As if tropical storms didn't get enough of a bad press, it now seems they can ramp up ocean acidification, putting the world's coral reefs under even greater pressure.
Seawater is becoming less alkaline as it absorbs ever more atmospheric carbon dioxide, levels of which are rising owing to fossil fuel use. As a result, seawater holds less calcium carbonate, so corals, molluscs and other creatures that use it to make their shells will struggle.
Corals were thought less at risk because they live in tropical seas rich in calcium carbonate, says Derek Manzello of the Atlantic Oceanographic and Meteorological Laboratories in Miami, Florida. But hurricanes could render them vulnerable.
Manzello and colleagues were monitoring two reefs off Florida when tropical storm Isaac swept through in August last year. The team found that the seawater's pH fell from 8.0 to around 7.8 as the storm moved through, and stayed at that level for a week afterwards. As a result, the water was significantly less rich in calcium carbonate over the same period. The main cause seems to have been rainfall and water run-off from the land.
With climate models predicting increased ocean acidification, Manzello calculates that by 2100, every hurricane will briefly push calcium carbonate levels so low that coral skeletons will begin dissolving.
"It's a double whammy, because the storm can also erode the reef," says Michael DeGrandpre of the University of Montana in Missoula, who was not involved in the study. He says the trigger for the damage need not even be a hurricane: a less extreme weather event like sustained heavy rain could have a similar effect on reefs.
Corals themselves can survive low carbonate levels, Manzello says, but if they don't build their skeletons, there could be knock-on effects. "The 3D reef framework structure is what protects shorelines from waves and storms, and provides habitat for the millions of species that reside on coral reefs," he says.