Can the Great Barrier Reef survive climate change?

Words by Daisy Dunne. Multimedia by Rosamund Pearce and Tom Prater.

“The insidious thing about climate change is there’s nowhere to hide from it,” says Prof Terry Hughes, director of the Australian Research Council's Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, a sleepy city in northern Queensland where even the winter sun beats down at 27C .

Hughes has spent the past three decades tracking changes on Australia’s Great Barrier Reef. It was in 1998 that the reef faced its first “mass bleaching event”. Bleaching occurs when some kind of stress – most commonly, unusually high sea temperatures – causes coral to release the colourful algae that lives inside its tissue, leaving it a ghostly white. This algae acts as the primary source of food and, without it, coral slowly starves.

Small-scale bleaching is a common sight on the reef, but 1998 marked the first year that saw widespread bleaching, Hughes says. That year, 42% of the reef turned white. The Great Barrier Reef is the world’s largest living structure – made up of 3,000 individual reefs spread over an area the size of Italy or Japan. The first mass bleaching affected around 4,200 sq km of coral, according to Hughes.

At the time, a newspaper article published in the Townsville Bulletin warned that “coral bleaching is killing the world’s coral reef ecosystems” and that “the future for even the Great Barrier Reef was bleak”. Speaking to the paper, marine ecologist Dr Terry Done said: “If the projections of global climate change do come about it’s likely we will see more years like this in the future.”

The second mass bleaching occurred four years later, in 2002. This time, 54% of the reef was affected.

“We had a very long gap – a 14-year gap – and we were just simply lucky,” Hughes says. The third mass bleaching episode came in 2016, this time causing 93% of the reef to bleach. Most of the damage was concentrated in the northern section of the reef, where sea temperatures were particularly high, according to aerial surveys undertaken by Hughes. Here, 81% of the coral bleached severely – while only 1% escaped bleaching altogether.

“Then, unfortunately, we were very unlucky because we saw widespread bleaching for the fourth time just one year later,” he says. Research recently published by Hughes shows that the 2017 bleaching event affected 83% of coral – a small decrease on the area affected in 2016. This is not necessarily a good thing, however, Hughes says. “In the north, the reason it didn’t bleach so much in year two is because all the heat-susceptible [coral] had [already] died.”

Last November, temperatures in Townsville rocketed to 41.7C – the hottest November day recorded in the city. In the days that followed, temperature records were broken across the state of Queensland, while unprecedented bushfires – fuelled by the heat – forced thousands from their homes.

“This is terrifying,” Hughes – an avid Twitter user – posted at the time. “An early summer heatwave breaks all records, lifting the chances of another episode of coral mortality on the Great Barrier Reef next Feb/March.” A few days later, he posted: “We’ve booked the boats in anticipation of more coral bleaching.”

However, just a few weeks later in January, Townsville faced unprecedented summer rainfall. In just one week, the city received the equivalent of its annual rainfall, causing widespread flooding that forced hundreds to evacuate.

The downpour also lowered the chances of mass coral bleaching for 2019, Hughes says: “Sea temperatures are cool – or close to normal – thanks to recent cyclones and monsoonal clouds and rain. We only have a few weeks more of summer before water temperatures peak and the drop off.”

The rise in mass coral bleaching is a consequence of rapidly rising temperatures, Hughes explains. (Carbon Brief analysis shows that temperatures above the Great Barrier Reef have risen by 0.6-0.9C since the industrial era began.) Hughes says:

“What we’ve seen here on the Barrier Reef mirrors the global trend. We’ve gone from a period before the 1980s where mass bleaching simply didn’t happen to an intermediate phase that lasted a couple of decades, where El Niños triggered bleaching.”

El Niño is natural weather phenomenon that develops in the Pacific Ocean. Every few years, a chain of events between the ocean and atmosphere causes sea surface temperatures to become unusually warm in the East Pacific. The warming shifts rainfall patterns, causing Australia to become drier and hotter in the summer. This can be followed by an unseasonably cool period, known as La Niña. The entire natural cycle is known as the “El Niño Southern Oscillation” (ENSO).

In the 1980s and 90s, the impact of El Niño, in combination with human-caused global warming, was enough to trigger mass coral bleaching in the summer months. However, in recent years, warming has intensified to such a degree that mass coral bleaching can also occur in summers without El Niño, Hughes says:

“We’re now seeing bleaching events throughout ENSO cycles, so even in some neutral years and in La Niña years, which historically were cooler. They’re still cooler than average, but, [during] La Niña periods now, the water temperature is hotter than it was during El Niño phases just 30 years ago.”

