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Ocean Conveyor Belt

 Are we destroying it?

By Nina Tombers Marine Conservation and Ecology student

In Charlie Chaplin’s “Modern Times» the little tramp works in a factory at a conveyor belt. As he struggles to keep up with the belt, he starts to make mistakes which has its effects further down the line. It ends in a big chaos and the whole machinery collapses. So, if only at one place of a conveyor belt something goes wrong it can jeopardize the whole system.

And it is not by chance that the ocean’s currents are also called the global ocean conveyor belt. Like a factory conveyor belt, we need it for our whole earth’s system, not only the ocean but also the climate, to work properly.

What is the global ocean conveyor belt?

The water in our oceans is not just still-standing like a lake might be, there is always movement which is caused by the different currents that formed in the oceans. These currents don’t just exist each for themselves but create one big belt moving water around the whole world and through the different oceans – the global ocean conveyor belt (Figure 1). The water does not only move two-dimensionally but three-dimensionally, at some places water will rise up while at others water will sink down to the bottom of the ocean. Therefore, the water in our oceans will keep moving around the world and exchange surface water with new nutrient rich bottom water.[1]

Warm water masses in the Atlantic move northward where they cool down and sink to the bottom and the ocean conveyor belt begins.

What is the driving force behind the conveyor belt?

In one word: the thermohaline circulation. The word thermohaline is a combination of the two ancient Greek words thermos which means warm and háls meaning salt. Ocean water’s salinity, the amount of minerals dissolved in the water, and the water’s temperature directly affect its density [2]. The more salt there is in the water, the heavier it gets and the warmer it is the less dense it gets as it expands. As we know, less dense material flows on top of denser one, this is also true for water masses in the ocean. So, if water is transported into the pole region for example in the North Atlantic it will cool down because of the cold temperatures in the Arctic. Furthermore, if you ever have had the chance to lick a piece of ice from an iceberg you will have noticed that it consists of freshwater even if it formed out of seawater. This is because when ice is created the salt does not freeze and stays behind in the water, which means that the water in areas where ice is formed contains more salt than usually. Both these processes lead to a higher density of the water therefore, the water will sink down. While the sinking water on top is replaced by water moving into its place on the surface, the water in the deep will move southwards and a thermohaline current starts. It then crosses the Atlantic moving south where it splits into two while moving around Antarctica, one going up again into the Indian Ocean while the other moves up into the Pacific Ocean. Both warm up while getting closer to the equator which makes them come up to the surface again. The water at the surface will once more move south- and westward and then back up to the North Atlantic again, where the whole cycle starts again. The time till the same water will end up at the same place again is estimated to be 1,000 years.

How does it influence our climate?

In South Africa we can perfectly experience the impact ocean currents can have on the climate because it lays between the cold Benguela current on the west coast and the warm Agulhas current on the east coast. Since water takes a lot longer to heat up or cool down, as a consequence of its higher heat capacity, than air, warm water can act as a heat store when air temperatures go down in winter. This is the reason why winters along the east coast of South Africa are a lot more temperate. The air can get energy in form of heat from the water and doesn’t cool down as much.

Total rainfall (mm) for period July 2011 to June 2012

Furthermore, the warmer the air is the more moisture it can hold which then again leads to more clouds forming so more rain can fall. This means that the west coast with its cold current is getting less rain than the east coast with the warm Agulhas current, which can be seen in a map of rain distribution in South Africa (Figure 2).

At the west coast we have the opposite situation. The cold water of the Benguela current cools down the air and therefore there is almost no moisture and that leads to a dry climate, which explains the Namib desert.

So, water currents firstly are able to change air temperature drastically and secondly have a huge influence on how moist the air is which therefore leads to how much rain a place will get. The warm and cool currents of the conveyor belt can have exactly the same influence on the climate elsewhere as we can experience the currents here in South Africa.

One of the most famous examples is probably how the Gulfstream provides western Europe with warmer climate. If you compare for example the climate of Great Britain and Canada, they are on the same latitude and should experience a similar climate but because of the warm water coming from the equator Great Britain has the much warmer climate

The gulf Stream carries warm water to the east coast of the USA and Canada and on to Western Europe

Does climate change put the global ocean conveyor belt in danger?

Climate change influences many different parts and processes on our globe, and it quite possibly will also have an effect on the ocean conveyor belt. With climate change there will be more rain over the North Atlantic, more meltwater from melting glaciers and sea ice will be added to the ocean and there will be less sea ice being formed. All of this leads to more freshwater and hence less sinking of cold and salty water. This then could lead to a slowdown or even stop of the ocean conveyor belt. And as we know western Europe for example depends on the warm water of the ocean currents for their climate, so it would quite possibly mean a radical change in temperature.[1] So, the global ocean conveyor belt as a whole system is crucial to keep the world’s climate in check, but with climate change and the shrinking ice masses we are getting closer and closer to destroy one of the driving forces. And as we seen in “Modern Times” by Charlie Chaplin, we know that even a small error in such a big system can bring it to the downfall. So, climate change is not only about a world warming up but could really mean the collapse of the ocean conveyor belt and possibly a new ice age for Europe.

Watch this short video on How Ocean Current Work

Resources:

[1] https://www.sciencelearn.org.nz/videos/147-the-ocean-conveyor-belt [2]https://oceanservice.noaa.gov/education/tutorial_currents/05conveyor2.html [3]htps://de.wikipedia.org/wiki/Thermohalin#cite_note-rahmstorf-1 [4]https://scijinks.gov/gulf-stream/ [5]https://oceanservice.noaa.gov/education/tutorial_currents/05conveyor3.html#:~:text=The%20global%20conveyor%20belt%20is%20a%20strong%2C%20but%20easily%20disrupted%20process.&text=If%20global%20warming%20results%20in,sinking%20of%20cold%2C%20salty%20water.

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