of the glaciers and ice sheets has two major impacts. First, areas that rely on the runoff from the melting of mountain glaciers are very likely to experience severe water shortages as the glaciers disappear. Less runoff will lead to a reduced capability to irrigate crops as freshwater dams and reservoirs more frequently go dry. Water shortages could be especially severe in parts of South America and Central Asia, where summertime runoff from the Andes and the Himalayas, respectively, is crucial for fresh water supplies (6). Also, in areas of North America and Europe, glacial runoff is used to power hydroelectric plants, sustain fish runs and irrigate crops as well as to supply the needs of large metropolitan areas. As the volume of runoff decreases, then the energy, urban, and agricultural infrastructures of such locations are likely to be stressed (7).
In addition, the melting of glaciers and ice sheets adds water to the oceans, contributing to sea level rise, as explained in the next subsection.
Most of the world’s coastal cities were established during the last few millennia, a period when global sea level has been near constant. Since the mid-19th century, sea level has been rising, likely primarily as a result of human-induced climate change. During the 20th century, sea level rose about 15-20 centimeters (roughly 1.5 to 2.0 mm/year), with the rate at the end of the century greater than over the early part of the century (8, 9). Satellite measurements taken over the past decade, however, indicate that the rate of increase has jumped to about 3.1 mm/year, which is significantly higher than the average rate for the 20th century (10). Projections suggest that the rate of sea level rise is likely to increase during the 21st century, although there is considerable controversy about the likely size of the increase. As explained in the next section, this controversy arises mainly due to uncertainties about the contributions to expect from the three main processes responsible for sea level rise: thermal expansion, the melting of glaciers and ice caps, and the loss of ice from the Greenland and West Antarctic ice sheets (11).
Image from NASA
Causes of sea level rise
Before describing the major factors contributing to climate change, it should be understood that the melting back of sea ice (e.g., in the Arctic and the floating ice shelves) will not directly contribute to sea level rise because this ice is already floating on the ocean (and so already displacing its mass of water). However, the melting back of this ice can lead to indirect contributions on sea level. For example, the melting back of sea ice leads to a reduction in albedo (surface reflectivity) and allows for greater absorption of solar radiation. More solar radiation being absorbed will accelerate warming, thus increasing the melting back of snow and ice on land. In addition, ongoing break up of the floating ice shelves will allow a faster flow of ice on land into the oceans, thereby providing an additional contribution to sea level rise.
There are three major processes by which human-induced climate change directly affects sea level. First, like air and other fluids, water expands as its temperature increases (i.e., its density goes down as temperature rises). As climate change increases ocean temperatures, initially at the surface and over centuries at depth, the water will expand, contributing to sea level rise due to thermal expansion. Thermal expansion is likely to have contributed to about 2.5 cm of sea level rise during the second half of the 20th century (11), with the rate of rise due to this term having increased to about 3 times this rate during the early 21st century. Because this contribution to sea level rise depends mainly on the temperature of the ocean, projecting the increase in ocean temperatures provides an estimate of future growth. Over the 21st century, the IPCC’s Fourth Assessment projected that thermal expansion will lead to sea level rise of about 17-28 cm (plus or minus about 50%). That this estimate is less than would occur from a linear extrapolation of the rate during the first decade of the 21st century when all model projections indicate ongoing ocean warming has led to concerns that the IPCC estimate may be too low.
A second, and less certain, contributor to sea level rise is the melting of glaciers and ice caps. IPCC’s Fourth Assessment estimated that, during the second half of the 20th century, melting of mountain glaciers and ice caps led to about a 2.5 cm rise in sea level. This is a higher amount than was caused by the loss of ice from the Greenland and Antarctic ice sheets, which added about 1 cm to the sea level. For the 21st century, IPCC’s Fourth Assessment projected that melting of glaciers and ice caps will contribute roughly 10-12 cm to sea level rise, with an uncertainty of roughly a third. This would represent a melting of roughly a quarter of the total amount of ice tied up in mountain glaciers and small ice caps.
