Industrial activities, such as fossil fuel burning and other human activities such as tropical deforestation have increased greenhouse gas concentrations in the atmosphere. Increasing global temperatures are likely to have extreme effects on global climate and may result in species extinction, changes in agricultural production and deleterious effects on health. Studies have been conducted in recent years on the effects of increase of greenhouse gases on the thermohaline circulation. There are several hypotheses that state that the prolonged effect of global warming could eventually “shut down” the thermohaline circulation and lead to cooling in certain regions in the North Atlantic Ocean. Several ocean-atmosphere models have been used to predict the effect of increase of carbon dioxide (a greenhouse gas) concentration on ocean circulation. Two such models are discussed and their results are analyzed.
 Global temperatures have seen a dramatic increase since the Industrial Revolution. Several climate models have projected an increase of between 1.1° C to 6.4° C in the global average temperature due to the continued effect of global warming (IPCC (2007)). Apart from the resulting adverse effect on global climate, increasing global temperatures may result in species extinction, changes in agricultural production, deleterious effects on health, rise in the sea level, reductions in the ozone layer and disruption in the ice shelf. Another possible outcome of global warming is what is now termed as the “shutdown of the thermohaline circulation”. Wallace S. Broeker, the man responsible for the term “Global Conveyor Belt”, called the thermohaline circulation the “Achilles heel of our climate system” (Broeker, 1997). There is much research that focuses on the effect of greenhouse gases on ocean circulation. Two such models are discussed in future sections. Some research has shown that the transfer of heat from regions around the equator to the poles is due to the thermohaline circulation in the ocean. This implies that Europe does not have the same climate as the poles because of the thermohaline circulation. The thermohaline circulation therefore plays an important role in regulating the amount of sea ice in the Polar Regions. There are several schools of thought (Seager, Battisti, Yin, Gordon, Naik, Clement and Cane (2002)) that attribute this climate in Europe to its position with respect to the ocean basin and the warm atmospheric waves that blow up north from the tropics. Rhines and Häkkinen (2003) challenged this claim. According to Rhines and Hakkinen, “it is the existence of the oceanic heat transport that allows the maritime effect to operate in the northern North Atlantic and to create a milder European climate than in the North America; without the heat transport, ice would likely extend over much greater areas of ocean and land”. Much research is currently focused on the role of ocean circulation in the supply of heat to Europe.
2. Thermohaline Circulation
 Ocean circulation is commonly divided into two parts: the thermohaline and the wind driven circulation. In other words, circulation in the oceans is partly due to wind stress, and also partly due to changes in density because of changes in temperature and salinity. The term “thermohaline” originates from thermo for heat and haline for salt, which together determine the density of the water mass.  Thermohaline circulation originates in specific areas of the North Atlantic and in the Weddell Sea of the Southern Ocean. In the North Atlantic, the evaporative cooling effect of winter is responsible for cooling the upper layers of seawater, increasing the salinity thereby increasing density and causing sinking. The sinking cool water is the North Atlantic Deep Water (NADW). The denser NADW flows southwards into the ocean basins. The bulk of...