Scientists say warm seas have killed two-thirds of a 700km (435 miles) stretch of coral on the Great Barrier Reef. The damage is concentrated in the northern section of the reef. (The central and southern sections remain almost intact.) “This one is by far the most extreme and we’ve seen three of these events now with just one degree of global warming. So, two degrees of global warming, which is the international target, is going to be a very challenging place for the Great Barrier Reef,” says Terry Hughes, director of the Australian Research Council.