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 reveals
We measured the effects of five heatwaves on Great Barrier Reef over the last three decades: in 1998 and 2002, 2016, 2017, and 2020. These extreme temperatures have made the Great Barrier Reef a patchwork of bleached coral reefs with different histories.
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
Climate change is rapidly changing the frequency, intensity, scale, and extent of climate extremes. This includes the record breaking marine heatwaves that 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 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.
Heatwaves: Ecological memories
The majority of reefs today are still 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. It takes approximately a decade for the fastest growing corals to recover, while it takes longer for slower-growing species. Many of the coral reefs along the coast that were severely damaged in 1998 have not fully recovered.
Each bleaching event has had a different geographical footprint. Drawing upon satellite dataTo understand why different parts were affected by each of the five events, we measured the heat stress intensity and duration that the Great Barrier Reef experienced each year.
The history of previous bleaching strongly influenced 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 (one to four year apart). The earlier event had prepared the Great Barrier Reef for subsequent impacts.
In comparison to 1998 and 2016, it took more heat in 2017 and 2018 to trigger bleaching levels similar to those in 2018. For reefs that had previously experienced heat stress, the threshold for bleaching was higher.
Similar to the previous bleaching events, southern corals were spared in 2016 and 2017 and were more vulnerable in 2020 than central and northern reefs.
These historical effects or ecological memories could be generated by many different mechanisms. One is heavy losses of the more heat-susceptible coral species during an earlier event – dead corals don’t re-bleach.
There is no hiding place
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.
The theory is that a network of climate-resistant, well-protected reefs could help to repopulate the area. broader seascapeIf greenhouse gas emissions are reduced to stabilize temperatures later in the century, this will be possible.
The unbleached southern coral reefs are too small and far from the Great Barrier Reef to support long-distance recovery.
Future replenishment of coral populations that are declining is more likely to occur locally. 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 tend to experience less heat stress than people in shallower water.
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 show that we don’t have the luxury of studying climate-related events that were previously rare or unprecedented. Instead, as the world gets hotter, it’s increasingly important to understand the effects and combined outcomes of sequences of rapid-fire catastrophes.