Ocean currents and the carbon cycle

The Meridional Overturning Circulation

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The oceans are a vital part of Earth systems. Underpinning their importance are the ocean currents, huge systems of moving water that join the different oceans together. They transport heat, salt, carbon, nutrients, and fauna, and they connect the world’s marine ecosystems.

The most important of these current systems is called the ‘meridional overturning circulation’ (MOC). In the past abrupt changes in the MOC caused extreme and rapid climate change, and it is possible that human activity will cause these currents to change in the future.

The MOC (which used to be called the thermohaline circulation) connects all the oceans, but the main action happens in the Atlantic and Southern Oceans (Figure 1). In the Atlantic Ocean, warm, shallow water travels north towards Greenland and Europe.

Figure 1. The MOC

When water reaches the Arctic Ocean (Nordic Sea on Figure 1) and Labrador Seas it gets cooled by the atmosphere and then sinks into the deep ocean, where it flows back southwards and into the Indian and Pacific Oceans. Eventually, the water is pulled back to the surface near Antarctica, forming a giant, complicated loop (Figure 2). The sinking water is the main supplier of oxygen to the deep ocean, making it critical to the existence of deep marine ecosystems.

Figure 2 Cross-section of the Atlantic Ocean – south to north

Movement of heat

The MOC is one of the most important systems of heat transport in the global climate system. This is largely due to the Atlantic Ocean circulation. The northward flow of warm, shallow water takes heat from the southern hemisphere to the northern hemisphere (Figure 2). This heat transport, combined with the positions of major surface currents (Figure 1) is why western Europe has mild winters compared to the east coast of the USA at the same latitude. This extra heat can also affect the extent of Arctic Sea ice and the melting of the Greenland ice sheet.

Carbon storage

The ocean holds about 50 times more carbon than the atmosphere, and because it can exchange its carbon with the atmosphere quickly, changes in ocean circulation can have important effects on atmospheric greenhouse-gas concentrations. Today the oceans absorb about one third of all the carbon dioxide (CO₂) that are being emitted by burning fossil fuels. Carbon is added to the ocean directly from the atmosphere as CO₂ is dissolved in sea water which then becomes a form of carbonic acid (ocean acidification).

Phytoplankton that live in the surface layers of the ocean use sunlight to photosynthesise, taking up some of the dissolved carbon and using it to grow. This organic material is the source of most of the life in the oceans. Once an organism dies, its body decays, and the carbon is added back into the sea water.  Crucially, some of the organisms sink before they decompose, so the carbon contained within them gets taken from the surface into the deep sea, where it can no longer be exchanged with the atmosphere.

Marine organisms then carry that carbon into the deep sea. This transfer of carbon is known as the ‘biological pump’, and it causes the deep sea to be highly enriched in carbon. As sea water moves away from the North Atlantic in the deep part of the MOC, decaying organic material from the water column is added to it and it gradually accumulates more and more carbon. In pre-industrial times, when this carbon-rich water rose back up to the surface, it contained so much carbon that carbon dioxide was released back into the atmosphere. Nowadays, the atmosphere contains a lot more carbon dioxide (in a sense more saturated with the gas), so less leaks out from the ocean.

At high latitudes where the atmosphere is very cold during winter, heat is lost from the ocean, reducing the water temperature. At the same time, when sea water freezes into sea ice, some salt gets left behind, making the rest of the water saltier. Cold, salty water is dense, so it sinks. This happens in the North Atlantic as well as in parts of the Southern Ocean, such as the Weddell Sea.

However, these density changes are not enough to drive the whole MOC. The deep and shallow layers of the ocean are constantly mixing, but this process is too slow to explain the volume of water that must rise back to the surface. It is the wind that gives the ocean enough kinetic energy to complete the global loop. Very strong winds blow from west to east all the way around Antarctica. Due to the Earth’s rotation, this wind forces surface ocean water to flow northwards, pushing it away from Antarctica and pulling water up from the deep ocean to fill the void left behind.