The third process that can cause sea level to rise is the loss of ice mass from Greenland and Antarctica. Were all the ice on Greenland to melt, a process that would likely take many centuries to millennia, sea level would go up by roughly 7 meters. The West Antarctic ice sheet holds about 5 m of sea level equivalent and is particularly vulnerable as much of it is grounded below sea level; the East Antarctic ice sheet, which is less vulnerable, holds about 55 m of sea level equivalent. The models used to estimate potential changes in ice mass are, so far, only capable of estimating the changes in mass due to surface processes leading to evaporation/sublimation and snowfall and conversion to ice. In summarizing the results of model simulations for the 21st century, IPCC reported that the central estimates projected that Greenland would induce about a 2 cm rise in sea level whereas Antarctica would, because of increased snow accumulation, induce about a 2 cm fall in sea level. That there are likely to be problems with these estimates, however, has become clear with recent satellite observations, which indicate that both Greenland and Antarctica are currently losing ice mass, and we are only in the first decade of a century that is projected to become much warmer over its course.
Image from wildwildweather.com
The Sea Level Rise Debate
Because the model simulations underestimate the sea level rise observed during the 20th century, significant debate has developed within the scientific community about IPCC’s projections of sea level rise for the 21st century. The accuracy of the projections has been questioned for a variety of reasons, particularly relating to limitations of the model representations of the ice sheets, which do not account for the increase in ice sheet movement (i.e., dynamics) that occurs as ice sheets warm, mainly because the physics are not well understood.
There are also problems projecting how rapidly and how much global temperature will increase during the 21st century, in part due to the range of possible emissions. Because rising temperatures play a key role in all three of the terms that contribute to sea level rise, uncertainties in projections of global warming lead to uncertainties in projections of sea level rise (9).
Regarding thermal expansion, there remain questions about the amount of heat that has been taken up by the oceans. Part of the problem results from the various types of instruments that have been used over time to measure ocean temperatures- different instruments create different results. At present, simulations of 20th century heat uptake by the oceans and of the amount of sea level rise do not fully match, making it more difficult to project the amount of thermal expansion that can be expected in the 21st century.
Second, the uncertainties in the increase in temperature affect the ability to project the rate of melting of mountain glaciers and ice caps. Observations of the retreat of glaciers have been, in a number of situations, more rapid than models have simulated. Whether this is a result of inadequacies in the modeling or a possible increase in the rate of melting prompted by deposition of soot, or both or possibly other factors, is not yet clear.
Third, and most important, are uncertainties relating to the potential loss of ice from the Greenland and West Antarctic ice sheets. The dynamics of ice sheet movement are not well understood—some ice streams are moving very rapidly, suggesting the potential for contributions to sea level rise of order 10 mm/year or even larger, a rate that is far larger than any of the other terms. There seems even the possibility of a collapse of one or both ice sheets, especially if there is rapid loss of buttressing ice shelves that would reduce the resistance to ice stream flows (9). Capturing these processes accurately in climate models is extremely difficult, while omitting the process that is likely the most important contributor to sea level rise presents quite a quandary—the result being that IPCC’s projections of sea level rise during the 21st century and beyond may be significantly too low.
Impacts of sea level rise
While there are obviously many challenges to projecting future sea level rise, even a seemingly small increase in sea level can have a dramatic impact on many coastal environments. Over 600 million people live in coastal areas that are less than 10 meters above sea level, and two-thirds of the world’s cities that have populations over five million are located in these at-risk areas (12). With sea level projected to rise at an accelerated rate for at least several centuries, very large numbers of people in vulnerable locations are going to be forced to relocate. If relocation is delayed or populations do not evacuate during times when the areas are inundated by storm surges, very large numbers of environmental refugees are likely to result.
According to the IPCC, even the best-case scenarios indicate that a rising sea level would have a wide range of impacts on coastal environments and infrastructure. Effects are likely to include coastal erosion, wetland and coastal plain flooding, salinization of aquifers and soils, and a loss of habitats for fish, birds, and other wildlife and plants (11). The Environmental Protection Agency estimates that 26,000 square kilometers of land would be lost should sea level rise by 0.66 meters, while the IPCC notes that as much as 33% of coastal land and wetland habitats are likely to be lost in the next hundred years if the level of the ocean continues to rise at its present rate. Even more land would be lost if the increase is significantly greater, and this is quite possible (11). As a result, very large numbers of wetland and swamp species are likely at serious risk. In addition, species that rely upon the existence of sea ice to survive are likely to be especially impacted as the retreat accelerates, posing the threat of extinction for polar bears, seals, and some breeds of penguins (13).
