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5 major heatwaves over 30 years have made the Great Barrier Reef a bleached checkerboard

5 major heatwaves over 30 years have made the Great Barrier Reef a bleached checkerboard

Only 2% of the Great Barrier Reef is left unbleached since 1998, and 80% of individual reefs have been bleached severe once, twice, or three times since 2016. our new studyToday’s revelations

The effects of five different marine heatwaves over the Great Barrier Reef were measured over the past 30 years: in 1998, 2002 and 2016, 2017, 2017 and 2020. We found that these extreme temperatures have turned it into a grid of bleached reefs, each with a different history.

The global temperature rise will determine whether we have a functioning Great Barrier Reef in decades to come. The bleaching events we’ve already seen in recent years are a result of the world warming by 1.2℃ since pre-industrial times.

The climate summit in Glasgow will see world leaders make more ambitious pledges to dramatically reduce greenhouse gas emissions. It’s vital for the future of corals reefs, and for the hundreds of millions of people who depend on them for their livelihoods and food security.

Coral in a warmer climate

The Great Barrier Reef is made up of more than 3,000 individual corals that stretch for miles. 2,300 kilometresSupports more than 60,000 jobs in the reef tourism.

A diver looks at bleached coral

The Great Barrier Reef is made up of more than 3,000 individual coral reefs that stretch for 2,300 kilometres.
ARC Centre for Coral Reef studies/Greg Torda via AAP

Climate change is causing rapid changes in the frequency, intensity, and scale of extreme climate events. This includes the record-breaking marine heatwaves which cause corals bleaching. Overheated corals cause bleaching, which is a stress response that causes corals to lose their colour and make it difficult for them to survive.

If all COP26 pledges made by individual countries are kept, the projected increase in global warming could be lowered. to 1.9℃. In theory, this would put us in line with the goal of the Paris Agreement, which is to keep global warming below 2℃, but preferably 1.5℃, this century.

It is however not enough to stop the ongoing degradation of the world’s coral reefs. The damage to coral reefs caused by anthropogenic heat is already evident. And further warming will only continue to increase the danger. ratchet downReefs can be found throughout the tropics.




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Heatwaves: Ecological memories

Today, most reefs are in their early stages. recovery modeAs coral populations begin to rebuild after the last bleaching events in 2016, 2017, and 2020, this is called coral re-building. For the fastest-growing corals, it takes around ten years for a decent recovery. It takes much longer for slower-growing species. Many of the coral reefs along the coast that were severely damaged in 1998 have not fully recovered.

The fringing reef flat of Orpheus Island, central Great Barrier Reef, before mass coral bleaching.
Bette Willis, Andrew Baird. Author provided

After further bleaching in 2002, the same reef flat was found on Orpheus Island.
Bette Willis, Andrew Baird. Author provided

Each bleaching event has had its own geographic footprint. Drawing upon satellite dataWe measured the duration of heat stress experienced by the Great Barrier Reef each summer to understand why different parts were affected by each event.

The recent history of bleaching had a significant impact on coral bleaching. For this reason, it’s important to measure the extent and severity of bleaching directly, where it actually occurs, and not rely exclusively on water temperature data from satellites as an indirect proxy.

The most vulnerable reefs were those that hadn’t bleached in the past ten years or more. However, the heat threshold for severe bleaching rose when successive episodes occurred within a short time span (between one and four years). This means that the Great Barrier Reef’s earlier events had made it more resilient to future impacts.

Bleached coral

Overheated corals can cause bleaching, a stress response.
Shutterstock

In 2018, 2017 and 2002, for example, it took more heat to cause similar levels of bleaching than in 2016 and 1998. For reefs that had previously experienced heat stress, the threshold for bleaching was higher.

In 2020, the most vulnerable corals in 2020 were the southern corals. They had escaped severe bleaching in 2016/2017.

These historical effects or ecological memories can be generated by many mechanisms. One is heavy losses of the more heat-susceptible coral species during an earlier event – dead corals don’t re-bleach.




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bats flying


There’s no place to hide

Only one cluster of reefs is left unbleached in far south, downstream of the Great Barrier Reef. This small area has remained cool throughout the summer months of all five mass bleaching events. These reefs are located near the Great Barrier Reef’s outer edge. There, upwelling cool water may provide some protection from heatwaves.


Map of the Great Barrier Reef showing cumulative bleaching levels in 2016, 2017, and 2020. The colours indicate the intensity of bleaching. They range from zero (category 1) to severe bleaching that affected over 60% of corals (category 4).
Author provided

The theory is that a network of climate-resistant, well-protected reefs could help to repopulate the area. broader seascapeIf greenhouse gas emissions can be reduced to stabilize temperatures in the second century, then this is possible.

The unbleached southern coral reefs are too small and far from the Great Barrier Reef to support long-distance recovery.

Future replenishment is more likely local. It would be from the millions of larvae that are produced by adults who have been rehabilitated on nearby reefs. Or corals living in deeper waters, which are less susceptible to heat stress than those in shallower water.




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The future recovery of corals will be more temporary and incompletive, before being interrupted by the inevitable next bleaching event. As a result, corals will continue to decrease under climate change.

Our findings demonstrate that we don’t have the luxury to study individual climate-related events which were once rare or unheard of. Instead, as the world gets hotter, it’s increasingly important to understand the effects and combined outcomes of sequences of rapid-fire catastrophes.


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