(CNN) Australia's Great Barrier Reef has experienced its most widespread bleaching event on record, with the south of the reef bleaching extensively for the first time, a new survey has found.

This marks the third mass bleaching event on the reef in just the last five years and scientists say that the rapid warming of the planet due to human emissions of heat-trapping gases are to blame

Aerial analysis conducted by Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, and others from the Great Barrier Reef Marine Park Authority, found that coastal reefs along the entire length of the iconic reef -- a stretch of about 1,500 miles (2,300 kilometers) from the Torres Strait in the north, right down to the reef's southern boundary -- have been severely bleached.

"We are all in shock really at how quick this has happened," said Hughes. "Three severe bleaching events in five years is not something we anticipated happening until the middle of the century."

Warm ocean temperatures are the main driver of coral bleaching, which is when corals turn white as a stress response to water that is too warm. This happens because they are expelling the algae that grows inside them, which is their main energy source and gives them their color.

Bleaching doesn't kill coral immediately. But if temperatures remain high, eventually the coral will die, destroying a natural habitat for many species of marine life.

Hughes said he took about 11 flights over nine days in March criss-crossing the full length of the Great Barrier Reef, surveying 1,036 reefs from the air to measure the extent and severity of the coral bleaching.

What he saw was unprecedented.

"For the first time, severe bleaching has struck all three regions of the Great Barrier Reef -- the northern, central and now large parts of the southern sectors," he said.

Of the reefs surveyed this year about a quarter were severely affected, while a further 35% had modest levels of bleaching.

The bleaching event this year is not only the largest, in terms of the area affected, but also second most severe on record, the scientists found, with the damage likely to be lasting and irreparable.

In 2016, bleaching killed more than half of the shallow-water corals on the northern region of the Great Barrier Reef. A second mass bleaching in 2017 meant the coral could not recover.

This year, the cumulative footprint of bleaching has expanded further south, affecting more fragile and heat-sensitive corals.

Hughes said they won't know the full extend of the loss of corals until they go back to the same reefs conduct underwater surveys in October or November.

Bleaching is when corals turn white as a stress response to warm water temperatures.

Bleaching events getting more frequent

Coral reefs are some of the most vibrant marine ecosystems on the planet -- between a quarter and one-third of all marine species rely on them at some point in their life cycle.

And none is more vital than the Great Barrier Reef.

It's also a vital resource to Australia's economy, contributing more than $5.6 billion annually and supporting tens of thousands of jobs.

As bleaching expands and becomes more frequent, corals are at greater risk of dying off -- and that will be devastating not only for the region's biodiversity, but for the thousands of people whose life and livelihood depend on the reefs.

Mass bleaching events on the Great Barrier Reef in 2016, 2017 and 2020. (Credit: ARC Centre of Excellence for Coral Reef Studies)

The scientists' main concern this year is the southern region, which escaped the bleaching during 2016 and 2017 as water temperatures were close to normal, Hughes said.

Because it has not been bleached before, this portion of the reef has more coral that is sensitive to the heat. Most susceptible to dying off are ecologically important species such as the staghorn, or branching, corals that are ideal habitats for an array of species of fish and other marine life.

"When we go back underwater in a few months time, we anticipate significant mortality or loss of those corals," Hughes said.

He anticipates that as much as half of these "red reefs" that suffered the most severe bleaching this year to have died because that's what happened in the northern reefs in 2016.

Another concern is the shrinking gap between one mass bleaching and the next. The first recorded bleaching event along the Great Barrier Reef occurred in 1998 -- then the hottest year on record.

Four more severe bleaching events have occurred since, in 2002, 2016, 2017, and now in 2020.

This year, February saw the highest monthly sea temperatures ever recorded on the reef since records from the Australian Bureau of Meteorology began in 1900. Many reefs experienced temperatures that were 3°C above the normal summer maximum.

"That's incredibly destructive. Those extreme temperatures can kill the coral very quickly," Hughes said. "We really are on uncharted territory here in terms of rising temperatures."

The bleaching event in 2020 was the most widespread on the Great Barrier Reef ever recorded.

Fewer opportunities for recovery

Past bleaching events have typically occurred in years with a strong El Nino-Southern Oscillation, a climate phenomena that can increase the odds of a host of extreme weather events around the globe.

But as summers get hotter year on year in Australia, scientists found that bleaching can occur even when El Nino is not active.

That could have a huge impact on whether the reefs can recover.

Hughes said it takes about a decade for the fastest growing corals to make a full recovery. As bleaching events become more frequent, there are fewer opportunities for the corals to rebound.

"Normal recovery rates for the reef are being impinged by the scale of the loss of the adult root stock -- that's the grown up corals that make the babies," Hughes said.

Reefs are important because they protect shorelines and coastal regions from erosion and extreme weather events. They are also source of food security for millions of people around the world.

According to a study by the United Nations on the Economics of Ecosystems and Biodiversity, coral reefs benefit about 850 million people worldwide, with at least 275 million depending directly on reefs for livelihoods and sustenance.

Already in Australia, fish stocks on the Great Barrier Reef are declining because of loss of habitat, Hughes said.

You take out the coral, the ecosystem collapses and marine life dies.

This photo taken on March 2020 shows coral bleaching on the Great Barrier Reef in Australia.

The mass bleaching conditions were also observed in late March by Coral Reef Watch, which uses remote sensing and modeling to predict and monitor for signs of bleaching.

Dr. C. Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration's (NOAA) Coral Reef Watch, told CNN last month that because of the massive amounts of heat the world's oceans have already absorbed, the reef likely won't have the chance to recover before it bleaches again.

"If it takes decades for a reef to recover ... what chance do we have for reefs recovering when events are coming back this fast?" he said.

Though researchers around the world are exploring ways to revive reefs , Eakin says those efforts will not be enough if we don't address the root cause of their demise -- human-caused climate change.

"We have to address climate change if we want to have coral reefs in the future."