The world's biggest risks
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The Earth's dying oceans threatened with mass extinction

Aerial view of Hardy Reef near the Whitsunday Islands in North Queensland, Australia.

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An ecosystem in turmoil

A study released earlier this year found that we may soon see a mass extinction of ocean life.

That's alarming, since oceans comprise nearly 70 percent of Earth, provide habitat for more 200,000 known species (and potentially millions more unknown ones) and are integral to all known life on our planet, as well as climate and weather patterns.

The World Wildlife Fund estimates the total value of the ocean's assets at around $24 trillion and said if the ocean were measured as an economy, it would have an annual gross domestic product of $2.5 trillion — the seventh largest economy in the world.

In the United States alone, what the government calls "the ocean economy" — six economic sectors that depend on the ocean and Great Lakes — contributed more than $282 billion to the U.S. GDP and provided more than 2.8 million jobs in 2011, according to the National Oceanic and Atmospheric Administration (NOAA).

Here are five of the biggest challenges the world's oceans face — and how humans can tackle them.

—By Robert Ferris, science reporter,
Posted 20 November 2015

David Messent | Getty Images