Cyclones can damage even distant reefs

Here’s some sobering news with the Atlantic hurricane season just days away. New research shows that strong cyclones, hurricanes and typhoons can harm coral reefs from as far as 1000 kilometres away.

Conventional modelling used to predict their impact assumes that wave damage occurs primarily within 100 kilometres of its track.

Marji Puotinen from the Australian Institute of Marine Science (AIMS) and colleagues weren’t so sure, however, and decided to first look closely at how Scott Reef, an atoll reef off northwest Western Australia, fared as a result of Cyclone Lua in 2012.

Although the area of the cyclone producing the most intense winds came no closer than 500 kilometres to the reef, the high seas it whipped up battered it with waves four to 20 metres high for three and a half days.

The researchers found that at its most exposed sections, Scott Reef lost 50% of its massive and robust Porites corals and virtually all its more fragile branching Acropora coral species.

Similar damage was found on another reef, a further 300 kilometres distant, and models predicted damaging waves could be felt up to 1000 kilometres away.

Puotinen and colleagues from AIMS, the University of Western Australia and the Indian Ocean Marine Research Centre next collected data on cyclone size and frequency gathered between 1985 and 2015 for 150 coral reef ecoregions around the world.

Position, strength and size for each cyclone was recorded every six hours, allowing variations to be plotted in detail.

They found that more than 70% of the ecoregions had experienced at least one impact by a cyclone at peak strength and size during the 30-year period. Some, however, experienced them roughly every five years, and others roughly every 10.

“Coral reefs have been living with cyclones for millions of years, but recovery after a big battering is a slow process, which can take a decade or more,” Puotinen says.

“This means that many coral reefs around the world will not have time to fully regrow before the next cyclone hits.”

Estimates of wave damage from cyclones involve highly complex calculations because they change constantly, varying in strength, size and speed over time. The largest waves occur from storms that move slowly, and have the highest winds spread over the largest area.

The total number of cyclones occurring in any given period may well not increase, Puotinen says, but that’s not necessarily good news for vulnerable reefs.

“Changes in the atmosphere mean it will be harder for cyclones to form in the first place, but warmer ocean water, which fuels their intensity, means it will be easier for them to strengthen once they do.”

The researchers say their findings should inform reef management strategies.

“Coral reef communities around the world are under increasing threat from a range of stressors, and we must understand which parts of the reef should be the focus of conservation efforts,” says AIMS’s James Gilmour, a co-author of a paper published in the journal Global Change Biology.

“In particular, it is the combination of cyclones with exposure to rising water temperatures that is the most significant emerging threat to reefs globally.”