Another study by Hughes found that, worldwide, severe coral bleaching is now five times more frequent than 40 years ago. The real danger of this rise is that it leaves coral with little time to recover, Hughes says. After coral is bleached, it can survive – if it regains its colourful algae. But, if temperatures are high enough, coral can “literally cook to death”, Hughes says. During the 2016 bleaching event, 30% of the reef’s coral died, many of them in just two to three weeks. (While bleached coral is bright white, dead coral is a lifeless brown.)

‘Ecological homogenisation’

The repercussions of mass coral die-off across the reef are hard to quantify. The reef is home to more than 1,600 species of fish and 10% of the world’s total fish population. The underwater ecosystem supports dozens of whale and dolphin species and six out of seven of the world’s sea turtle species. It also plays host to more than 20 seabird species, which fish and nest on the reef’s islands.

The reef’s biodiversity attracts two million visitors a year – providing jobs for around 65,000 Australians. This tourism generates an annual sum of £2.8-3.4bn (AUS$3.6-4.3bn).

Fish counts undertaken from 2014 to today suggest that numbers of parrotfish, butterflyfish and damselfish have declined in some parts of the reef. However, it is difficult to tell whether this decline is a direct result of coral bleaching, or some other environmental stress, such as overfishing.

There is, however, evidence to suggest that bleaching could be changing the makeup of species found on the reef. A survey taken eight months after the 2016 event found that, following bleaching, the mix of wildlife found across the reef appeared to be less diverse.

“What you are seeing here is ‘ecological homogenisation’ – or flattening. It is a reduction in the diversity of corals that make up the community,” Dr Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch programme, told Carbon Brief in April, 2018. “Less diversity of coral means less diversity of the fish, crabs, shrimp, clams, worms and other organisms that live on the reef.”

Another study looked at animal diversity across the reefs of Lizard Island – in the northern part of the Great Barrier Reef – before, during and six months after the 2016 bleaching event. It found that while most coral species bleached during the event, some types of coral faced higher rates of mortality than others.

“Soft coral” – coral without a hard exoskeleton, such as devil's hand and leather coral – were the most likely to have been killed by bleaching, the study’s lead author Laura Richardson, a PhD candidate at the Centre of Excellence for Coral Reef Studies at James Cook University, tells Carbon Brief. “In the case of our study, the sharp and sustained increase in water temperature was too much for the soft corals.”

Her research team also recorded significant ecological homogenisation following the bleaching event. “The immediate impacts of this increased similarity on the functioning of the coral reef ecosystem is unknown and research is needed to understand it,” she says. “But, from what we know so far, it doesn’t bode well.”

This is because, in a reef ecosystem, each organism provides a unique ecological service, Richardson says. As the community of organisms found on the reef becomes less diverse, some of these vital services could be lost.

Coral die-off could also be having indirect effects that may go unnoticed by the average snorkeller, research suggests. A study published in 2018 found that damselfish, which normally use coral to hide from large predatory fish, choose not to shelter in dead coral – despite it providing the same level of protection as living coral.

“It appears that the presence of live coral rather than shelter per se is the necessary cue that elicits the appropriate behavioural response to potential predators,” the authors write in their research paper.

A second study published in 2018 found that, following coral die-off, butterflyfish – which feed on coral – not only ate less often, but also changed their behaviour towards each other.  Out on the reef, butterflyfish often guard patches of coral that they feed on. If another fish approaches their patch, they can become aggressive and chase their rival off.

However, after a coral bleaching event, the amount of aggressive behaviour displayed by butterflyfish can fall by up to two-thirds, according to the research. This is likely because, following bleaching, the fish have less to eat and so lack the energy to show aggression, the authors say.

A loss of aggression could cause the territories normally held by butterflyfish to break down, the authors say. “Territorial breakdown could lead to dampened dispersal among reefs, together altering community dynamics.” Shifts in the community of coral species found on reef could increase the risk of local extinctions, they add.

Out of sync

For coral, bleaching is not the only consequence of climate change. Ongoing research suggests that changing sea temperatures could be influencing the time that some corals choose to “spawn”, or release eggs.

Though immobile, corals are animals and reproduce by “external fertilisation” – a fusion of sperm and eggs that takes place outside of the mother organism’s body. In the Great Barrier Reef, hundreds of species take part in “mass spawning” – the release of millions of sperm and eggs at the same time. This event sparks a feeding frenzy for small fish and other animals that eat coral eggs.

Scientists studying the phenomenon have discovered that mass spawning usually occurs around one week after a full moon. However, the exact month and time that spawning occurs is likely to be controlled by something else – potentially sea surface temperature.

For the past 20 years or so, Prof Andrew Baird, a coral reef ecologist from the Centre of Excellence for Coral Reef Studies at James Cook University, has visited sites across the reef to record the month and timing of spawning for a group of hard corals known as Acropora – or staghorn corals. The group includes more than 150 species globally and is typically the most common coral group found in the Great Barrier Reef.

