Ocean Circulation and Climate
"The oceans are a key element in the climate system because they transport heat and fresh water and exchange these with the atmosphere."
World Ocean Circulation Experiment
Surface and deep currents in the world's oceans combine to form what is known as Thermohaline Circulation, or more simply the Global Conveyor Belt (Figure 1). This large-scale ocean circulation system transports heat from the Equator to the Poles, one component of which is the movement of the surface waters known as the Gulf Stream and its northern extension the North Atlantic Drift. Without the warming influence of these waters, Britain and western Europe would lose their temperate climate, and become significantly colder – similar in fact to that of Moscow, which is on the same latitude.
Research by a consortium of European and North American Oceanographers has found that the North Atlantic Drift moves at approximately 8.5 million cubic metres per second, supplying an average of 313 million megawatts of power, releasing 6.5°C of heat into the atmosphere (Dickson and Dye, 2007; www.whoi.edu/oceanus Interrogating the "Great Ocean Conveyor").
A major part of the Global Conveyor Belt in the northern Hemisphere is the formation of dense water masses that act as pumps driving the start of the global system. As the North Atlantic Drift moves northwards, the warm water cools by wind driven evaporation, leaving the original water mass much cooler and saltier. As the water mass moves into the Arctic region and sea ice forms, the exclusion of salt from the ice mass makes the water significantly denser, and so it sinks beneath the less dense warmer waters that are still coming up from the tropics. These dense waters then begin their long global journey, travelling southwards at depth, completing the three dimensional loop of ocean current circulation.
However, as the triggering of these pumps is driven by density and temperature, any small changes to this delicate balance, for example an increase in fresher / warmer water from increased river input or ice melt, or a decline in the sea ice cover (Figure 2) could potentially significantly alter the flow of the Global Conveyor Belt, and so the climate in northern Europe and Britain.
Abrupt Climate Change
Abrupt Climate Change occurs when the climate system is forced at a rate more rapid than natural external forcing factors such as the Milankovitch solar orbiting cycles. The Committee on Abrupt Climate Change define it as "an abrupt change that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it". Fossil evidence shows that such climate changes can happen within a decade.
Scientists have identified 25 possible historical climate shifts over the last 100,000 years that have been called "Dansgaard-Oeschger Cycles". The most recent of these was the Younger Dryas Event which began approximately 13,000 years ago with the return of glacial conditions to parts of the Northern Hemisphere before abruptly returning to warm and wet conditions around 11,500 years ago. One possible explanation of this event is the sudden influx of water into the Arctic Ocean from Lake Agassiz (Figure 3), a glacial lake in North America covering an estimated 440,000 square km's. It is suggested that this influx of freshwater upset the Global Ocean Conveyor, either dampening or completely shutting down the Gulf Stream and North Atlantic Drift. Within 10 to 20 years of this happening, ocean temperatures around Britain had dropped by an average of 10°C, and sea ice reached as far south as 45°N – well beyond the UK's southern coast.
A similar episode called the Antarctic Cold Reversal occurred 14,500 years ago when a meltwater pulse from the Antarctic Ice Sheet caused a period of significant cooling following the gradual warming of the Antarctic during the deglaciation at the end of the Last Glacial Maximum – a case where warming resulted in abrupt cooling as the ocean circulation system was interrupted.
Often associated with the Dansgaard-Oescher Cycles are Heinrich Events. These are global climate fluctuations that coincide with the release and melting of large sections of the Northern Hemisphere ice shelves. The onset of climate reactions to these releases can occur over a matter of years (Maslin et al. 2001), and scientists believe the effects last up to 750 years. Work by Hemming in 2004 dates the last 7 Heinrich Events as occurring at approximately 12, 16.8, 24, 31, 38, 45 and 60 thousand years before present. If correct, it would indicate that the frequency of these climatic events that impact on the ocean circulation system is increasing.
It is impossible to say whether our present period of warming could cause the same thing to happen again. We don't have a large ice sheet or glacial lake situated on the North American landmass, but the changing climate is causing the ice caps at both Poles to melt. Not only does this release fresher meltwater into the surface waters of the Global Ocean Conveyor, it also reduces the amount of white ice surfaces to reflect the incoming solar radiation back out into space, so increasing the warming of the ocean waters even further.
It was known that the driving pumps of sinking dense water in the North Atlantic were slowing, with the hypothesis being that generally warming air was causing polar ice to melt, increasing the addition of less-dense water into the system (Figure 4). However, a research team from Woods Hole Oceanographic Institution reported in December 2008 (Vage and Pickart, 2008, Nature Geoscience) that data collected by the ARGO Float system showed a sudden turning on of deep water formation and sinking in the winter of 2007-2008. It is suggested that lower winter temperatures combined with changing wind and storm patterns drawing colder air to the south may have helped trigger these pumps.
What is certain is that their research highlights the complexity of the ocean atmosphere system, where global warming can cause regional instabilities resulting in abrupt cooling.