Most environmental problems are concentrated in the area where the pollution is produced. This is good, because it’s a lot easier for a single city or country to deal with an environmental challenge than it is for the international community.
There are two huge exceptions to this. The first is global warming, which (as the name implies) affects everyone. The second is the world’s oceans, most of which are not claimed as the territory of any nation or the property of any individual. For this reason, they suffer from what economists call the tragedy of the commons. Each actor has an incentive to consume as much of the oceans’ bounty as they can, since they know that if they don’t, someone else will. The inevitable result is that unless something is done, the world’s seas -- home to more than half of the planet’s life — will be irrevocably despoiled.
The most immediate global oceanic threat comes from overfishing. As early as 2011, it was estimated that 90% of fisheries were either fully exploited or overexploited.
By some estimates, the number of fish in the oceans has declined by half since 1970. This represents a loss of biodiversity, as well as a threat to a major source of protein consumed by humans. One culprit is the subsidies that countries -- most of them in East Asia -- give to their fishing fleets. Beyond ending these subsidies, the main weapon against overfishing is catch shares, a cap-and-trade program for fisheries that has been successful at restoring the health of many fisheries in the U.S.
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