Most of the world’s existing basic forms of life first appeared in the seas some 550 million years ago, and even now many new species probably live undiscovered in the ocean depths. In some ways life is even more diverse at sea than on land.

The immensity of the oceans once made them seem invulnerable: after all, they cover over 70 per cent of the Earth’s surface, and have depths greater than the height of the world’s tallest mountains. Yet overexploitation and careless, unregulated pollution place this astonishing environment in jeopardy. As a new century starts, there is an urgent need to change, learn from the indicators and halt the progressive destruction of a common asset.

Of course, no clear ‘line in the sand’ divides the oceans from ecosystems on land. Tropical coastal mangroves provide important feeding and nursery grounds for fish and other species but face destruction to make way for tourism, urban development and shrimp aquaculture. Similarly, the rivers of the ancient forests of North America’s Pacific coast provide the spawning habitat for salmon, yet the forests are being destroyed by clear-cut logging.

Human impact
Most of the world’s people live near the coast and that is where human impact is most keenly felt. Over three-quarters of all the seas’ pollutants come from human activities on land, or based there. Phosphates, nitrates, metals, radionuclides, organic chemicals and many other substances are discharged into the seas and are accumulating there and disturbing biological systems.

Toxic chemical effluent is pumped into the sea from outdated industrial processes, exported from industrialized nations to developing countries. Pesticides such as DDT are being increasingly used in agriculture and public health programmes in the developing world. Persistent organic pollutants do not just threaten human health and the environment where they are manufactured, but are carried by the oceans and the atmosphere to contaminate even remote polar regions: this can only be brought under control by a complete ban on producing and using organochlorines throughout the world. All these are just a few of the chemicals discharged to the marine environment: many cannot even be easily identified.

The oceans are a common heritage of humanity

Global warming is likely to have a big impact at sea. The oceans play a central role in shaping the Earth’s climate, absorbing carbon dioxide and other gases, and redistributing heat and water. Sea levels have risen by an estimated 10 to 25 centimetres over the last century, and as this continues the waters will cover land and coastal habitats in many countries. Bangladesh may lose 17.5 per cent of its land, and the Majuro Atoll in the Marshall Islands could lose 80 per cent. Already warmer waters and other effects of global warming are bleaching corals and having a severe effect on reefs.

Melting poles
Climate change is likely to be greatest at the poles. These regions play a fundamental role in ocean circulation patterns which, in turn, determine climate patterns over the rest of the globe. The Arctic is already warming faster than the rest of the world; both sea ice and glaciers there are shrinking, while rain and snowfall are likely to increase by 3 to 6 per cent for every degree of warming. Together these developments could make the upper layers of the Arctic seas less salty, possibly altering the thermohaline circulation which drives global ocean circulation and vital currents like the Gulf Stream. Meanwhile, Antarctic ice-shelves have collapsed in recent years.

Governments and industry alike must act responsibly, implement the Kyoto Protocol to the Convention on Climate Change and related agreements to reduce emissions of the greenhouse gases that cause global warming, and boost their efforts to develop and apply alternative, renewable energy technologies such as wind and solar power.

Fishing pressure
Sea fishing, the Food and Agriculture Organization of the United Nations (FAO) reports, generates approximately 1 per cent of the global economy and, together with related industries, supports the livelihoods of some 200 million people around the world. Yet most major fisheries are already overexploited, and the pressure that fishing exerts on the seas continues to increase.

Overfishing does not only affect the targeted fish stocks, but impacts whole ecosystems. A recent analysis of FAO fisheries data over the past 40 years by Daniel Pauly and his colleagues has revealed the trend of progressively fishing down food chains: when high-value, top predator species have been overfished and depleted, those lower in the chain are targeted. This strategy could bring wholesale changes to marine ecosystems: to take one hypothesis, the southern North Sea could become dominated by jellyfish, rather than commercially exploitable fish, as the top predators.

Every year some 27 million tonnes of unwanted fish are discarded after being caught – one-third of the total from shrimp fishing alone – and this also contributes to altering the species balance. Overfishing also affects whales, sea turtles and birds, as well as life on the seabed. Every year longlining in the Southern Ocean traps an estimated 100,000 seabirds; some albatross populations have been driven almost to extinction.

The consequences of overfishing, climate change, pollution, habitat degradation and other problems threaten to degrade the oceans irreparably. Yet they are a common heritage of humanity. Marine law may place much of the oceans under national control, but the problems facing the seas are common to us all.

Solving the environmental problems facing the oceans and ensuring sustainable fisheries is one of the greatest challenges facing humankind in the 21st century. No single nation or region can do this alone: it will require comprehensive international cooperation as required by the United Nations Convention on the Law of the Sea.