Changing Currents

Ocean circulation plays an important role in determining marine conditions and influences climates across the planet. The oceans of the world may seem like separate entities, and we call them by different names, but they are all part of what's known as a global circulation system.

Model of the global thermohaline circulation. Image courtesy of NASA from the public domain.

The basic processes of the global circulation system are easy to grasp. Here is a simplified explanation of how it works: surface waters travel westward across the Pacific, Indian and Atlantic Oceans, before plunging down great depths in the North Atlantic Ocean; cycling along the ocean bottom in the opposite direction, these waters finish their journey by up-welling along the Pacific Coast of North America. This current cycle takes over a thousand years to complete! This massive movement of water is driven by temperature and differences in salinity. That is how is gets the name thermhaline; thermo for temperature and haline for salinity.

The Meridional Overturning Current (MOC) is the major current found in the North Atlantic, and is the driving force behind this global oceanic circulation. It includes the Gulf Stream, which brings warm air from the Caribbean, warming the British Isles and parts of Europe. The MOC works by bringing less dense, warmer water northward; as it gets farther north the water cools, becomes denser and sinks, driving the downward movement of the larger ocean circulation pattern. The MOC is a density driven current system dependant on the temperature and salinity of the incoming water to sustain itself. When this system changes, climates are greatly affected throughout the North Atlantic [1].

Changing times

Oceanic currents are sensitive to temperature and salinity changes [1], and global climate change is altering both. Sea surface temperatures have increased worldwide, affecting numerous ocean ecosystems. Climate change also affects salinity in some areas by increasing the amount of freshwater that enters the marine environment. As the polar regions warm, sea ice melts and causes a large influx of freshwater to enter the oceans [2]. With the occurrence of these events, the MOC circulation pattern has already begun to slow down [2]. Because of this slowing of the MOC, there is less warm moist air from the south moving north, changing the climate of Western Europe.

Water running off a glacier in Greenland. The last several decades have seen more metling than freezing in many parts of Greenland. Image courtesy of NASA from the public domain [4].

This is not the first time this has happened in the history of our planet. Approximately 13,000 years ago, a warming period after the ice age caused the ice sheets covering North America to melt and eventually dump large amounts of freshwater into the North Atlantic, leading to the slowing down of the MOC. As a result, parts of Europe were temporarily thrown back into cold, ice age-like conditions [3].

Although the MOC is not likely to stop completely in the next few years, studies suggest that it will slow down [3]. North America is not presently covered in ice that could spill into the ocean, but Greenland has a large ice sheet that is being affected by warming temperatures.

The Greenland ice sheet has been losing mass steadily over the last decade, with a dramatic decrease occurring between 2004 and 2006. In the north of Greenland, the ice sheet appears stable and does not show an increased rate of melting, but in the south the loss of ice mass increased 250% over the two-year period. This huge increase in freshwater runoff will affect ocean circulation in the near future [4].

Changes to ocean circulation are highly debated today, even amongst climate change scientists. Some propose that the MOC will actually stop, while others think that it will just slow down. There is one thing we know for sure: regardless of the rate, changes in oceanic circulation will affect the well being of life across the planet.

Changing Currents lesson plan

1. Quadfasel, D., The Atlantic heat conveyor slows. Nature, 2005. 438 (1): p. 565-566.

2. Alley, R., et al., Climate Change 2007: The Physical Science Basis, Summary for Policymakers. 2007, Intergovernmental Panel on Climate Change. p. 18.

3. Gagosian, R.B., Abrupt climate change: should we be worried? 2003, Woods Hole Oceanographic Institution.

4. Velicogna, I. and J. Wahr, Acceleration of Greenland ice mass loss in spring 2004. Nature, 2006. 443 : p. 329-331.