Unfortunately, many of the nations that are most vulnerable to sea level rise do not have the resources to prepare for it. Low-lying coastal regions in developing countries such as Bangladesh, Vietnam, India, and China have especially large populations living in at-risk coastal areas such as deltas, where river systems enter the ocean. Both large island nations such as the Philippines and Indonesia and small ones such as Tuvalu and Vanuatu are at severe risk because they do not have enough land at higher elevations to support displaced coastal populations. Another possibility for some island nations is the danger of losing their fresh-water supplies as sea level rise pushes saltwater into their aquifers. For these reasons, those living on several small island nations (including the Maldives in the Indian Ocean and the Marshall Islands in the Pacific) could be forced to evacuate over the 21st century (11).
Image from globalwarmingart.com
Each year the oceans absorb the equivalent of about a third of human emissions of carbon dioxide (CO2), transferring most of it to the deep ocean (13). Over the past 200 years, the increasing CO2 emissions from fossil fuel combustion have led to an exponential increase in the net amount of CO2 being dissolved in the ocean. Dissolved CO2 creates carbonic acid, which reduces the ocean pH level, making it more acidic (15).
Acidity is measured using the pH scale, where items are given a numerical value between 0 and 14. A value of seven is neutral, with higher values being described as basic and lowers values as acidic. Historically, ocean pH has averaged around 8.17, meaning that ocean waters are slightly basic. But with the rising CO2 concentration causing acidification, today the pH levels are around 8.09, edging the waters closer to neutral (16).
Geological evidence and model reconstructions indicate that, over the past 300 million years, the average pH of the ocean has not varied by more than 0.6 from its present value (14). Thus, the marine ecosystems present today have evolved in a relatively stable pH environment. With the rising CO2 concentration over the last 200 years, ocean pH has been steadily decreasing. While the acidification of the oceans is not yet itself worrisome except in polar regions, the rate at which the pH is dropping is becoming alarming. This is because the rate of change is so much higher than the natural weathering processes that have, in the past, buffered changes in ocean pH. If the CO2 concentration continues to rise and the pH level continues to fall at current rates, the ocean pH could drop by as much as 0.5 during the 22nd century (14). Such a drastic change would very likely have a substantial adverse impact on ocean life.
Possible impacts/ Preventative Measures
The most direct impacts of ocean acidification will be on marine ecosystems. A decrease in ocean pH would affect marine life by lowering the amount of calcium carbonate (the substance created when CO2 is initially dissolved) in the water. Calcium carbonate is the substance used by many marine organisms (including coral, shellfish, crustaceans, and mollusks) to build their shells (17). If the pH drops by the expected 0.5 during this century, the resulting effect would be a 60% drop in available calcium carbonate (17). Such a decrease would put the productivity and even the survival of thousands of marine species at risk.
To prevent the rapid acidification of the ocean and hold the pH level within an acceptable range for marine life, the atmospheric CO2 concentration needs to be kept below no more than about 450 parts per million (ppm). With the current concentration at roughly 387 ppm, the concentration seems likely to be near 500 ppm by mid-century without sharp reductions in emissions. To keep the decrease in pH to less than 0.2 pH, which could help to protect critical marine ecosystems, will require keeping the CO2 concentration below about 450 ppm (18).
Another impact of glacial retreat is the possible effect fresh melt water will have on the thermohaline circulation. Driven by density gradients in ocean waters, the thermohaline (or deep ocean overturning) circulation is made up of the global flow of ocean currents. As ocean waters move around, different water masses are formed as evaporation removes fresh water and precipitation and river runoff add fresh water, each changing ocean salinity and therefore the density of the waters. Surface currents, which are largely driven by wind patterns, take the water masses to areas where they are warmed by high solar radiation (leading to lower density) or cooled in higher latitudes (leading to higher density). When surface water density becomes greater than for waters below, downwelling currents carry the denser surface waters down and push less dense, nutrient rich waters toward the surface, where winds bring them all the way to the surface and create areas rich with marine life. Thus, the density gradients created by temperature (cold water is more dense than water that is warm) and salinity (salt water is more dense than freshwater) are critical to both how ocean waters move and where there are nutrients that promote significant marine life (19). Because both temperature and salinity are influenced by changes in the climate, there are concerns about the ways in which the thermohaline circulation might be affected.
