Climate change is having an adverse effect on the marine environment. Oceans contribute to global weather patterns, sustain intricate food webs and buffer CO2. Sea level rise is evident with climate change but other changes to the marine environment due to climate change are not as well known. This review examines the global thermohaline circulation, how it is measured and the relationship with climate change. Future predictions of the movement of global thermohaline circulation is often conducted by climate models which may not be the most accurate
What is the Global Conveyor Belt
The Global Thermohaline Circulation or the 'Global conveyor belt' is a series of oceanic currents powered by varying levels of salinity and temperature. The circulation of these currents are propelled by the cooling of oceanic water at high latitude areas such as the polar caps and the warming of ocean water around lower latitude areas around the equator. In lower latitude areas the currents are also driven by a evaporation while around the polar regions are boosted by a net freshwater gain from polar ice. This is known as strong thermal forcing1.
The North Atlantic Ocean acts as the 'engine room' for the global conveyor belt. As the water in the North Atlanitc ocean moves towards the arctic is cooled by polar ice. The water gradually sinks deeper down as the brine of the Arctic Ocean increases the density of the water by increasing dissolved salt and with a decrease in temperature. The change in density and the replacement of the sinking water is the driving force of thermohaline circulation2.
The sinking water from the North Atlantic then flows southward towards the Southern Ocean where it then flows north upwelling near the equator. This upwelling is due to an increase in temperature that decreases the density of the water in the Indian and Pacific Oceans2. The current then travels north near the surface of the ocean towards the North Atlantic where the cycle starts again.
ocean_conveyor.jpg Fig.1 The Global conveyor belt. Source: Broecker, 1991, Climate Change 1995: Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Contribution of Working Group II to the Second Assessment Report of the Intergovernmental Panel on Climate Change. UNEP and WMO, Cambridge press university 1996
Global thermohaline circulation plays an important part in the distribution and transportation of organic and inorganic particles in the world's oceans. It constantly circulates inorganic carbon, small amounts of organic carbon, and nutrients such as phosphorous and nitrogen around the world's oceans. This carbon circulation plays an important part in the oceanic carbon cycle and is a key component is providing the nutrients for marine primary production3. Warm water upwelling in coastal regions provides the majority of inorganic carbon to productive regions, without this carbon productivity would decrease and lead to carbon dioxide pressure on these coastal regions3. This reduced pressure may lead to an increase in the diffusion of atmospheric carbon dioxide into the oceans that may allow the oceans to become a significant sink of atmospheric carbon dioxide3.
Measuring the Global Conveyor Belt
The enormity of the global thermohaline circulation makes it difficult to measure its efficiency. As it covers most of globe and in some places at great depths, it is nearly impossible to measure the system due to practicality, cost and immense water pressure at great depths. This has led to most of the scientific work focusing on the active areas of circulation where vertical sinking occurs in the North Atlantic and in tropics where upwelling occurs. Significant work on the overall flow between the poles has been undertaken and is referred to as meridional overturning circulation or MOC4.
Due to range of forces that drive the conveyor belt measuring all the components and factoring in the number of variables that allow it to function is extremely complicated. Numerous studies and current work on oceanic salinity, temperature and the transport of nutrients and water in ocean currents have been conducted. However, to gain a thorough insight into the capability of the global conveyor belt comparisons to older datasets on salinity and temperature taken from hydrographic records have been used in many studies5,6. This data can be inserted into various models and patterns emerge. Oceanic currents are measured in a few ways. The physical movement of water in the oceanic currents can be measured, flow rates and movement can be deducted from climate models and satellites can be used to trace perfluorinated acids.
Climate models create a global picture of the thermohaline circulation and are created to predict how it may react with future changes. Coarse resolution climate models are used to monitor heat, freshwater input and geochemical tracers in the ocean and in particular the Southern Ocean and Antarctic circumpolar current7. The Southern Ocean plays a vital role in the distribution of nutrients around the world. Many models focusing on the Southern Ocean also take into account sediment and ice cores which can show the activity of the thermohaline circulation through carbon and nutrient levels detected in core samples 8. There are many other models combining both ocean and atmospheric carbon dioxide levels and temperatures as well as marine nutrient levels and other water attributes transported by the thermohaline circulation, these models offer an insight into the complexity of the global conveyor belt9.
New techniques are constantly being developed to try to measure the rate of circulation. An example of this includes perfluorinated acids (PFA's) which due to a high solubility rate in water have been used in studying the transportation rate of the global conveyor belt10. It has been discovered that concentrations of these acids do follow the global thermohaline pattern10.
Another example of methods to measure the global conveyor belt is by satellites. Satellites are able to provide data at a global scale on sea temperatures, sea surface heights and most recently, ocean salinity levels . These when coupled with deep-sea pressure monitors allow the meridional overturning current to be adequately measured 11.
Decline in the Global Conveyor Belt's efficiency
Due to the complexity of the thermohaline circulation there may be a number of contributing factors to the decline in the rate of circulation.
