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Climate Change & the Oceans

Introduction

Global warming will accelerate sea-level rises due to thermal expansion of seawater and the melting of land-based ice sheets in Greenland and Antarctica. If current projections of climate models are reasonable, global sea level may stand at least half a metre higher by the end of the next century. This will seriously threaten many low-lying islands and coastal zones, rendering some countries uninhabitable. Other impacts include the increased risk of coastal flooding and salination of fresh groundwater supplies, exacerbated by an increasing occurrence of droughts or storms. Rapid sea-level rise would damage the coastal ecology, threatening many important fisheries.

Changes in ocean circulation are also projected to occur as a result of changing global climate. This will not only influence marine ecosystems (due to changes in upwelling zones), but will dramatically affect regional meteorological patterns (in response to the shifting heat balance). Palaeoclimatic studies have revealed that rapid circulation changes, within only a few decades, have occurred in the past.

Climate Change and Sea Level

The global mean sea level has already risen by around between 10-25 centimetres during the past century. According to a number of studies, the sea has been rising at the rate of 1-2 millimetres per year over the past 100 years. Measuring past and current changes in sea level, however, is extremely difficult. There are many potential sources of error and systematic bias, such as the uneven geographic distribution of measuring sites and the effect of the land itself as it rises and subsides.

It is likely that most of this rise in sea level has been due to the concurrent increase in global temperature over the last 100 years. Global warming should, on average, cause the oceans to warm and expand, thus increasing sea level. Climate models indicate that about 25% of the rise in sea level this century has been due to the thermal expansion of seawater. A second major cause of rising sea level is the melting of land-based ice caps. Presently, it is uncertain to what extent the melting of the Greenland and Antarctic ice caps has contributed to global sea level rise during the 20th century.

Forecasts of a rising sea level are based on climate model results, which indicate that the earth's average surface temperature may increase by up to 5.8°C over the next 100 years. Climate change is expected to cause a further rise of between 9 and 88 centimetres by the year 2100, depending on how future greenhouse gas emissions are uncontrolled. For most scenarios this expected rate of change is significantly faster than that experienced over the last 100 years.

Forecasting sea-level rise, however, involves many uncertainties. While most scientists believe that man-made greenhouse gas emissions are changing the climate, they are less sure about the details, and particularly the speed, of this change. Global warming is the main potential impact of greenhouse gas emissions, but other aspects of the climate besides temperature may also change. For example, some studies suggest that changes in precipitation will increase snow accumulation in Antarctica, which may help to moderate the net sea-level rise. Another complication is that the sea-level would not rise by the same amount all over the globe due to the effects of the earth's rotation, local coastline variations, changes in major ocean currents, regional land subsidence and emergence, and differences in tidal patterns and sea water density.

Nevertheless, some areas of Antarctica have warmed by 2.5°C during the past 50 years, a rate of warming 5 times faster than for the Earth as a whole. Whilst scientists believe this to reflect mostly regional changes in climate, the recent disintegration of the Larsen Ice Shelf has renewed speculation that climatic changes in the high latitudes have the potential to cause severe impacts via a rise in global sea level over the next 100 to 200 years.

Impacts of Changing Ocean Circulation Patterns

The oceans store an immense amount of heat energy, and consequently play a crucial role in the regulation of climatic patterns. Changes in global surface temperature, therefore may influence ocean circulation and further exacerbate climate changes due to greenhouse gas emissions.

At present, northern-western Europe, including the UK, is warmed by heat carried polewards by the Gulf Stream. When the warm water meets cold polar air in the North Atlantic, heat is released to the atmosphere and the water cools and sinks. The bottom water so formed, called the North Atlantic Deep Water (NADW), forms part of the global ocean circulation. A number of the theories have been put forward concerning the role of the NADW for future climate change. Some scientists believe that global warming may reduce the strength of the Gulf Stream, or divert its current path further south. This would have severe climatic consequences for those countries influenced by its warming effect, particularly in winter. Whilst the Earth may warm as a whole, a change in the Gulf Stream could make winters in the UK much colder than at present.

The El Niño ocean circulation, a natural phenomenon, affects the global climate every 2 to 7 years. El Niño is a warm body of ocean water in the eastern equatorial Pacific which brings heavy rainfall to South America and drought to Australia and south-east Asia. Whilst El Niño has been occurring naturally for hundreds, perhaps thousands of years, there is speculation that global warming may change its frequency and intensity. Some climate models have projected that if global warming continues as a result of mankind's greenhouse gas emissions, El Niño may return every year instead of every 5.

Impacts of Sea Level Rise

Low-lying Islands and Coastal Zones

Higher sea-levels would threaten small islands in the Pacific and Indian Oceans. The projected future rise in sea-level rise may be only tens of centimetres, but the forecasted rise would put millions of people and millions of square kilometres of land at risk. The most vulnerable land would be unprotected, densely populated, and economically productive coastal regions of countries with poor financial and technological resources for responding to sea level rise. Clearly, a rise in sea level would create irreversible problems for low-lying island nations such as the Maldives and the Pacific atolls. The costs of protecting this land from the sea and preventing constant erosion would be enormous. Additional investments would also be needed to adapt sewage systems and other coastal infrastructure. However, some localities, such as shallow ports, would benefit from a higher sea level.

Groundwater Salination

Groundwater in some coastal regions would become more saline. Rising seas would threaten the viability of freshwater aquifers and other sources of fresh groundwater. Communities may have to pump out less water to prevent aquifers from being refilled with seawater. Coastal farming would face the threat of inundation, freshwater shortages, and salt damage.

Wetlands and Estuaries

The flows of estuaries, coastal rivers, and low-lying irrigation systems would be affected, and tidal wetlands and mangrove forests would face erosion and increased salinity. Wetlands not only help to control floods, but they are critical to biodiversity and to the life cycles of many species. While many marshlands would be able to migrate inland as the sea rose, some species would suffer serious losses during the transition. Flat river deltas, which are often agriculturally productive, would also be at risk.

Storm Surges and Flood Damage

The damage caused by floods, storms, and tropical cyclones might worsen. Major harbour areas would experience more frequent flooding during extreme high tides and, in particular, during storm surges. Countries already prone to devastating floods, such as low-lying Bangladesh, would be the most affected. Warmer water and a resulting increase in humidity over the oceans might even encourage tropical cyclones, and changing wave patterns could produce more swells and tidal waves in certain regions.