Research finds current models underestimate the impact of hurricanes andtyphoons on coral reef communities

Big and strong cyclones can harm coral reefs as far as 1000 kilometres awayfrom their paths, new research shows.

A study led by Dr Marji Puotinen from the Australian Institute of MarineScience (AIMS) sounds a warning about the way strong cyclone winds buildextreme seas that affect coral reefs in Australia and around the world.

Conventional modelling used to predict how a cyclone, hurricane or typhoonmight impact corals assumes that wave damage occurs primarily within 100kilometres of its track.

To test this, Dr Puotinen and colleagues looked at Scott Reef, a well-studiedatoll reef structure off the northwest of Western Australia, and how it faredas a result of Cyclone Lua – a slow-moving weather event that developed offthe coast in 2012.

Although the area of the cyclone producing the most intense winds came nocloser than 500 kilometres to the reef, the high seas it whipped up batteredit 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 50per cent of its massive and robust Porites corals and virtually all its morefragile branching Acropora coral species. Similar damage was found on anotherreef, a further 300 kilometres distant, and models predicted damaging wavescould be felt up to 1000 kilometres away.

“This example demonstrates that if we assume damage from all cyclones occurswithin a 100 kilometre radius of a cyclone’s track, we will underestimate thespatial extent for big, strong cyclones by up to 10 times,” Dr Puotinen said.

“This could lead to making unfortunate choices when trying to prioritiseconservation targets.”

She added that estimates of wave damage from cyclones involve highly complexcalculations because they change constantly, varying in strength, size andspeed over time. The largest waves occur from storms that move slowly, andhave the highest winds spread over the largest area.

To test the consequences of using the standard distance-based model, she andcolleagues – from the AIMS node in Perth, the University of Western Australiaand the Indian Ocean Marine Research Centre – collected existing informationon cyclone size and frequency, crunching data gathered between 1985 and 2015for 150 coral reef ecoregions around the world.

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

They found that more than 70 per cent of the ecoregions had experienced atleast one impact by a cyclone at peak strength and size during the 30-yearperiod. Some, however, experienced them roughly every five years, and othersroughly every 10.

“Coral reefs have been living with cyclones for millions of years,” said DrPuotinen. “But recovery after a big battering is a slow process, which cantake a decade or more. This means that many coral reefs around the world willnot have time to fully regrow before the next cyclone hits.”

Climate change models present a complex picture for cyclones. The total numberoccurring in any given period may well not increase – but that’s notnecessarily good news for vulnerable reefs.

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

She added that her team’s findings carry lessons for reef management andconservation strategies.

“When deciding where on the Great Barrier Reef, for instance, to investmillions of dollars to repair or enhance reefs, you don’t want to select alocation likely to be regularly battered by cyclone waves,” she said.

Our research should make it easier for reef managers to choose betweencandidate reefs.”

Dr James Gilmour, also from AIMS, a co-author on the paper, said the findingsillustrated the complexity and severity of the threats facing reefs around theworld.

“Cor al reef communities around the world are under increasing threat froma range of stressors, and we must understand which parts of the reef should bethe focus of conservation efforts,” he said.

“In particular, it is the combination of cyclones with exposur e to risingwater temperatures that is the most significant emerging threat to reefsglobally.”

Unravelling the specific effects of cyclones, the researchers conclude, willprovide vital clues for the management of at-risk areas.

The research is published in the journal Global Change Biology.

Image: The same area of Scott Reef photographed in 2010, and again in 2012after Cyclone Lua. Credit: James Gilmour/AIMS

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