Impact of ocean currents on global climate

DEFINITION: Continuous surface and deep-level movements of ocean waters in certain directions

For environmentalists, the study of ocean currents relates directly to two major concerns: the potential impact of the currents on global climate and weather changes and how the currents can assist in the conservation and sustainable cultivation of food and other marine resources.

Scientists understand a great deal about the significant large-scale movements of all ocean waters, but the specific driving forces are still under investigation. Studies of ocean currents are usually divided between a focus on surface currents, which are largely driven by prevailing winds, and a focus on deep-level currents, which are driven by more complex factors, including saline density, water temperatures, the gravitational pull of the moon, and sea-bottom depths as well as shoreline formations. The impacts of marine life-forms on water circulation, including the movement of microscopic species, are also part of the study of ocean currents.

Ocean currents in the upper 400 meters (roughly 1,300 feet) of the ocean depths are largely driven by wind currents, which represents the movement of roughly 10 percent of the ocean water in total. Surface currents and winds have been studied and measured for centuries. While precise modern models are quite complex, in general scientists refer to prevailing winds, which blow from west toward the east in the latitudes between 30 and 60 degrees, and trade winds, which blow from east to the west closer to the equator (below 30 degrees latitude). These winds create the classic sailing routes, such as the routes between the eastern United States and Europe, where sailing more north assists in a heading toward Europe and sailing more south (toward the equator) assists in a heading toward the eastern US seaboard.

The study of ocean currents and flows, however, reveals that this general picture is made far more complex by the existence of related currents called meanders, rings, eddies, and gyres. These are relatively smaller patterns of flow within the larger general current pattern. For example, an oceanic gyre or ring is created when winds and limitations of shorelines create circular movement of currents (the circles can be smaller, rings, or involve entire sections of an ocean surface between major continents, such as the South Pacific Gyre, the Indian Ocean Gyre, and the North Atlantic Gyre).

These smaller-scale currents are important for sea life as well as for navigation. Circular rings and eddies, rather like air weather patterns of cyclones or tornadoes, can draw materials from the bottom of the ocean and mix layers of currents that provide nourishment for the development of microscopic life, starting an important that supports various levels of higher sea life. Many of these rings thus sustain areas historically known for specific species of sea life. Consequently, disruptions in these patterns have potential implications for cultivation of these traditional stocks.

It has become increasingly important for scientists to understand long-term cycles of shifting weather and ocean current patterns. Prevailing winds and currents have been noted to run in regular patterns for thirty to fifty years and then shift for an equal amount of time before returning again to the previous conditions. This has important implications for commercial fisheries, for example, because oceanographers must account not only for the human-made (anthropogenic) dangers of overfishing but also for the apparently natural shifts in current patterns that have impacts on the availability of certain species. These patterns have been found to have positive effects on some species and negative effects on others.

Vertical Currents

In contrast to wind-driven surface currents, which have been known for centuries, below-surface currents were not believed to be significant until relatively recently. In the twentieth and twenty-first centuries, however, deep-ocean currents have received considerable attention. In addition to the horizontal movements in ocean waters are the vertical movements usually referred to as thermohaline circulation. The term “thermohaline” comes from the Greek words for heat and salt, the two major influences on deepwater currents in addition to gravitational pull from the moon and the impacts of coastline formations. Thermohaline circulation is sometimes popularly referred to as the oceanic conveyor belt. The slow movement of these currents can extend into the hundreds of years.

Put simply, dense, and thus heavier, waters are created in the extreme cold conditions of the polar seas. These waters sink, following currents away from the poles and pushing up warmer waters to interact with the cooler atmospheric temperatures. Deep ocean currents affect 75 percent of ocean waters, which have a temperature from 0 to 5 degrees Celsius (32 to 41 degrees Fahrenheit). Deepwater circulation involves the vast majority of ocean waters as opposed to the surface, horizontal movements driven by prevailing winds.

Environmental Impacts

One of the most important implications of the vertical currents of the world’s oceans concerns the amount of carbon dioxide in the earth’s atmosphere. Colder water absorbs carbon dioxide, whereas warmer water releases it. Another important factor in carbon dioxide distribution is the process known as the biological pump, which moves carbon from the ocean surface to the seafloor. While the presence of carbon dioxide is part of the reason the earth is habitable (the is one reason the earth is warm enough for human occupation), there is concern that human activity has created abnormal levels of carbon dioxide. The rate at which carbon dioxide is being added to the atmosphere has not been exceeded in the past 420,000 years, and the increase since the mid-nineteenth century has been greater than any observed during the past 20,000 years. Two-thirds of this increase comes from the burning of fossil fuels; the last one-third can be accounted for by deforestation. If global temperatures continue to rise beyond the ability of the ocean currents and the biological pump to mitigate the effects, it is possible that ocean levels will rise enough to inundate low-lying coastlines around the world.

Scientists have also speculated about whether impacts on the atmosphere could actually interfere with the thermohaline circulation. Such interference could potentially have catastrophic impacts on both weather patterns and marine life in and near landmasses around the world. Many European nations, for example, have historically had relatively mild climates in part because of the warming impact of ocean currents on weather patterns. Sudden shifts in these currents could thus result in rapid changes in historic weather patterns. Oceanographic scientists do not all agree, however, regarding the likelihood of such catastrophic changes. An ongoing debate among scientists concerns the ability to measure long-term cyclical change versus anthropogenic change.

In addition, questions remain concerning impacts on the oceans and the energy sources that drive currents. The late Nigel Calder, a famous science writer, pointed out that whereas it was once presumed that levels and temperature are primary producers of currents, as recently as 1998 two American oceanographers suggested instead that winds and especially the moon’s gravitational pull have a much more significant impact on ocean currents, and particularly on the circulation of ocean waters and their interactions with the land borders. Calder concluded, “If the tidal story is correct, the value of 20th-century computer models of the ocean circulation, based as they were on heat-salt effects, is seriously in question.”

Oceanographers are increasingly taking greater numbers of variables into consideration in their analyses of ocean currents and the impacts of weather patterns. Ocean water reacts with sunlight, with various temperatures of air, and with temperature differences between layers of the open sea, and the ways in which changes in ocean currents and temperatures actually take place are not entirely understood. Most scientists agree, however, on the importance of learning more about the potential for change in ocean currents in the short and the long term. For example, in 2020, oceanographers discovered that climate change has accelerated ocean currents by about 15 percent from 1990 to 2013. At first, they believed faster winds were responsible for the change but later discovered the actual cause. They knew that oceans warm from top to bottom. However, a new modeling study showed that climate change is the clause because faster currents occur in warm water.

Bibliography

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Longhurst, Alan. Ecological Geography of the Sea. 2d ed. Boston: Elsevier, 2007.

Mann, K. H., and J. R. N. Lazier. Dynamics of Marine Ecosystems: Biological-Physical Interactions in the Oceans. 3d ed. Malden, Mass.: Blackwell, 2006.

Park, Chris C. “Oceans and Coasts.” In The Environment: Principles and Applications. 2d ed. New York: Routledge, 2001.

"Slowdown of the Motion of the Ocean." NASA Science, 5 June 2023, science.nasa.gov/earth/earth-atmosphere/slowdown-of-the-motion-of-the-ocean/. Accessed 21 July 2024.

Voosen, Paul. "Global Warming Is Speeding Up Ocean Currents. Here's Why." Science, 20 Apr. 2022, www.science.org/content/article/global-warming-speeding-ocean-currents-here-s-why. Accessed 21 July 2024.