Consequences of climate change on oceans (17)
Over the past century, most of the world’s mountain glaciers and ice sheets in both Greenland and Antarctica have lost mass.
By Climate Institute
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 percent of the worl’s land area and exist on every continent except Australia. They are the world’s largest reservoir of fresh water, holding approximately 75 percent.
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.
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 Kilimanjar’s glacial ice has decreased by about 80 percent.
If this rate of loss continues, its glaciers will likely disappear within the next decade. Similar glacial melting back are occurring in Alaska, the Himalayas, and the Andes.
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.
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.
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. Therefore, the mass balance of the entire Antarctic Ice Sheet is negative.
Sea level rise
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.
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.
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.
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.
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 breakup 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, with the rate of rise due to this term having increased to about three 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 (Inter-governmental Panel on Climate Change) Fourth Assessment projected that thermal expansion will lead to sea level rise of about 17-28 cm (plus or minus about 50 percent).
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 10-12cm to sea level rise, with an uncertainty of roughly a third. This would represent a melting of about 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 meters 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 meters 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.
Impacts of sea level rise
In summarizing the results of model simulations for the 21st century, IPCC reported that the central estimates projected that Greenland would induce about a 2cm rise in sea level whereas Antarctica would, because of increased snow accumulation, induce about a 2cm 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.
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.
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 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.
More land would be lost
The U.S. 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 percent 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.
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.
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.
For some island nations another possibility 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.
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.
An ecosystem (short for “ecological system”) is generally defined as a community of organisms living in a particular environment and the physical elements with which they interact. An ecosystem is an open functional unit that results from the interactions of abiotic (soil, water, light, inorganic nutrients and weather), biotic (plants, animals, and microorganisms usually categorized as either producers or consumers), and cultural (anthropogenic) components.
An ecosystem can be as small as a field or as large as the ocean. It is used to describe the world’’s major different habitat types. Terrestrial ecosystems include: arctic and alpine ecosystems, dominated by tundra with scarce vegetation; forest ecosystems, which can be subdivided into a whole range of types including tropical rainforests, Mediterranean evergreen forests, boreal forests, and temperate coniferous, deciduous and mixed forests; grasslands and savannas; and deserts and semi-arid ecosystems. Freshwater ecosystems include lakes, rivers, and marshlands. Marine ecosystems comprise an enormous range, from coral reefs, mangroves, sea-grass beds, and other shallow coastal water ecosystems, to open-water ones, including the mysterious, little-known ecosystems of the abyssal plains and trenches of the world’s oceans.
Ecosystems sustain human societies and allow them to prosper, due to the nutritional, environmental, cultural, recreational and aesthetic resources they provide. We all depend directly or indirectly on the products and services of ecosystems, including crops, livestock, fish, wood, clean water, oxygen, and wildlife.
In the over 25 years since its founding in 1986 as the first environmental organization on the Earth focused on climate protection, the Climate Institute has been instrumental in moving climate protection onto the international agenda, fostering collaboration between developing countries and richer nations, and in launching and implementing pioneering studies or initiatives on such subjects as environmental refugees. This article was complied by several writers, including institute president John Trapping.