Much about the oceans is still not known: widespread weather and climate measurements for the atmosphere go back 150 years, but scientists have only been systematically monitoring ocean temperatures for about 50 years (although ice-sheet cores and deep-sea sediments provide important clues about climate patterns and temperatures further back in time, much like tree rings do).
The Earth is a complex living system. All its individual parts depend on the others. Just as a person can hobble along for years with a weak heart or damaged lungs, but cannot live if either fails, the Earth can sustain only so much disruption. This evidence is in the geologic record — punctuated with massive and widespread extinctions caused by disruptions to the Earth’s systems.
The Earth’s oceans and atmosphere are akin to the human body’s respiratory and circulatory systems. The oceans cover 70 percent of the Earth’s surface, store 1,000 times more heat than the atmosphere, and is the planet’s largest reservoir of water. Through evaporation, the ocean transfers huge amounts of water vapor to the atmosphere, where it cools, condenses and eventually falls to the ground as rain or snow (a process known as the hydrological cycle).
The ocean’s currents play a fundamental role in regulating the Earth’s climate and circulating life-sustaining nutrients around the globe. Just as blood vessels and capillaries bring oxygen from the lungs and nutrients to cells throughout the human body, the ocean’s currents distribute oxygen, nutrients and heat worldwide. The ocean distributes 25 to 50 percent of the energy received from the sun, and the ocean conveyor is a major source of heat to the North Atlantic.
Salt and heat: The conveyor’s “motors”
The oceans are a mix of salty and fresh water, and how they move and mix provide important clues to climate dynamics. The process that propels the conveyor works like this:
The ocean’s system of currents takes 1,000 years to go full cycle. Warm water is chilled in the far North Atlantic and sinks. The cold, salty current flows south near the bottom. PHOTO: Argonne National Laboratory.
At the equator, the sun warms surface waters and triggers evaporation. As water evaporates, the tropical waters get saltier.
The warm, salty water is carried northward along the East Coast of the United States by the Gulf Stream, a tributary of the Conveyor, and then over towards Europe.
As it travels, this current releases a huge amount of heat to the atmosphere in the North Atlantic
As this great volume of water becomes colder and denser, it plunges downward to the ocean depths (salty, cold water is denser than fresher, warm water).
As cold, salty water sinks in the North Atlantic, it pulls warm, salty tropical waters northward to replace the sinking colder waters.
This massive plunge of water drives the ocean’s “conveyor belt,” sending deep currents traveling along the ocean bottom to surface elsewhere around the world. Eventually these currents resurface in the Atlantic.
This cycle takes centuries to complete, unlike global wind and air circulation patterns, which take place over days or weeks. It’s called the “thermohaline circulation” from the Greek words “thermos” (heat) and “halos” (salt).
The release of the heat from the warm waters of the Gulf Stream tempers Northern Europe’s winter temperatures. It also generally distributes heat more evenly around the planet, moderating extremes of both between cold northern latitudes and hot equatorial regions.
A food source for fish, zooplankton (tiny swimming animals) are part of the food chain that relies on upwelling for nutrients. PHOTO: Matt Wilson/Jay Clark, NOAA
Delivering oxygen and nutrients around the globe
This system of currents delivers more just heat. “Deep beneath the surface, this cold, dense, salty water flows south, turns east around the Cape of South Africa, and then slowly warms and, centuries later, rises to the surface in the Indian Ocean and western Pacific Ocean,” explains Environmental Defense marine ecologist Rod Fujita in his book Heal the Ocean. “It brings oxygen and carbon dioxide to deep waters and nutrients to surface waters where phytoplankton can use them.”
Cold, nutrient-rich waters rise to the surface seasonally, and mix with sunlit surface waters. These upwellings trigger the growth of phytoplankton (microscopic marine plants). In turn, phytoplankton are eaten by zooplankton, which are eaten by fish and other sea animals up the food chain. The areas where these upwellings occur are often rich fishing grounds, the sea’s “gardens of Eden” where an abundance of marine life flourishes. The waters off the Channel Islands, the west coast of Africa, and the west coast of South America are good examples.
In Heal the Ocean Fujita writes of this drama at sea: “the magnificent large fishes of the Pacific were not wandering about aimlessly; they, and the albatrosses, turtles and sharks were focusing on patches of highly concentrated food. The sought-after squid and fishes were in turn congregating where the water was rich in plankton. The plankton in turn was concentrated where deep, nutrient-rich waters were upwelling and where eddies, gyres, and fronts were pushing nutrients, plankton, fish, and squid together.”