Gulf Stream

Of all the ocean currents, the Gulf Stream is one of the strongest. It brings warmth to Europe and North America.

In fact, average annual temperature in north-west Europe is about 9°C above the average for this latitude because of the Gulf Stream.

How does it work?

Surface water in the north Atlantic is cooled by winds from the Arctic. It becomes more salty and more dense, which makes it sink to the ocean floor. The cold water then moves towards the equator where it slowly warms. To replace all this cold water, the Gulf Stream moves warm water from the Gulf of Mexico to the North Atlantic.

Could global warming affect the Gulf Stream?

When the last Ice Age ended (10,000 years ago) and huge amounts of ice melted, the water in the North Atlantic became less salty because of all this new freshwater. As a result, the ocean water in the North Atlantic was less dense, and did not sink. This caused the Gulf Stream to shut down. Temperatures in north-west Europe fell by 5°C in just a few decades. Something similar could happen today: an increase in global temperatures and precipitation could add a lot more freshwater to the north Atlantic.

Implications

A reduced Gulf Stream would mean lower temperatures in north-west Europe and north-east America. But the increase in temperatures from global warming gas emissions is much greater than the potential cooling effect. Substantial cooling because of a reduced Gulf Stream is unlikely this century.

How the oceans transport heat

The 'Global Conveyer Belt' diagram below shows how the oceans move heat energy from the tropics to the poles and back again to moderate the Earth's climate.

On the diagram, warm surface water is orange and the cold bottom water is purple. The ocean is generally divided up into 2 layers: a warm upper layer that is much thinner than the cold heavier bottom layer. The cold bottom layer is important because it holds a large amount of nutrients that are the basis for the marine food chain.

Ocean water at the surface is warmed at the tropics and then moves towards the poles, driven by winds and the Earth's rotation, where it loses heat, becomes saltier, denser, sinks, and continues its circulation through the oceans. It takes up to 1,000 years for water to completely circulate throughout the oceans.

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