Oceans, seas, islands and coastal areas form an integrated and essential component of the Earth's ecosystem and are critical for global food security and for sustaining economic prosperity and the well-being of many national economies, particularly in developing countries.

The oceans are a highly productive system which continuously recycles chemicals, nutrients and water through the "hydrological cycle", which powers climate and weather, and which regulates global temperature by acting as a giant heat reservoir from the sun. About two-thirds of  the world's population live within 60 kilometres of the  coast,  and almost half of the world’s cities with more than one million people are sited in and around the tide-washed river mouths known as estuaries.

From a human point of view, oceans are also a major source of food and employment, and provide natural routes for communication, transportation and  trade. But the resources of the oceans, both living and non-living, are not finite and show signs of stress from human and natural actions. To draw attention to the risks which threaten the sustainability of the oceans and their resources for future generations, the United Nations General Assembly declared 1998 as the International Year of Ocean. 

The oceans' tremendous presence causes it to have a huge effect on the planet and our civilization. It is greatly responsible for the climate of the Earth. It regulates air temperature and supplies moisture for rainfall. The ocean also provides us with food, energy, minerals, and a cheap method of transportation. Without the oceans, the Earth wouldn't be able to sustain life.

The state of the world’s oceans continues to deteriorate. As new threats to the health and viability of the oceans emerge, most of the problems identified decades ago have still not been solved and many have become worse, according to a study carried out in 2001 by the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection. At risk are the vast resources of the oceans and the many economic benefits that humanity derives from them, estimated to be about $7 trillion per year. Coastal areas  the most productive marine environments are the most affected. Currently more than half of the world’s population lives within 100 kilometers of the coast, with two thirds of all cities with over 2.5 million inhabitants. By 2025, it is expected that 75 per cent of the world’s population will live in coastal areas. The large-scale movements of populations to coastal areas have been coupled with a significant increase in economic activity and industrialization along the coastline such as oil and gas exploration, mining, fish farming, tourism, development of ports, marinas and coastal defenses  putting enormous pressure on coastal areas. Pollution, the overexploitation of marine resources and the destruction of marine environments are the greatest threats to the oceans. About 80 per cent of all pollution entering the oceans comes from land-based sources: this includes both land-based discharges and discharges through the atmosphere. The rest is due to maritime transportation, dumping and offshore production.

Marine experts of United Nations organizations listed 20 issues of global concern regarding deterioration of the marine environment:

• Harmful algal blooms

• The effects of classical contaminants (sewage, metals, persistent organic substances, petroleum hydrocarbons, radio nuclides)

• The effects of deforestation

• The effects of increased or decreased mobilization of sediments

• The demise of coral reefs

• The loss of wetlands

• Declines in mangroves

• Habitat destruction

• The transfer of harmful species into coastal areas

• Climate change

• Sea-level rise

• Inundation as a consequence of physical alteration

• Increased risks to human health

• Reduced biodiversity

• Endocrine-disrupting chemicals

• Over fishing

• Destructive fishing practices

• The effects of the exploitation of coastal mineral resources, particularly sand and gravel and

• Litter

Pollution from Land-based Activities

While the amount of some pollutants discharged into the seas has been reduced, and some forms of pollution are now thought to pose less of a threat than before, the amount of waste  municipal, industrial and agricultural introduced into the sea is growing worldwide. These pollutants include sewage, persistent organic pollutants, radioactive substances, heavy metals, oils, nutrients and litter. Also growing is the use of pesticides, fertilizers and other agrochemicals — all substances that are washed or blown off the land into the oceans. Sewage, or improperly treated domestic wastewater, poses one of the gravest hazards to coastal environments worldwide. The enormous inputs of nutrients that sewage introduces into the marine environment can destroy the very sensitive and fertile environments of coral reefs, lagoons and seagrass beds.

It leads to changes in species diversity and causes excessive growth of algae. It also causes extensive economic losses by ruining large areas used for fisheries, recreation and tourism. Human health is also threatened by pollution from sewage, which causes frequent outbreaks of gastrointestinal diseases such as cholera, typhoid and infectious hepatitis, which in turn has precipitated a health crisis with massive global implications. Bathing in polluted seas is estimated to cause some 250 million cases of gastroenteritis and upper respiratory disease every year, costing societies worldwide about $1.6 billion per year, according to a recent study sponsored by the World Health Organization (WHO) and the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection. The toll from consuming contaminated shellfish is even greater.

The same study estimates that eating uncooked sewage-contaminated shellfish causes some 2.5 million cases of infectious hepatitis each year, at a cost of some $10 billion annually. Sewage also introduces significant amounts of plastics and other marine debris to coastal waters, threatening marine life through entanglement, suffocation and ingestion. Plastic bags are often mistaken by sea turtles for jellyfish and eaten — blocking their digestive systems and potentially killing them. Fishing lines and nets, six pack rings, ropes and other litter can wrap around fins, flippers and limbs, resulting in drowning or amputation. Some debris can keep killing for decades.

To address problems caused by pollution from land-based activities, more than 100 countries in 1995 adopted two international documents: the Washington Declaration on the Protection of the Marine Environment from Land-based Activities, and the Global Programme of Action for the Protection of the Marine Environment from Land-based Activities (GPA). The latter addresses the impacts of land-based activities on the marine and coastal environments, covering issues such as contaminants, the physical alteration of marine and coastal environments, sources of pollution, the protection of habitats critical for endangered species, and the protection of ecosystems such as breeding and feeding grounds. In addition, the World Bank, through the Global Environment Facility (GEF), has in place programmes to reduce pollution due to non-treated sewage, as well as initiatives to reduce nitrogen pollution.

Pollution from Ships

Threats to the marine environment from shipping activities, while not as prevalent as pollution originating on land, can also arise from accidents, operational discharges, and physical damage to marine habitats. While in tonnage terms the main pollutant entering the marine environment due to shipping operations is oil, the greatest threat to the marine environment stemming from shipping activities arises from the introduction of harmful alien species into new environments through ships’ ballast water. It is estimated that 3,000 species of animals and plants are transported every day around the world in the ballast water of ships or in their hulls. In response to these threats, the International Maritime Organization (IMO) a United Nations specialized agency has developed a number of international rules and standards, such as the International Convention for the Prevention of Pollution from Ships. In 2001, to address the use of toxic anti-fouling paints on ships hulls, the IMO adopted the Convention on the Control of Harmful Anti-fouling Systems on Ships. Regulations for ballast water management, to prevent the transfer of harmful aquatic organisms in ballast water, are under development.

Dumping

Until recently, ocean dumping was an accepted method of waste disposal in many regions of the world. But in recent years, dumping of substances considered to be threats to the marine environment, as well as incineration at sea, have been phased out as a result of the establishment of international and national norms that promote more environmentally friendly disposal methods. These changes have substantially reduced the amount of pollutants dumped into the oceans. The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (the London Convention), adopted in 1972, and its 1996 Protocols, contain the key international rules and standards dealing with dumping. Other instruments have also been adopted at the regional level.

Source: http://www.un.org/Depts/los/convention_agreements/convention_20years/oceanssourceoflife.pdf