Temps de lecture :6 minutes
The oceans and seas of the world, which cover 71% of the earth’s surface, still remain the least known territories. Only 10% of deep ocean cartography has been indexed and to date, only 2% of the ocean floor has been explored. According to scientific estimations, these ecosystems provide a habitat for 90% of the biomass and 80% of the biodiversity in the world – of which, as yet only 5% are known (274 000 marine species).
In addition to their ecological importance, oceans feed 3.5 billion human beings. The fishing and aquaculture industries employ 35 million people in the world, and the produce of the sea makes up at least half of mineral provisions and animal proteins for Africa and South Asia. Marine territories also provide support and resources for human activities: transport, tourism, minerals (bromine, magnesium etc.) and energy (gas, petrol, oil, tides, currents, etc). 30% of petrol production in the world is extracted from the ocean floor.
In less than a century the volume of maritime traffic has exploded: from 550 million tonnes of goods transported in 1950 to nearly 6.8 billion in 2004. 90% of commercial international traffic travels by sea. Greece and Japan are the biggest commercial maritime powers. Maritime tourism represented about 3% of global commercial trade in 2005.
Two-thirds of humans live in coastal areas, and this is where 80 to 90% of marine resources are concentrated: indeed, the impact of urban and industrial development is particularly heavy here. Marine areas are therefore very polluted: according to a study by S. Halpern for Marine Life Census (1), around 40% of seas and oceans have been severely deteriorated, and only 4% are free from pollution or other damage. However, only a tiny amount (less than 1%) of the ocean’s surface is located within a protected zone.
The Climatic Function of Oceans
The ocean plays the part of a huge temperature regulator between the poles and the equator. It absorbs heat when atmospheric temperature is higher than its own, and it heats air for conduction, or for evaporation and then condensation. Further, the heat is transferred into time and space: the heat accumulated in summer is retransmitted in winter, and high latitude (polar) regions receive heat stored by water near the surface in tropical regions. By these mechanisms, the ocean redistributes 25% to 50% of the energy that the planet gets from the sun, and is responsible for half the earth’s climate.
In absorbing atmospheric heat, the earth’s waters are subjected to a thermal expansion and increase in volume. This is the main cause of a normal rise in sea levels. This phenomenon is currently being reinforced by climate change. According to the most pessimistic estimations of the Intergovernmental Panel on Climate Change (IPCC) (2), sea levels could rise as much as 1.2m by the end of the century (3). If this trend is not stopped, such a rise in sea levels would be catastrophic for countries bordering the sea, especially the poorest countries such as Bangladesh. Their land flooded, several hundred million people in the world would be displaced and become environmental refugees.
The transfer of temperatures from one ocean to another is caused by marine currents. There are two types: surface currents, created by regular winds (the Labrador, Cape Horn, Kuroshio, and Caribbean currents, the Gulf Stream, North Atlantic Drift, etc.) and deep ocean currents (4). As another of their thermal functions, the majority of these currents play a vital role for marine flora and fauna in feeding and proving minerals for phytoplankton and mixing oxygen from different ocean beds.
Caused particularly by the melting of ice in the Arctic, the level of fresh water in oceans is currently increasing. This phenomenon disrupts the circulation of marine currents. According to Hervé Mercier, director of laboratory research of ocean physics in Plouzané, the Gulf Stream could slow down by 25% by 2100 (5). Such a change would be followed by a reduction in temperature (an average of 5 to 6°C) in Europe and on the east coast of the United States, as well as a weakening of marine life. (6)
Oceans and Carbon Dioxide
Oceans absorb an average of 30% of the carbon in the atmosphere every year (less during El Niño) through the process of dissolution. The Southern Ocean (or Antarctic Ocean) alone is responsible for capturing 30% to 50% of this carbon gas (7). CO2 is dissolved in the water and then absorbed by phytoplankton and aquatic plants or blended into calcium carbonate by becoming part of the shells and carapaces of zooplankton, crustaceans and molluscs. When these organisms die, the calcium carbonate formed is deposited at the bottom of the ocean before very slowly transforming in to sedimentary rock. Thanks to these biological and geographical phenomena, oceans form the largest carbon sinks in the world. However, this is now being affected by human activity and their capacity to absorb CO2 is diminishing.
