Climate change is the greatest global threat to coral reef ecosystems. Scientific evidence now clearly indicates that the Earth’s atmosphere and ocean are warming, and that these changes are primarily due to greenhouse gases derived from human activities.
According to the Intergovernmental Panel on Climate Change, so far the oceans have taken up 90% of the excess heat generated by human-caused global warming. Even if emissions are aggressively curtailed, the oceans will continue heating at an accelerating rate for decades. What’s more, the oceans are acidifying. They’ve soaked up an estimated 20–30% of human carbon emissions; as carbon dioxide dissolves into these waters, their pH plummets.
Warming and acidification are stressors for corals (and for many other marine organisms). Heat causes coral to lose its algae and bleach. At the same time, increasing acidity makes it difficult for individual corals, typically millimeters in size, to build the calcium carbonate deposits that form large reef structures. If the pH is low enough and the corals unhealthy enough, reefs can even start to dissolve, making them vulnerable to shattering during storms.
Unhealthy reefs threaten not only the organisms that inhabit them but also the livelihoods of the people who depend on them. Reefs are the backbone of near-shore ecosystems around the world, providing a home for thousands of species of fish as well as mollusks, crustaceans, sea turtles, and countless other creatures. Without their associated reefs, nearby fisheries are at risk of collapse. The world’s reefs are valued in the tens to hundreds of billions of dollars annually. Each year, for instance, the Great Barrier Reef contributes about A$5.6 billion (US$3.84 billion) to Australia’s economy.
Scientists around the world are looking for all kinds of ways to protect and maybe even revive corals. One option is to create more marine protected areas—essentially national parks in the ocean. Scientists say creating marine refuges, where fishing, mining, and recreating are off limits, make the reefs healthier, and so more resilient.