The influences can operate in various ways. First, ocean circulation could be influenced by changes in runoff from glaciers and ice sheets. As glaciers melt and release fresh water into the ocean, the influx dilutes saltier waters, likely reducing the rate of bottom water formation because relatively fresh water will not be able to sink (even at higher latitudes where it becomes cold and dense), thus affecting deep ocean currents (20). With the rate at which glaciers are melting and the amount of freshwater that might be introduced into the ocean changing, it is thus quite possible that the intensity of the thermohaline circulation could be reduced.
Climate change will not only affect salinity levels, but will also affect ocean temperatures and circulation patterns. First, as ocean temperatures increase, thermal expansion will cause the density to decrease and so increase the volume of ocean waters, raising sea level. Because surface currents are driven by the winds, warm surface waters moved by the winds are generally replaced by the colder waters underneath, with the upwelling bringing up nutrient-rich colder waters that promote flourishing marine life (19). As ocean surface waters warm and become less likely to sink, a smaller amount of cold water is brought up to the surface, impacting circulation patterns and marine life. In addition, warmer temperatures will lead to more evaporation. When the water evaporates, the salt stays behind. An increase in salinity changes the density of the water, and therefore affects circulations patterns (21).
Given the interactions of these processes, there are increasing concerns that climate change will reduce the overall intensity of the thermohaline (deep-ocean) circulation. Should the increase in freshwater or the increasing ocean temperatures drastically alter density levels, the path of the thermohaline circulation could be altered or even significantly disrupted. Because the circulation plays a key role in ocean temperature patterns around the globe, weather patterns are also likely to be disrupted.
Image from UCAR
Changes such as these could be quite important for northern European countries. The Gulf Stream carries warm water from the tropics to the North Atlantic, and the heat it gives off to the atmosphere contributes to the mild temperatures in the region, even though Europe is located at a relatively high latitude. With sufficient cooling, the water sinks near Greenland and further north, pulling more warm waters northward from the tropics. If ocean warming slows the thermohaline circulation, less warm water would be transported north and Europe would likely experience less warming or even a cooling (21).
Such a cooling event may have occurred during the Younger Dryas about 12,000 years ago when meltwater release from rapid deglaciation of North America freshened the North Atlantic, likely shutting off the deep ocean circulation (22) and disrupting weather and ocean circulation patterns (23). Within a decade of the shutdown of the thermohaline circulation, global climate patterns were altered significantly and European and North American temperatures dropped by as much as 15ºC. Such a rapid and dramatic shift in climate has not happened since, but with melting of Greenland beginning, there is an increasing risk of a similarly sudden shift in the future (24).
Conclusion
As CO2 emissions and climate change continue, risks to the health of the ocean will become a more prominent concern. With accelerated melting back of glaciers and ice sheets and the subsequent rise in sea level, with further decreases in oceanic pH, and with deceleration of the thermohaline circulation, there are many ways in which the delicate balance of ocean dynamics and ecosystems are being put at risk. These factors, combined with the uncertainty in predicting exactly how these impacts will interact, are causing changes in the ocean: an increasingly problematic issue for future generations.
Figure 1: Causes of sea level rise from climate change. (2002). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 17:10, March 26, 2008 from http://maps.grida.no/go/graphic/causes-of-sea-level-rise-from-climate-change.
Additional Resources
Nicholls, R.J., P.P. Wong, V.R. Burkett, J.O. Codignotto, J.E. Hay, R.F. McLean, S. Ragoonaden and C.D. Woodroffe, 2007: Coastal systems and low-lying areas. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 315-356. http://www.ipcc-wg2.org/index.html
EPA. “Coastal Zones and Sea Level Rise.” Updated 08 February 2008
Union of Concerned Scientists. "Highlights from the First Section of the IPCC Fourth Assessment Report."
Interactive sea level map: http://flood.firetree.net/
CMAR. Sea Level Rise: Understanding the past -- Improving projections for the future
Climate Institute Articles on Rising Sea Level
The High Stakes for Small Islands (Autumn 2009 Climate Alert)
Polar Ice Melting Spurs Interest in Climate Stabilization
Sea Level Rise and Severe Storms in the Chesapeake Bay Region: A Canary in the Mineshaft?
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America’s Coast Under Global Warming Threat