In constantly absorbing growing amounts of carbon gas, oceans are becoming more acidic. This change in pH disturbs the formation of shells and carapaces for marine micro-organisms (8). Further, climate change is indirectly leading to an increase in wind speed, in turn mixing marine waters. Water near the surface mixes with CO2-rich deep water and loses its ability to absorb carbon gas. According to the latest studies from Nicholas Metzl and his team in the LOCEAN laboratory, the efficiency of the North Atlantic to function as a carbon sink has decreased by half between 1996 and 2005. Similarly, that of the Southern Ocean is 10 times lower than previous estimations: 0.05 gigatonnes of carbon per year, instead of 0.5 gigatonnes (9). Finally, the warming of surface waters can lead to a reduction in phytoplankton activity and could provoke a release of CO2. Carbon gas is less soluble in warm water.
Since the industrial revolution, human activity has taken a heavy environmental toll on the oceans and seas: stocks of fish are overexploited, coral reefs threatened, habitats destroyed, etc. (10). Around 6 million tonnes of petrol are released into the ocean every year through offshore extraction or oil tanker transport. Accidental oil spills, in reality, only represent a tiny part of pollution in the oceans, much less significant than that linked to the emptying or cleaning out of oil tanker reservoirs, for example. But it is mainland human activity that forms the majority of ocean pollution. According to the United Nations Environment Programme (UNEP), 90% of substances and waste found in the sea come from the land (11). Carried by rivers, wind (for 20 to 30% of pollutants) or dumped directly into the sea, they are from agricultural, industrial, domestic, and transport based origins.
Regulated by the Stockholm Convention, pollution from persistent organic pollutants (POPs) has decreased over the last two decades. It was reduced by half in the Baltic Sea but still represents a large problem for the numerous coastal regions of Africa, East Asia or the Indian Ocean, where countries depend on agriculture and the chemical industry. Further, since 1960 the number of marine dead zones has doubled. This change is linked to excessive amounts of nutrients coming from agricultural mineral fertilizers in particular. By 2030, UNEP estimates that nitrogen levels in oceans will be 14% higher than in 1990. Another form of sea pollution is from waste water, reaching new levels in developing countries. In Latin America, East Asia and Central East Africa, at least 80% of waste water is discharged without treatment.
Insidious and long lasting, pollution from carrier bags and other plastic litter also worries scientists. Richard Thompson and his marine biology team at the University of Plymouth have discovered that coastal sediment, shallow water, and beach sand are polluted by microscopic particles of plastic, including nylon, polyethylene, and polyester (12). Such contamination could remain for centuries, if not thousands of years. Biologists have found significant traces of these particles in balanomorpha (a type of barnacle) and in sand hoppers, suggesting that these elements are already in the food chain. It remains to be seen how many of these may be toxic for ecosystems and human health.
Once or twice per decade, the seasonal marine current El Niño warms the waters of the East Pacific level with Chile and Peru. This variation in temperature disturbs regional marine circulation and stops the cold and mineral rich deep waters from resurfacing. Deprived of their food sources, marine flora and fauna weaken, causing a significant fall in fishing stocks. This phenomenon is also responsible for climatic upheavals showing themselves as persistent droughts in Indonesia and Australia, cyclones in Polynesia, torrential rains in Peru and Ecuador and exceptional cold spells. El Niño is one of the manifestations on a regional scale of a disruption in atmospheric circulation between the poles and the equator.
(10) United Nations Environment Programme report, October 2006
The Global Programme of Action for the Protection of the Marine Environment from Land-Based Activities
(12)UICN – Marine Programme
– Greenpeace – pollution marine par hydrocarbure
– United Nations Environment Programme –
– Guellec Jean, Lorot Pascal, Planète Océane, L’essentiel de la Mer, 2007