Global Warming and Oceanic Circulation

Thermohaline Circulation - Robert Simmon, NASA

Ocean currents (collectively known as the Ocean Conveyor) driven by temperature and salinity differences move heat from Antarctic to Arctic regions via the Gulf Stream (see diagram). This is known as ‘thermohaline circulation’. 

As the heat is lost to the atmosphere in the Northern Hemisphere, the current becomes cooler and more saline (due to evaporation). As a result, it sinks and moves south. Models created by Manabe and Stouffer (1993) predicted that increasing the atmospheric CO2 concentration fourfold was enough to completely 'break down' this conveyor mechanism.

Melting of Polar Ice Caps Could Disrupt Ocean Currents

Climatologists believe that this will be caused by a large influx of fresh water, possibly from the melting of polar ice caps, which could disrupt the Gulf Stream and the North Atlantic Drift, a branch of the Gulf Stream that travels to Europe. Fresh water is less dense than salty water, and could lie on top of the warm salty water as the Ocean Conveyor moves north, preventing it from giving up its heat to the atmosphere.

The fresh water could also dilute the salty water, making it less dense and so preventing it sinking. Thus the density/salinity driving force behind the current would no longer exist. Gagosian (2003), states that the North Atlantic region has indeed been freshening dramatically in the last decade.

This slowing down or shutdown of the Ocean Conveyor could possibly result in a reduction in heat transport to Western Europe. Some climate models have predicted that Northern Hemisphere temperatures will be drastically reduced with a complete shutdown of circulation. Moreover, according to Rahmstorf (2010), the resulting climate would also be much drier.

The Ocean Conveyor was in fact disrupted during the 'Younger Dryas' era (around 12,700 years ago), causing temperature decreases of up to 5°C. In this period, severe winters lasted for years, glaciers advanced and sea ice spread, causing a large reduction in biodiversity.

Greenhouse Emissions and the Ocean Conveyor - A More Realistic Scenario

Some climatologists, however, claim that the conditions responsible for the climate disruption during this era are not comparable to those present today: during the Younger Dryas era, for instance, there was a massive influx of fresh water from the collapse of the Laurentide ice sheet rather than from polar ice melting.

Indeed, the IPCC's third Assessment Report in 2001 states that 'even in models where the thermohaline circulation (THC) weakens, there is still a warming over Europe'. Moreover, according to Rahmsdorf, a total shutdown of the THC is extremely unlikely and it is more feasible that the thermohaline circulation is merely weakened by around 20-50%.

Wood and Vellinger (2003) concur with this view, stating that the THC will ‘weaken or remain unchanged over the next century’ as a result of increased greenhouse gases and that a complete shutdown of the Ocean Conveyor would be a ‘low probability /high impact’ event.

However, even a slow disruption to the Ocean Conveyor could cause major changes in rainfall patterns in the tropics, with more extreme El Nino events, causing widespread changes in ecosystems and the expansion of deserts in some areas. Nutrient concentrations in the upper ocean may also be reduced, in turn affecting marine biodiversity and resulting in a general reduction in surface plankton. Global warming could also result in reduced overturning of the ocean around Antarctica, due to reduced sea-ice formation.

The Importance of Carbon Emission Reductions

Interestingly, Michael Schlesinger and his colleagues at the University of Illinois have used climate models to predict the percentage chances of a complete shutdown of the Ocean Conveyor with and without any global change in climate policy. They have concluded that there is a 70% chance of complete shutdown between now and 2205 in the absence of carbon taxes and a 25% chance of this occurring if these taxes are effectively implemented.

Whether these predictions are accurate or not, it is clear that additional control measures, such as carbon capture and sequestration may therefore be required to further reduce the likelihood of disruption to the thermohaline circulation.

References

Gagosian, R., 2003, ‘Abrupt climate Change: Should We Be Worried?’ Woods Hole Oceanographic Institute, whoi.edu

IPCC, 2001, IPCC Third Assessment Report, Climate Change 2001, ipcc_tar

Kloeppel, J, 2005, ‘Global Warming Could Halt Ocean Circulation, With Harmful Results’, eurekalert.com

Manabe and Stouffer, 1993, ‘Century-scale Effects of Increased Atmospheric C02 on the Ocean–atmosphere System’, Nature Publishing Group, nature.com

Rahmsdorf, S., 2010, ‘The Thermohaline Ocean Circulation – A Brief Fact Sheet’, Potsdam institute for Climate Impact Research’, pik-potsdam.de

Wood and Vellinga, 2003, ‘Global Warming and Thermohaline Circulation Stability’ Royal Society publishing.org