If geography is the science of space, then few spaces are more important to humanity than the world-ocean. The world-ocean—the term preferred by geographers for the “seven seas,” which, in fact, constitute one unified geophysical system—covers 71% of Earth's surface and contains 97% of the planet's water. More than 20% of the world's petroleum is derived from offshore sources, and 95% of world trade by weight, or two thirds by value, is carried by ship. Eighty percent of the world's fish catch comes from the ocean, supporting the livelihoods of 140 million people. Economists have calculated that the world-ocean provides services to humanity valued at $21 trillion per year, as opposed to only $12 trillion per year provided by land. This entry reviews some ...

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