Oceans and Climate
The oceans have a profound influence on the world’s climate, most strikingly in the way they absorb solar energy and redistribute it around the world in warm surface currents. Cold currents also produce local climatic effects, while alterations in currents are associated with climatic fluctuations such as El Niño. The future behavior of the oceans is crucial to the future course of climate change, as they are an important store for carbon dioxide, the principal greenhouse gas.
Five or six major surface currents carry heat away from the tropics and subtropics toward the poles, giving some temperate regions a warmer climate than they would otherwise enjoy. A prime example is the effect of the warm Gulf Stream and its extension, the North Atlantic Drift, on Europe. The North Atlantic Drift carries heat originally absorbed in the Caribbean Sea and Gulf of Mexico across the Atlantic, where it is released into the atmosphere close to the shores of France, the British Isles, Norway, Iceland, and other parts of northwestern Europe. As the prevailing westerly winds blow this warmed air over land, these countries benefit from a milder climate than equivalent regions at similar, or even lower latitudes, on the western side of the Atlantic. For example, winter temperatures are typically higher in Reykjavik, the capital of Iceland, than in New York. Similarly, in the northwest Pacific, the Kuroshio Current warms the southern part of Japan, while in the extreme southwest Pacific, the East Australian Current gives Tasmania a relatively mild climate.
In some instances, the climatic effect of cold currents is simply to produce a cooler climate than would otherwise be the case. For instance, the west coast of the US is cooled in summer by the cold California Current. Cold currents also affect patterns of rainfall and fog formation. In general, the various cold currents flowing toward the equator on the eastern sides of oceans—combined with upwellings of cold water from the depths in these regions—cool the air, reduce evaporative losses of water from the ocean, and cause downdrafts of drier air from higher in the atmosphere. Although clouds and fog often develop over the ocean in these areas (as what little moisture there is condenses over the cold water), these quickly disperse once the air moves over land. Thus, cold currents contribute to the development of deserts on land bordering the eastern sides of oceans, such as the Namib desert in southwestern Africa.
Carbon in the oceans
The oceans contain Earth’s largest store of carbon dioxide (CO2)—the main greenhouse gas implicated in global warming. Huge amounts of carbon are held in the oceans, some in the form of CO2 and related substances that readily convert to CO2, and some in living organisms. The oceanic CO2 is in balance with the atmospheric content of the same gas. For many years, the oceans have been alkaline, and acted as an important store for the excess CO2 released by human activity. Biological and chemical processes turn some of this CO2 into the calcium carbonate shells and skeletons of organisms, other organic matter, and carbonate sediments. However, the increasing CO2 concentration is beginning to acidify the oceans, threatening shell and skeleton formation in marine organisms, as acid tends to dissolve carbonates. Further, some scientists fear that the rate at which the oceans can continue to absorb CO2 will soon slow down, further aggravating global warming.