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How Oceans Shape Climate

Most discussions of climate change focus on what is happening above ground — because warmer temperatures in the part of the atmosphere where we live affect us the most. But the oceans— one of the most important working parts of the climate system — get very little attention.

The oceans are a reservoir which can and do store vast amounts of heat energy. The sunlight reaching Earth heats the atmosphere, the land, and the ocean surfaces. Many weather and climate patterns are formed as sun-heated air moves in global circulation patterns to distribute heat from warmer places to colder ones. In fact, once heat energy from the sun reaches this planet, it is exchanged many times between land, air, and water.

Basic climate science suggests that more heat will be trapped in the atmosphere if concentrations of manmade greenhouse gases continue to build. How long will this take? That’s where the oceans come in. The oceans will absorb much of the heat as Earth’s temperature rises. And if you’ve ever waited for a teapot to boil, you know it takes a long time to heat water. Now think of a teapot the size of the Pacific. No, global warming won’t make the oceans boil — but it could take centuries for a human-enhanced greenhouse effect to raise the ocean temperature even a few degrees.

So the heating of Earth by a human-enhanced greenhouse effect will take a lot longer because the process will not be over until the oceans have heated up. And it will take the oceans a lot longer than the atmosphere to heat up, because they have much more heat capacity than the atmosphere does. This may be good news, but only temporarily. The ocean-caused delay in global warming could encourage people to ignore the problem and to delay doing something about it.

That’s just the beginning. The more scientists learn about climate, the more they discover about how important the oceans are in shaping weather on time-scales ranging from days to centuries. The hurricanes that menace the Atlantic and Gulf coasts of the United States every August get their energy from the warmed-up surface layer of the Atlantic.

The water in the ocean circulates. That is, it moves in slow, steady currents that span the globe. These currents and circulation patterns can do a great deal to determine climate. For example, the Gulf Stream and its extensions make the climate of Europe warmer than it would otherwise be.

Ocean currents are driven by convection (the chimney effect), among other things, just as air currents are. Both hot air and hot water tend to rise because they are less dense. Salinity and air currents also drive ocean circulation. Changes in sea ice in polar regions (which can be caused by global warming) can affect water salinity, and vice versa. So the climate system interacts with ocean currents and the forces that drive them in a number of ways.

There is some evidence in ocean sediments that ocean currents can change rather quickly, and that changes in ocean currents are associated with (and perhaps have caused) climate change during prehistoric times.

In recent years, scientists have rapidly increased their understanding of some of the natural cycles in ocean conditions and currents. The El Niño is just one example of a semi-regular oscillation in the ocean that affects climate. Others, such as the Pacific Decadal Oscillation and the North Atlantic Oscillation, also affect climate. Better understanding of these forces will eventually make it easier to distinguish “natural” climate variation from climate change caused by humans.

Still another important climate-related role oceans play is that of helping regulate the amount of carbon dioxide (the principal greenhouse gas) in the atmosphere. The oceans are constantly both releasing dissolved carbon dioxide into the atmosphere and absorbing carbon dioxide from the atmosphere. The amounts exchanged are vast — estimated at greater than 100 billion metric tons of carbon a year moving in each direction. This compares with a human contribution from fossil-fuel-burning of slightly more than 5 billion metric tons of carbon. But oceans are believed to be a net “sink” for carbon, absorbing roughly 2 billion tons of carbon more than they release.

Many things can affect the rates of carbon dioxide exchange between atmosphere and ocean — including changes in currents and surface temperature. Little is known about how global warming might affect ocean currents, but changes in currents have the potential for making global warming either better or worse. Carbon dioxide dissolves less readily when water is warmer. If warming of the oceans reduces their ability to remove carbon dioxide from the atmosphere, this could produce a “positive feedback” which would actually intensify global warming.

Scientists have made great strides in recent decades in understanding the linkages between the oceans and the climate system. The more they learn, the more complex and important those connections seem to be.

Climate Change Update is produced by the National Safety Council’s Environmental Health Center (EHC) under a cooperative agreement with the U.S. Environmental Protection Agency. It is available on request. Write to Climate Change Update, Environmental Health Center, 1025 Connecticut Avenue, NW • Suite 1200, Washington, DC 20036, or e-mail ehc@nsc.org.

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Environmental Health Center
A Division of the National Safety Council
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September 4, 2001 | Disclaimer/Policy