Two thirds of Australia’s Great Barrier Reef has been severely affected by two consecutive years of mass bleaching, according to new aerial surveys.

Bleaching is caused by abnormal environmental conditions, such as heightened sea temperatures, which causes coral to drive out the algae known as zooxanthallae that gives them colour, leaving the coral bleached and looking white.

Last year mass bleaching left 35 per cent of the northern and central parts of the coral either dead or dying. It is possible for coral that has been bleached to recover if the algae can recolonize it, but it can take years and requires the water temperature to drop. This year’s bleaching marks the fourth time the Great Barrier Reef has experienced severe bleaching since 1998, and the second mass bleaching that has happened in the space of 12 months.

Professor Terry Hughes and Dr James Kerry at the ARC Centre for Excellence for Coral Reef Studies at James Cook University carried out the aerial surveys that covered more than 8,000km. This year, only the southern third of the reef has been left untouched.

Mr Hughes claimed that while the reef is struggling with multiple impacts, such as the effects of Tropical Cyclone Debbie that recently hit the area or last year’s El Nino, the record-breaking temperatures driven by global warming is the most pressing problem.

Speaking to The Guardian, Mr Hughes said there is concern over the frequency of bleaching of the reef when it takes around 10 years for some of the coral to recover from these events.

“The significance of bleaching this year is that it’s back to back, so there’s been zero time for recovery,” he said, adding that while it is too early to tell the extent of the damage that will be caused by this year’s bleaching, the effects will be seen up to 500km south of the areas damaged last year.

He told the BBC this is the shortest gap that has been seen between mass bleaching. “The sooner we take action on global greenhouse gas emissions and transition away from fossil fuels to renewables, the better.”

Randi Rotjan, a marine biologist at Boston University in Massachusetts called the news “really scary,” and likened the bleaching to the reef “losing its insurance policy”.