The slow-moving category four tropical storm, which crossed the north Queensland coast on Tuesday afternoon, is likely to have left a trail of extensive damage to reefs in its path, much like Cyclone Yasi in 2011, said David Wachenfeld, director of reef recovery at the Great Barrier Reef Marine Park Authority.

"The primary concern will be the immediate physical damage in that central part of the storm with high wind speeds," Dr Wachenfeld said. "It's had lots of time for the wave energy to be built up by the winds and for that wave energy to hit the tops of the reefs."

Cyclones can be a mixed blessing for corals. While reefs near the storm's centre can be badly damaged, the tempest can also bring much-needed mixing of relatively hot, stagnant waters.

The rain and subsequent cloud cover can also help relieve the heat stress that causes the bleaching that has hit the Great Barrier Reef for an unprecedented two years in a row.

Cyclone Debbie's arrival, though, has interrupted the aerial surveys the park authority is conducting with James Cook University to determine the extent of this year's bleaching. Another summer of abnormally warm waters prompted many corals to expel the algae that gives them both colour and the bulk of their energy.

"We've got two different styles of extreme weather events delivering different coral impacts - but nonetheless killing corals in two different parts of the reef," Dr Wachenfeld said.

"There is some overlap between them but essentially, each of the three events [the 2016 and 2017 bleaching and Cyclone Debbie] is covering a different large area of the Great Barrier Reef," he said. "The three of them in conjunction will have delivered a really serious impact in just over a year."

Initial findings suggest the worst of this year's bleaching may be in the central part of Great Barrier Reef, including near the tourism centres such Cairns.

Last year's bleaching was worst in the relatively pristine northern third of the reef, with two-thirds of the corals dying - a subject raised by the Greens in the Senate on Wednesday.

Climate, other risks

Dr Wachenfeld said there are reports of bleaching again this year "right through to the Torres Strait", of varying severity.

"Way up there, this cyclone is going to make no difference at all to the temperatures of the water," he said.

Reefs closer to the area hit by Cyclone Debbie, such Orpheus Reef near Townsville, could do with cooler conditions. (See chart below from the Australian Institute of Marine Science, showing on-going unusually warm waters.)

The need to do more to protect the reef - such as curbing greenhouse gas emissions and investing to reduce other threats including nutrient-rich run-off from farms - is "only reinforced by the current events", he said.

Global warming is increasing the background temperatures of the world's oceans, increasing the likelihood that natural weather fluctuations will push corals beyond heat-stress thresholds. Longer term, rising acidity from the increased absorption of carbon dioxide in the oceans also threaten corals and other creatures such as shell-fish.

Climate change is also increasing the likelihood of more intense cyclones even if - at least in the Australia region - the number of cyclones will probably be reduced, according to scientists such as Jonathan Nott of James Cook University.