Ocean currents play an important role in controlling climate around the world. These currents act like a giant conveyor belt, warming and cooling parts of the Earth as water circulates. Melting ice caps, caused by global warming, could affect the conditions that cause ocean waters to circulate and have a dramatic effect on climate.

There are a number of ocean currents around the world and these currents are known collectively as a global ocean conveyor. One of the most significant driving forces in the circulation of the ocean waters is thermohaline circulation, where the density of water, affected by temperature and salinity, causes water to circulate. These ocean currents have an impact on climate. The Gulf Stream in the Atlantic, for example, transports warm water with a high level of salinity and low density from equatorial regions further north on the ocean’s surface, warming countries such as the United Kingdom. The further north the water travels, the cooler it gets. Cold water becomes denser, falls further towards the bottom of the ocean and is carried back further south. This causes a continual ocean current in the North Atlantic.

One of the effects of global warming is that polar ice caps are beginning to melt. As ice caps are composed of freshwater only, continual melting would cause the level of salinity in surrounding ocean waters to dilute. Changes in salinity levels could affect thermohaline currents by preventing water from achieving enough density to sink to the bottom of the ocean. More seriously, ocean currents could stop completely.

If ocean currents were to stop, climate could change quite significantly, particularly in Europe and countries in the North Atlantic. In these countries, temperatures would drop, affecting humans as well as plants and animals. In turn, economies could also be affected, particularly those that involve agriculture. If these effects were to continue, Europe, North Atlantic countries and parts of North America could experience long periods of freezing conditions. However, if ocean currents halted as a result of global warming, these temperatures would also be affected by other aspects of the global-warming phenomenon.

Rocks and ice provide evidence of ocean currents stopping for periods of time in history. One example can be found around 13,000 years ago when the warmth experienced at end of an ice age caused large masses of ice to melt into the sea. The resulting changes in water density stopped ocean currents from flowing and caused freezing conditions in some parts of the world for over 1,000 years.