World Oceans Day.

On Tuesday the second annual official World Oceans Day will be celebrated. First suggested by the Canadian government at the 1992 Earth Summit in Rio de Janeiro, it had been celebrated unofficially until receiving UN endorsement in 2009. It's an opportunity to not only consider the world's oceans but also reflect on our relationship with them. Some 74% of the Earth's surface (or roughly 361 km²) are covered by a continuous body of water, conventionally divided into several oceans and seas. It's a vast quantity — some 1.3 billion cubic metres, more than half of which over 3,000 metres deep — and its sheer size may very well be one of the reasons we tend to take it for granted.

In the Solar System, Earth is the only planet with liquid water on its surface. While we may one day discover water on some of the Solar System's moons, or even on planets in other systems, it may prove a long and in many ways impractical trek for a drink. Never mind that access mightn't be unrestricted; even on Earth we have great difficulty sharing and distributing the oceans seemingly infinite resources. Not that the oceans simply serve as a reservoir: they're the very cradle of life on our planet and home to a myriad of creatures. Life evolved in the oceans three billion years prior to the occurrence of plants and animals on land. Thus the oceans are, in a sense, the ancestral home of all life on Earth.

The oceans generate most of the oxygen we breathe, they absorb carbon-dioxide, and have a significant effect on our climate. The evaporation of ocean water is the source of most rainfall, while the temperature of the oceans determine climate and wind patterns. Yet the vitality of the world's oceans is threatened by our activities and the climate changes they've brought about. Over-exploitation, illegal and unregulated fishing, destructive fishing practices and pollution — particularly from land-based sources — threaten vulnerable marine ecosystems. Increased sea temperatures, sea-level rise, and acidification caused by changes in the climate further threaten not only marine life, but coastal and island communities as well as national economies.

The oceans area also essential to trade: roughly 90% of the world's non-bulk, non-perishable cargo is shipped across the seas. Despite being slow, shipping remains significantly less costly than other types of transcontinental transport. Because of this, the oceans are also affected by criminal activity. Piracy may look cool at the cinema, but in reality armed robbery threatens the lives of mariners and the safety of shipping. Smuggling of contraband and trafficking of people endanger lives, security, and peace not only across the oceans but eventually across lands as well.

Serving as the world's transport medium of choice entails other hazards: though silicone-based paints are now used to keep fouling organisms (like barnacles) off ships' hulls, the majority of the more than 51,000 commercial vessels that ply the world's oceans have hulls treated with biocides that poison and contaminate marine life, including commercial fish species — placing human health at risk. Ships also produce almost as much air pollution as half of the total number of cars on the planet (some 300 million), propelled by cheap sulphur-rich "bunker-oil" which produces tiny particles of soot and sulfuric acid known to damage lungs, and believed to be responsible for as many as 60,000 deaths a year worldwide.

The oceans are essential to the health and survival of all life on the planet, they provide nutrients that help sustain us, and are of great economic importance to everyone who relies on fishing, tourism, and other marine resources for income. So take some time on Tuesday to celebrate the oceans and their inhabitants, the nourishment they provide, their importance for trade and communication. (It could be as easy as watching Finding Nemo with the kids.) While The Ocean Project's theme this year is Oceans of Life, the UN's theme is Our Oceans: opportunities and challenges, and though most events in Canada will be held along our ocean coasts, those of us inhabiting the largely landlocked prairies should take time to consider what's downstream the many rivers that provide our needs. Each and every on of them eventually empties into the oceans. Along with whatever we put in them.

Note: this is a slightly updated version of an article originally published on June 7, 2009.