First the bad news. The planet's coral reefs are up against it. They face a barrage of threats: burial by sediment eroding from deforested lands, strangulation by algae thriving on fertiliser runoff, overfishing of species vital for reef health, damage by anchors, toxic pollution - the list goes on and on. As if that wasn't bad enough, soaring levels of carbon dioxide are making seawater more acidic, which will make it harder and harder for coral polyps to build their rigid skeletons. Worst of all, when seawater gets abnormallyrnhot, the brightly coloured symbiotic algae that live within tropical corals and produce most of their food disappear, leaving their hosts vulnerable to starvation and disease. With global warming kicking in, such "bleaching" episodes are becoming more and more common. As waters warm, some tropical coral species are at least expanding their ranges northwards and southwards. However, the amount of suitable habitat, for instance areas with clear water, is limited. Overall, the long-term outlook for corals isrnnothing less than catastrophic. If CO_2 levels and temperatures continue to rise, half of all coral-associated species could become rare or extinct. And it's not just the incredible diversity of life that will vanish. As reef growth slows, erosion will start to outstrip reef building, and existing reefs will crumble away.
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