To do this, Baird drives out on a motorboat and packs only a snorkel, chisel and a waterproof notepad. On a hot spring day in November, he heads out to the shallow reefs off the shores of Magnetic Island, a spot popular with tourists within the Great Barrier Reef marine park thanks to its unique wildlife. (It is one of the last remaining habitats for wild koalas in Australia and is home to a population of semi-tame rock wallabies.)  

The spot he chooses to survey is a reef where staghorn coral lives interspersed with tall, weedy macroalgae. He snorkels above a colony and uses the chisel to crack the branches of the coral to expose the developing eggs. (It takes “less than two weeks” for the animals to recover from the intrusion, he says.)

Once cracked open, the coral’s developing eggs become visible. If the eggs have some colour, they are mature, and will spawn after the next full moon. If the eggs are white, they are immature and spawning is at least one month away. If the coral contains no eggs, it has either spawned recently or is not likely to spawn anytime soon, he says.

In 2016, Baird and his colleagues published the results of a 12-year study looking at drivers of coral spawning across the Indo-Pacific region. To do this, the researchers recorded the month of coral spawning at 34 reefs across the region, including in Australia, Indonesia, India and Egypt.

They then compared this data to a range of factors that may trigger coral spawning, including sea temperature, rainfall and wind speed. (Previous research had suggested that coral may have evolved to avoid rough seas caused by high winds, which can have a negative impact on fertilisation.)

However, the research by Baird’s group found that the best predictor of coral spawning time was sea temperature. Specifically, a sharp increase in sea temperature from one month to the next.

Researchers are still not sure why coral could be timing their spawning to coincide with sharp changes to sea temperature. One theory is that sea temperature could affect sperm’s swimming ability, while another theory suggests that temperature could help regulate the production of sperm and eggs.

One important implication of these findings is that, as the climate changes, the month of coral spawning could become “decoupled” from other important ecosystem processes, Baird says.

If the annual rise in sea temperature on the Great Barrier Reef begins earlier in the summer as a result of climate change, then corals could spawn earlier. This could have a knock-on effect on species that depend on coral spawning. Some reef fish, for example, time their reproduction to match the annual feast provided by mass coral spawning. The authors say:

“Rising sea surface temperatures, shifted currents, altered nutrient distributions and increased frequency of extreme events are projected for the oceans over the coming decades. For corals that cannot adapt through behavioural [changes] or rapid evolution, decoupling is a real danger.”

Climate change could also pose a risk to coral by driving “ocean acidification” – a phenomenon that occurs as seawater absorbs CO2 from the atmosphere.

Of the CO2 released into the atmosphere by humans, around 30-40% of it dissolves in the oceans, while the rest remains in the atmosphere or is absorbed by living things on land. This has caused oceans, which are alkaline, to become more acidic over time. The overall pH of seawater has fallen from 8.2 to 8.1 from the start of the industrial era to present day.

The chemical reactions associated with ocean acidification also drive a reduction in the availability of calcium carbonate – a compound that hard corals use to build their tough outer shell. With less calcium carbonate available, hard corals find it more difficult to repair or grow their skeletons.

Research released in 2016 found that some parts of the Great Barrier Reef are “highly vulnerable” to the impacts of ocean acidification. A second study published at the same time found ocean acidification is already harming parts of the reef. It estimated that, at One Tree Island in the southern Great Barrier Reef, the rate that corals can rebuild their skeletons is now 7% lower than during pre-industrial times.

It is not only climate change that threatens the reef’s survival. The other “main threats” to the reef are overfishing and pollution, Prof Hughes says. “Those haven’t gone away. In fact, they’re still escalating in most places.”

A large driver of pollution is nitrogen fertiliser run-off from sugar-cane plantations and other types of farming. Research has found that, once washed into rivers, farming pollutants can travel up to 450km to reach and contaminate reef waters. The nitrogen in fertilisers can spark the growth of algae blooms, which “smother” coral, preventing them from receiving sunlight.

A report by WWF released in December, 2018 found “alarming” levels of pollutants present in the blood of sea turtles. Analysis of the animals’ blood and cells found traces of metals such as cobalt, antimony and manganese. A possible source of these pollutants could be nearby industrial activity, such as mining, the authors say.

The reef is also threatened by habitat destruction for new developments. One such development is the Carmichael coal mine, a controversial project headed by Indian mining firm Adani. As well as contributing to climate change by causing the release of CO2, the mine would also require a shipping terminal at Abbot Point – which is opposite the reef – to expand. This could cause coral to face more pollution, including from coal dust and, potentially, deadly collisions with boats. Hughes says:

“The Great Barrier Reef, today, after two back-to-back bleaching events, is in poor condition. The amount of corals out there on the reef is the lowest we’ve ever measured since monitoring began in the 1980s. Now is not the time to develop new coal mines in Australia or anywhere else.”

