Satellite view of Arctic ice in 2008

Source: NASA/Goddard Space Flight

Center Scientific Visualization Studio

The oceans are a logical place to start to understand how climate change is affecting our planet. The oceans have a central role in protecting Earth. But ocean acidification, rising sea levels and melting ice caps, particularly in the polar region, are endangering our planet. Since water circulates over the globe in a predictable pattern, changes in the great ocean conveyor belt affect worldwide climate and the ocean’s inhabitants. Learn more about these factors as well as the potential for extreme weather and intensified bleaching of coral reefs.



Arctic sea ice

Source: NOAA

Climate Change and the Oceans

What is the ocean’s role in Earth’s climate? Earth is often called the blue planet because the Atlantic, Pacific, Indian, Arctic and Southern oceans cover 71 percent of it. The Pacific alone covers more than half the planet, and more than all the land areas combined. Oceans contain nearly 1.34 billion km3 of water, and Earth’s waters are overwhelmingly salt water—97 percent—plus 2 percent ice and 1 percent fresh water. The ocean is our planet’s largest heat sink. By absorbing, storing and then slowly releasing large quantities of heat, the ocean buffers the climate of the nearby land and, over time, the entire planet.

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What is ocean acidification?

Oil refinery smokestacks

© UCAR / Photo by Carlyle Calvin

Ocean acidification is different from climate change, but related to it because both are caused by CO2. The ocean is the earth’s largest carbon reservoir, containing more than 50 times as much CO2 as the atmosphere. As more man-made C02 has entered the atmosphere, more of the gas has entered the ocean. But the uptake of excess CO2 comes at a high cost — ocean acidification.

In a chemical reaction, CO2 dissolves in the ocean, raising the level of acidity. Since the Industrial Revolution, the oceans have become 30 percent more acidic; it is possible that by the end of the century, the surface oceans could become 150 percent more acidic.

Ocean acidification is particularly damaging to the many organisms that use calcium carbonate to build protective shells. Collectively called “calcifying organisms,” they include some phytoplankton, and many invertebrates such as corals, sponges, marine worms, mollusks, and crustaceans. Increased acidity makes it harder for them to form shells, which will cause further negative changes in many marine ecosystems as the decline of calcifying species affects other species that depend on them for food. So climate change does not directly cause acidification of the oceans per se; rather climate change and ocean acidification are two separate and serious problems that are related because both are caused by excessive emissions of carbon dioxide.

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Are sea levels rising?

When water warms, it expands and takes up more volume. This effect is called "thermal expansion." Long-term measurements demonstrate that sea levels are rising worldwide both from thermal expansion caused by warming temperatures and from the addition of water from inland glaciers, which are melting nearly everywhere at accelerating rates. Increased melting is also occuring at the ice caps in Greenland and West Antarctica. 

Many scientists now think that sea levels will rise by at least one to two feet by 2100. A rise of two to six feet is possible, if emissions of greenhouse gases remain unchecked and significant melting of the ice caps occurs. A rise in sea level of just a foot or two could have significant negative consequences for islands in the Caribbean and the Pacific and for low-lying coastal areas along the continental U.S., such as the eastern shoreline of Cape Cod, the barrier islands protecting North Carolina, most of southern Florida and the city of Boston. Since most of the world's major cities also lie along ocean coastlines, sea level rise has major implications for those important population centers, where erosion, flooding and rising groundwater levels will threaten buildings, roads, subway systems and other essential services.

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What are the effects of melting ice caps?

Black soot released as pollution may

accelerate melting sea ice by settling

on snow and absorbing the sun's rays.

Source: NASA/Goddard Space

Flight Center Conceptual Image Lab

Additional rises in sea level could be set off by the melting of the world’s major ice caps. Rising global temperatures are believed to be melting Greenland’s massive icecap and the seaward fringe of the Western Antarctic ice sheet faster than ever before, with slabs of ice breaking off as icebergs. Loss of ice around the fringes makes it easier for blocks of continental ice to slip toward the ocean; there is the possibility that ice sheets could collapse, raising sea levels dramatically. (Greenland and Antarctica are "ice caps" on top of solid land. The Arctic ice is floating in the sea; significant melting in the Arctic will not affect sea levels, although other serious adverse effects are likely. See below for more information.)

While it does not appear likely that they or the larger Eastern Antarctic ice sheet will melt substantially during this century, it has recently been estimated that even a partial melt could increase the upper range of sea level rise to six feet. This would have devastating effects for low-lying areas throughout the world.

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What about melting sea ice in the Polar region?

According to the Arctic Climate Impact Assessment Report (2005), climate change in the Polar region is expected to be some of the largest and most rapid, and will cause major physical, ecological, sociological, and economic impacts, especially in the Arctic, Antarctic Peninsula, and Southern Ocean. In the Arctic, warming is occurring faster than the global mean. From 1978 to 2004, perennial sea ice cover in the Arctic declined by 7.8 percent each decade. If these trends continue, late-summer sea ice could disappear from the Arctic as early as early as 2030.

Illustration by Steve Deyo, ©UCAR, based on research by NSIDC and NCAR

Due to its light color, Arctic sea ice reflects most of the sunlight that reaches it back into space. In contrast, dark ocean water absorbs most of the sunlight. As sea ice continues melting, it can cause further warming, leading to more ice melt and reinforcing the melting cycle. As summer melt increases, wintertime recovery becomes more difficult.

The animal most vulnerable to sea ice melt is the species at the top of the Arctic food chain, the polar bear, which hunts primarily on the sea ice. Penguins in the Antarctic are also affected by the disappearance of sea ice.



