Term Paper: Effects of Climate Change on Ocean Circulation

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Oceanography

The Effects of Climate Change on Ocean Currents

The Pentagon Report was released to the world media in 2004 predicting that the north Atlantic current would stop in the near future, bringing global catastrophe. It is known that in the past the ocean currents have been affected by changes in climate, although the lack of direct observations make it difficult for scientists to discover the exact nature of changes which are currently being observed. There have however been changes noted in currents in the Pacific and in the Atlantic Oceans related to climate change, suggesting that the events predicted in the Pentagon Report may occur at some stage in the future, although it is difficult to predict exactly when. If such changes in ocean current did occur, it is likely that the changes would impact on a global scale.

Introduction

When the Pentagon Report was released to the media in 2004 it suggested that there would be catastrophic events which would occur over the course of the next few years as a direct result of global warming and climate change (Schwartz and Randall, 2004). The report predicted that at some time in the near future the currents in the Atlantic Ocean would stop, which would create all manner of impacts around the world, threatening U.S. national security. Since the report was released there have been all manner of stories which have been published either confirming or refuting the claims made in the report. This study aims to research the scientific basis of the claims made in the report and assess whether the event is likely to occur based on the available evidence.

Theory

The ocean system is one on which a large number of interrelated outside driving forces have an influence. These include currents, upwelling processes, ambient temperature and winds. These factors may all be influenced by climate change (Lutjeharms and Ruijter, 1996).

There are indications that marine systems respond to climate change. For example temperature variations influence local currents and water column stratification. In the Pacific Ocean studies have been conducted into the consequences of El Nino Southern Oscillation (ENSO) events. ENSO is a result of the cyclic warming and cooling of the surface ocean of the central and eastern Pacific. During El Nino years the influence of upwelling cold waters decreases which causes the surface waters of the central and eastern Pacific to heat up. In comparison, when the upwelling of cold water is more intense than usual the La Nina event takes place. Climatic disturbances have also been observed in the western Mediterranean where the increase of summer temperatures of 2-3°C has resulted in the deepening of the thermocline. A similar climate induced disturbance has also been observed in the Indian Ocean and along the Caribbean coast (Danovaro et al., 2001).

It is difficult to assess, however the extent to which the current observations in the ocean systems indicate a problem. The short history of modern observations from the ocean mean that there is a lack of history from which to evaluate how the earth's current and circulation may have behaved in the past. This then makes it very difficult to determine whether any observed changes in the ocean currents may be part of a normal pattern of change, or related directly to climate change (McManus, 2006).

Results

Figure 1 - Ocean Currents across the globe. Taken at http://www.wbgu.de/wbgu_sn2006_en/wbgu_sn2006_en_voll_2.html#Heading7

Discussion

It is unknown whether the changes which can be observed in the oceans occur cyclically, stochastically, episodically or are long-term trends. Evidence of responses in the ecosystem to climate change is difficult to obtain, particularly in the deep sea (Danovaro et al., 2001). The usefulness of climate estimates and models depends entirely on the current level of understanding of the processes which drive ecological systems.

The basic problem which is expected to arise in the change in ocean currents is related to the response to the current freshwater influx. The Pentagon Report, which was released to the media in 2004 (Schwartz and Randall) presented a worst case scenario. This scenario involved the North Atlantic Current halting its flow, which would lead to severe cooling in the North Atlantic region within a few years. This situation has however been evaluated as speculative and extremely unlikely by other researchers. At the current time there is very little evidence to support the speculation that such an abrupt change will happen in the near future. However there is a possibility that with continued climate change these current changes may develop later in this century.

The way in which the currents work is that large masses of water sink from the surface far down into the deep ocean in certain areas of the Atlantic. The water then flows at a depth of up to 3km into the Southern Ocean. To balance the loss of water from the northern regions warm surface water flows from the south northwards to replace the water lost to the depths. This continual flow and replacement of water all results in large scale turnover of water in the region.

Global climate change affects the water flow associated with these currents in two ways. Firstly, the temperature change causes a thermal expansion of the water and secondly increased precipitation and meltwater dilute the seawater with freshwater. Both of these reactions decrease the density of the seawater, which leads to the risk of preventing the water from sinking. If the water were to stop sinking, this would then also prevent the resultant reactions to replace the sinking water. Overall this would stop the ocean currents from flowing, which is the effect predicted in the Pentagon Report (Schwartz and Randall, 2004).

Bryden et al. (2005) reported that circulation in the Atlantic may already have decreased by around 30%. The interpretation of their data has come under scrutiny from other experts who have claimed that the calculations presented do not correspond with other modeling calculations. There have been other reports however that the currents in the northern Atlantic Ocean are in decline. Hakkinen and Rhines (2004) used TOPEX/Poseiden altimeter data gathered during the 1970s, 80s and 90s to show that the subpolar current circulation was weaker in the 1990s than in the 1970s and 1980s. The data which they described from their observations also indicates that the decline extends deep into the water column, which suggests that the changes are long-term or permanent changes. Loder et al. (2006) also demonstrated that the water masses and currents in the Orphan Basis of the North Atlantic had changed over recent years. The data from observations taken between 2004 and 2006 showed that the intermediate and deep waters in that area had become both warmer and saltier over the time period, which had led to observed differences in the current compared to those expected from previous periods of observation.

There is no doubt that should the North Atlantic Currents stop at any point in the future there will be severe adverse effects across the globe. The global current systems can be seen in Figure 1. The currents are all interlinked at various points across the globe, and it is likely that should any one current stop, every other major current would also be affected. A critical effect in determining the events which will occur is the amount of freshwater that enters the northern Atlantic. This depends largely on the rate at which the ice sheet around Greenland melts, and this is difficult to predict. It is likely that with continued global warming this will happen at an accelerated rate, resulting in more freshwater entering the northern Atlantic and bringing about the predicted changes in Atlantic Ocean currents (WBGU, 2006).

Conclusions

It appears clear that there may be changes observed in the oceans at the present time that climate change is having an impact on… [END OF PREVIEW]

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