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.”

A techno-fix?

Another possibility not considered is that researchers could develop some of kind of technology that could lessen the impact of climate change on the Great Barrier Reef.

Shortly after the 2017 coral bleaching event, more than a dozen researchers signed a letter published in Nature Ecology & Evolution that called for “new interventions” to “save coral reefs”.

“For coral reefs to remain resilient and their services sustained, we argue that new and potentially riskier interventions must be implemented,” the researchers say.

The comment article concentrates on two “emerging” techniques: “assisted gene flow” and “synthetic biology”.

“Assisted gene flow” would involve introducing coral from other parts of the world where seas are typically warmer than in the Great Barrier Reef. The researchers suggest taking coral from the Persian Gulf, where the bleaching threshold is “3-4C higher” than in the Indo-Pacific region. The hope is that the transplanted corals would breed with native corals and, so, spread their genes for heat tolerance.

However, the authors concede that this technique could come with risks. These include that the transplanted corals could bring diseases that native corals are not familiar with, which could spark an epidemic.

Another risk is “maladaptation” – the idea that the transplantation process could unintentionally pass on other genes that would cause native corals to become less suited to their environment. For example, transplanted corals could pass on genes that code for a preference for the saltier water found in the Persian Gulf.

“Synthetic biology” refers to the idea of using gene editing to create heat-tolerant corals. This could be achieved with “Crispr-Cas9” technology, the authors say. Crispr is a tool that allows scientists to introduce new genes into an organism’s DNA through a simple “cut and paste” method. (Crispr has previously been used to create long-life mushrooms and to reduce deafness in mice.)

But using such a technique on a scale as large as the Great Barrier Reef would require “extensive societal consultation”, the authors say:

“We acknowledge that systems like the Great Barrier Reef...may be perceived to be compromised by interventions like large-scale assisted gene flow [and] synthetic biology. However, the opportunities these interventions offer to sustain functioning, albeit altered, coral reefs worldwide must be weighed against the alternative of continual decline.”

A major drawback of both technologies is that developing them at scale would require a significant amount of research and investment, the authors note. “Only a subset of the million species on coral reefs could feasibly be made climate tolerant.”

A separate project headed by researchers from the University of Sydney and the Sydney Institute of Marine Science is exploring another type of technology that could be used to tackle coral bleaching. Known as “marine cloud brightening”, this technique would use water cannons to shoot saltwater into the air above the reef. Once airborne, the salt particles would facilitate the formation of marine clouds, the research team says, which could shield coral from incoming sunlight.

The proposal has never been tested – but research using computer modelling suggests that this shielding effect could be enough to prevent coral bleaching in the Great Barrier Reef. Several barriers to testing the technology remain, however. Some scientists are concerned that altering the climate above the reef with marine cloud brightening could lead to unfavourable changes to the climate elsewhere, for example.

Several other ideas to protect the reef from climate change have been put forward – although not all of them have been subject to peer-reviewed research.

In 2017, the Australian government announced plans to spend $2.2m on a controversial scheme to introduce giant fans above a one-sq-km patch of coral in the northern part of the reef. The three-year pilot project would see solar-powered fans mounted onto floating pontoons in attempt to drive water currents that could bring cool water to the sea surface. This could protect corals during bleaching, according to the project’s leader.

However, an independent review of the scheme by scientists called it a “major departure from reality”, according to documents seen by the Guardian. The review added that the fans could pump warm water towards deep sea corals, exacerbating the impacts of bleaching.

“The notion that we can protect the barrier reef from the next bleaching event by installing fans on one of the 3,000 individual reefs that makes up the entire [reef] is pretty ludicrous,” says Hughes, who was not involved in the review.

Another project is trialling the possible use of “artificial reef sunscreen” – a thin layer of calcium carbonate that would sit on the ocean surface above coral, shielding it from incoming sunlight. Preliminary laboratory experiments find the sunscreen can reduce the amount of sunlight reaching corals by 30%, according to the scheme’s benefactors.

The project is being funded by the Great Barrier Reef Foundation, an organisation that came under scrutiny after it was handed $443m in government funding in 2018. At the time, the foundation had just six full-time employees and a chair’s panel made up of executives from mining and oil companies, according to Buzzfeed News.

One problem with the “artificial sunscreen” proposal is that strong waves and currents could easily break down the ultra-thin film, Hughes says. “You would need it to be immobile, the size of Italy and to last for about two months for it to be effective.”

A wider issue of many of the technologies proposed to help the Great Barrier Reef is that they do not tackle climate change head on, but rather seek to minimise its impacts, Hughes says. “The root causes of the problems of the Great Barrier Reef are pollutants running off from agricultural land, which we can deal with, and climate change – and that’s the elephant in the room which Australia is refusing to deal with.”