The shrinkage of the Arctic ice will not affect sea level, however, since the Arctic ice cap is a sheet of ice “floating” in the water, much like ice cubes in a glass of water. If the ice cubes melt, they do not increase the level of water in the glass. In contrast, Greenland and Antarctica consist of large masses of ice resting on top of solid land, and melting of these ice sheets will raise sea level.

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What is the great ocean conveyor belt? How does climate change affect it?

In addition to wind and tides, ocean currents are driven by differences in water density. Dense cold salty water at the surface of the North Atlantic sinks and pushes currents through the deeper parts of the world’s oceans to Antarctica, then to the northeastern Pacific Ocean, where it rises to the surface, continues south and west past Indonesia, around Africa’s Cape of Good Hope and north through the South Atlantic and North Atlantic oceans. While it is on the surface, the current becomes steadily warmer and saltier by evaporation.

On the last part of its journey, it loops into the Gulf of Mexico, north along the edge of the continental shelf of eastern North America, then east to Europe where its heat and moisture warms countries from Great Britain to Norway. (This is why the U.K. and much of Europe are warmer than much of the U.S. even though these countries are at a higher latitude.) After releasing its heat into the atmosphere, the cooled but still salty water then flows east to Greenland and Newfoundland, where it sinks to begin another 1,000-year cycle of the great ocean conveyer belt.

Dramatic climate change can occur from the reduction of large-scale mixing of water — thermohaline circulation — throughout the ocean. Increased precipitation and widespread melting of ice caused by global warming could create a larger layer of fresh water that would slow or prevent normal thermohaline mixing and would affect the currents offshore from Greenland and Newfoundland.

Worldwide thermohaline circulation has abruptly shut down and recovered in the past, causing climate to flip-flop from warm to cold to back again. If such an event happened today, there could be crop failures in Canada, England, and northern Europe. The current consensus among scientists is that thermohaline circulation is not likely to be disrupted if global warming is limited to 2 degrees C; however with 3 to 5 degrees C of warming, a number of experts interviewed by the German Advisory Council on Global Change (WBGU) think the risk is greater than 50 percent.

Source: Grid Arendal and UNEP

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Is global warming causing more extreme weather?

Even small temperature increases can significantly increase the frequency and severity of extreme weather events, such as hurricanes or high-precipitation rainstorms. Warming of the tropical sea-surface by 0.5º C during recent decades may explain the fact that hurricane energy has increased globally by about 70%. The IPCC report of 2007 considers it likely that global warming will increase intense tropical cyclone activity through most of the 21st century and very likely will increase the frequency of heavy precipitation over most areas.

The increasing cost of hurricane damage is primarily due to the increased amount and monetary value of rapid development that has taken place in coastal areas. The high winds and waves associated with intense storms cause severe physical damage to shorelines and shallow water marine communities including beaches, mangrove forests, sea grass communities, coral reefs and shallow bottom-dwelling communities. High rainfall can cause temporary flooding, erosion and decreased salinity, as well as runoff of nutrients and silt.

Global warming is predicted to increase the frequency of severe storms at the same time that sea level rise magnifies their impact on low-lying coastlines and islands. New England is particularly vulnerable to extreme weather events from nor’easters and hurricanes to flooding low-lying coastlines and islands due to sea level rise.

Source: NCDC

This chart from The National Climatic Data Center (NCDC) shows severe weather events over the last three decades. NDCC tracks and evaluates climate events in the U.S. and globally that have significant economic and societal impact.

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Coral reef in the Pheonix Islands

Protected Area, one of the Aquarium's

major conservation projects

How are coral reefs affected by warmer temperatures and acidification?

Coral reefs may host the highest biodiversity on Earth. Reef animals yield antibiotics, anticancer drugs and other medical products, and pharmaceutical companies are actively prospecting for more. Tourism and recreation, for example at the Great Barrier Reef and in the Caribbean, add to the economic value of reefs. Fringing reefs protect shorelines from storm surges and erosion. Coral reefs may be the most vulnerable to all the climate stressors — rising temperatures, acidification, invasion by pathogens, higher sea levels and increased frequency of severe storms.

Coral bleaching occurs when zooxanthellae abandon corals in response to high temperatures. Zooxanthellae, which are photosynthesizing algae and cyanobacteria combined, provide more than 95% of the food of their coral hosts and give corals their particular coloration. Under stress, corals expel these zooxantheallae, which leads to a bleached appearance. Corals can survive brief periods of coral bleaching but sustained bleaching can lead to coral death.

The U.S. National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching hot spots. In 1998 and 2002, the Great Barrier Reef and other coral reefs experienced the most severe bleaching ever recorded. The Belize Barrier Reef has also suffered bleaching due to higher temperatures. While some corals are more resistant to bleaching, and others can recover after a disturbance of this sort or even migrate, rising ocean temperatures will probably cause more bleaching events in the future.

Ocean acidification is another serious ocean-related issue, caused by some of the same emissions that cause climate change. As man-made C02 enters the atmosphere, more gas enters the ocean, which has become more acid. Cold-water corals, which are found at depths from 200-1000 meters, are particularly vulnerable to the effects of acidification. The corals appear to live for hundreds of years, forming large reef systems. Ocean acidification may affect cold-water corals earlier and more strongly than warm-water reefs, because at greater depth, seawater is less saturated with carbonate. Wherever they do occur, cold-water reefs are biodiversity hotspots that serve as refuges, nurseries and feeding grounds for hundreds of marine species.

These very slow growing corals are extremely vulnerable to man-made disturbances, such as bottom-trawl fishing, which has already severely damaged reefs in the northeastern Atlantic. Acidification will make them even more vulnerable. The hardier cold-water corals will be severely stressed by 2040 and two-thirds could be in a corrosive environment by the end of the century.

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