“We no longer have the luxury of studying individual climate-related events that were once unprecedented or very rare.”
“Instead, as the world gets warmer, we need to understand the effects of sequences of fast-paced disasters, as well as their combined effects.”
If global warming is kept at 1.5 degrees, the maximum increase in average global temperature that was the focus of the United Nations COP26 climate conference, the coral mix on the Barrier will change, but it could still thrive, said Prof Hughes .
“If we can keep global warming up to 1.5 degrees from global average warming, then I think we’ll still have a vibrant Great Barrier Reef.”
Bleaching is a stress response of overheated corals, which causes them to lose color, and many struggle to survive.
Differences span the two percent of reefs that “overall escaped bleaching,” to the 80 percent that have severely bleached at least once since 2016.
To understand how the Reef will succeed in the future, a better understanding of the compound effects of several disturbances across time and space is needed, Prof Hughes said.
“For the first time, in 2020, we have seen severe bleaching across the entire length of the Reef – in parts of the northern, central and especially the southern region.”
But each whitening event has a different “geographical footprint”.
The northern Reef avoided damage in 1998 and 2002 before being the hardest hit region in 2016. The south escaped in 2016 and 2017.
Scientists have found that the responses to extreme heat depend on the recent history of bleaching.
In 2002 and 2017, more heat was needed to achieve similar levels of bleaching to the events in 1998 and 2016.
“To our surprise, we found that the threshold for bleaching was much higher on reefs that had experienced an earlier episode of heat stress,” said co-author Mark Eakin.
“Consequently, the most vulnerable reefs each year were the naive ones that haven’t bleached lately.”
When consecutive bleaching episodes were only one to three years apart, the earlier event may have hardened affected areas to further effects, co-author Sean Connolly said.
“But more frequent, severe bleaching events will only undermine the resilience of coral reef ecosystems,” said Professor Connolly.
“Corals still need time to recover from another round of heat stress so they can make babies that will disperse, settle and recover from the depleted parts of the Reef.”