Sea change

Though the reef may be in “poor condition” today, its future could be even more bleak.

In October of last year, the Intergovernmental Panel on Climate Change (IPCC) – the United Nations body for assessing the science related to climate change – published a report looking at how the world would differ at 1.5C and 2C of global warming. (In 2015, the world’s political leaders signed the Paris Agreement, a pact to keep warming to “well below” 2C with an aspirational aim of limiting warming to 1.5C.)

Among its headline findings, the IPCC report found that 2C of global warming would lead to the loss of 99% of the world’s tropical reefs. And, even if warming is limited to 1.5C, around 70-90% of tropical reefs could disappear.

It is worth noting that, at present, the pledges made to tackle climate change by individual countries are not enough to meet either of these targets. If countries fulfill their promises, global average warming is likely to reach 3.3C above pre-industrial levels by the end of the century, according to analysis from independent research group Climate Action Tracker. Without any climate action, global warming could reach as much as 5C.

The findings are based on a review of recent scientific research papers, says Dr Michelle Achlatis, a researcher from the Coral Reef Ecosystems Lab at the University of Queensland and contributing author of the report. She tells Carbon Brief:

“All statements are accompanied by a ‘confidence level’, which is based on the combination of the scientific evidence and the degree of scientific agreement for that statement. Importantly, the IPCC has assigned the category of ‘high confidence’ to this prediction. This is the highest confidence level that the IPCC uses.”

The estimates in the IPCC report are based on a set of studies that use modelling to project how climate change will impact tropical corals. These models consider how climate change could lead to increased sea temperatures and more frequent episodes of extreme ocean heat, known as “marine heatwaves”.

They also consider how corals have reacted to past bleaching events. One factor to take into account is the time that it takes coral to recover from one episode of bleaching. Several of the studies used in the projections include an “optimistic” recovery time of around five years, the report says. (Some argue that it can take one to two decades for coral reefs to completely recover from bleaching.)

Another important issue to consider is “thermal adaptation” – the possibility that, over time, coral species could evolve to become more resistant to extreme ocean heat and so less likely to bleach. This could occur as heat-sensitive individuals die off, leaving heat-tolerant individuals to reproduce and pass their genes along to their offspring.

The report says that the projections expect “rapid thermal adaptation” – another “optimistic” assumption. “Adaptation to climate change at these high rates has not been documented,” the report says. However, it is possible that as global warming intensifies, there will be more pressure on corals to adapt and so the overall rate will get faster. Achlatis says:

“The ability of corals to adapt to change – and the speed with which they can adapt – is currently under debate. Some experiments show that corals can partner with [algae] symbionts that are more tolerant to temperature changes so that they too can hopefully become more tolerant of ocean warming. Other experiments show that today’s corals fare better under pre-industrial conditions than present-day conditions – suggesting that corals have not adapted much to environmental changes.”

It is important to note that the IPCC projections only consider the impacts of coral bleaching and ocean acidification on coral reefs. This means that any impact from pollution, overfishing or habitat destruction would be in addition to the predicted damage from climate change.

How, then, can anyone expect the reef to survive through the decades ahead? Prof Hughes is more optimistic about the reef’s future. “I think the figures – the 70-90% gone with another half a degree of global average warming above the 1C we’ve already experienced – are on the more pessimistic end of the spectrum.”

The reason for this, says Hughes, is that the report’s projections do not take into account how the reef ecosystem as a whole could adapt to higher levels of warming. Hughes says:

“When bleaching occurs, it’s actually incredibly selective. In the science literature, we distinguish between species that are so-called ‘winners’ versus ‘losers’. The losers are the heat-susceptible ones. In 2016, about half of those species were killed on the Great Barrier Reef. But the so-called ‘winners’ are much more resistant.”

During the bleaching event of 2017, water temperatures were warmer than during the 2016 event in many parts of the reef – but, overall, less bleaching was recorded, he says: “The reason it didn’t bleach so much in year two was because all the heat-susceptible ones had [already] died. But the tougher corals that bleached mildly in year one [survived].”

It is this “filtering effect” that could ensure the reef’s survival, according to Hughes: “We’re seeing a very rapid change in the mix of species because of the filtering mechanisms – and so that’s why I think we will have a reef in 50 or 100 years time if we can control extreme climate change.”

“I agree with Prof Hughes that heat-tolerant species on the GBR will survive, and that this gives us reason to be hopeful,” Achlatis says. “However, as Prof Hughes pointed out, coral reefs formed only from heat-tolerant species could look very different from the reefs of today. What these reefs will look like – and what the consequences will be for the other reef inhabitants, such as fish – remains to be explored.”