Coral reefs set to lose tolerance to bleaching as oceans warm

There's probably a few factual bits below but it's mostly just modelling crap.  They at least acknowledge that corals do adapt -- which is a great leap forward for them

The future is not looking bright. Coral reefs are set to become more vulnerable to bleaching as rising temperatures cripple their self-defence mechanisms.

Bleaching occurs when warm waters strip away the colourful photosynthesising algae that provide nourishment to corals.

This happens during unusually warm periods, such as during El Niño years, but doesn’t always kill coral, which can recover when waters cool again.

Corals are often able to survive heatwaves by developing resistance during periods of milder warming, when water temperatures rise and cool off again, says Tracy Ainsworth of the Australian Research Council Centre of Excellence for Coral Reef Studies. The corals are essentially given a warning for what’s about to come, a sort of practice run.

A little stress can help corals

"Corals that undergo smaller stress prior to a bleaching event are able to retain more symbionts within the tissue, those algae which are crucial for nutrition," says Ainsworth. "This has major implications as to whether or not it can survive."

Now that climate change is driving up ocean temperatures, there are fears that these acclimatisation periods will become shorter or disappear completely.

To get an idea of how warming waters might affect corals, Ainsworth and her colleagues studied patterns of sea surface temperatures at Australia’s Great Barrier Reef over the last three decades.

They found that during that time, 75 per cent of heatwaves were preceded by moderately warm temperatures. These can help cut coral mortality by 50 per cent.

More heat, more stress

They then modelled future scenarios and found that this proportion may drop to 22 per cent if sea surface temperatures rise by 2 °C, as could occur by 2100.

What’s more, they found that an increase in local water temperature of just 0.5 °C can lead to loss of this adaptation mechanism.

"We will no longer be getting a situation where corals have a small stress, a period of recovery due to water cooling, and then a big stress," says Ainsworth. "What we’ll see is an accumulation of one big stress."

Survival strategies

Their experiments also confirmed the importance of practice runs, showing that corals developed a number of heat resistance strategies as the water warmed up.

"They upregulated their heat shock responses and all these other molecular mechanisms that prevented damage to the cells during the next stress," says Ainsworth.

But increasing sea temperatures caused by climate change will see that gap between the preparation period and the peak stress disappear, says study co-author Scott Heron of the US  National Oceanic and Atmospheric Administration.

Not too late

"Those temperatures will no longer drop below the stress levels," says Heron. "So instead of a gap to recover between the preparation period and the peak stress, the corals have an extended period of stress."

If these predictions are born out, coral cover in the Great Barrier Reef could dwindle to less than 5 per cent by the end of the century.

Nevertheless, it is not too late to turn things around. The researchers’ modelling studies demonstrated that aggressive efforts to curb greenhouse gas emissions would result in no net decline in coral cover by the end of the century.

"I think we do still have hope, we should never give up," says Ainsworth.

SOURCE