Primarily the major cause believed to be responsible for any decline is the degradation of the polar ice caps. The influx of freshwater from melting ice into the ocean is believed to disrupt the halocline environment in the North Atlantic. Where previously the dense saline water would gradually sink to greater depths the added amount of freshwater reduces the density, causing a significant slowdown in the rate of overturning and gyre circulation. Some models suggesting that a shutdown of the thermohaline circulation is imminent 12.
Melting of the polar caps is thought to be a direct result of human induced climate change. Increased amounts of freshwater will be introduced to the system due to an increased amount of rain and snow in the North Atlantic area caused by larger amounts of evaporation from increased sea surface exposure11. As carbon dioxide increases in the atmosphere, polar ice caps continue to melt which in turn leads to a shutdown of the thermohaline circulation. Studies have shown using the GFDL climate model show that a small increase in carbon dioxide may lead to reduced circulation with the capacity to recover, although over time if carbon dioxide levels are too high the global thermohaline circulation may not recover.
While the global conveyor belt is a driving force of global climate, the carbon cycle and marine primary production there is much debate within the scientific community on the declining efficiency of the belt or if there is even a risk of decline at all. Many scientists such as Wallace Broecker, a pioneer of the belt's relationship with global climate, believe that the shutdown of the thermohaline circulation will have an immediate and catastrophic effect. Broecker believes that if the conveyor belt shut down immediately, temperatures would fall in the Northern Atlantic region by 20 degrees Fahrenheit within 10 years13. Disruption of thermohaline circulation has also been detected from geological records at the end of the last ice age, which is thought to of started the younger Dryas, a cold snap that turned Scandinavian forests into tundra12.
Since studies on the global thermohaline circulation started there has been a lack of decisive evidence that circulation is in decline. Many results are inconclusive or have high error rates in sampling. Measuring thermohaline circulation as previously mentioned is highly complex, with many different parameters, most of the thresholds of these parameters are largely unknown 14. This gives a notion that instead of the conveyor being fully functional, there is simply a lack of evidence that it is slowing. What is largely agreed upon though, is the fact that a reduction will no doubt have significant climatic impacts.
Implications on global climate
When discussing the effect which a decline of the Global Conveyor Belt will have on the global climate, it is hard to look past popular media, and in particular the 2004 blockbuster film "The day After Tomorrow". In this film, melting Greenland and polar ice causes a shutdown in the global thermohaline circulation which causes catastrophic super storms and deadly freezing periods in a matter of days.
Although many scientists agree that the shutdown of the Global Conveyor will alter climates, it is near impossible that any changes will occur at the intensity and short time frame portrayed in the film. The majority of literature and information of the subject refers to the major impact of the collapse as being a dramatic cooling of the area around the northern Atlantic and in particular the countries of Northern Europe.
The basis for a possible European 'ice age' is through the deterioration of the Gulf Stream. It is accepted that the Gulf Stream is a primary source of heat for the North Atlantic region12 as it transports large amounts of warm water northward past Western Europe. The shutdown of this warm current may send Europe into an induced glaciation period or mini ice-age13. A study which effectively showed the sensitivity of the Thermohaline circulation and the climatic effects which could be caused by its deterioration found that as the northern hemisphere cools, the southern hemisphere may work in the opposite direction and in fact warms15.
In the same study, it was found that Europe would receive on average 20-30cm more snowfall per year and the snow cover in central and northwest Europe will last on average 2-3 months longer than usual after the Thermohaline Circulation has been stopped for ten years15.
Another study concluded that by the end of the 21st century, the earth would be 2-6 degrees Celsius warmer than before the industrial revolution. The study concluded that if global warming increased, the earth would experience extreme weather phenomenon such as tornadoes, heat waves and major sea level rise16.
Work done by scientists Michael Vellinga and Richard Wood used the climate model HadCM3 to predict climate change caused by a shutdown of Thermohaline circulation17. It was determined that shutdown would cause an average of -1.7ËšC in the northern hemisphere17. It is also believed that after the shutdown, the already present global warming may restrict the formation of new ice caps and so continue the problem further17. Another change may be a dramatic rise in sea levels that may affect lower lying countries and coasts17.
To gain a significant insight into global thermohaline circulation constant monitoring of temperature, ocean depths, salinity and deep water pressure are required over a long time period. One of the major problems in accessing the conveyor are the large amounts of unknowns such as what rate of freshwater influx from ice cap melting will be a turning point in the stoppage of the conveyor. As well as this, there are many variables such as the speed of which the ice caps are melting. Slower melting could lead to a slow, gradual reduction in the belt whilst fast melting may cause an abrupt stop11. Much of the research being carried out is largely speculative using climate models and estimations due to no clear method of measurement developed. Although the outcomes and causes of a decline in the belt are mostly agreed upon, there is no strong or reliable evidence of a significant slow in circulation efficiency.
