How Global Heating Affects Our Coral Reefs and Marine Life

Wondering what rising ocean temperatures, coral bleaching and ocean acidification means for our reefs? Wendy’s dug into the science with a little help from marine biologist Professor John Pandolfi.

The first time I dove I was blown away by the incredible underwater world completely invisible from just 15 metres away above the surface of the water. The idea that reefs could disappear in my lifetime seems incomprehensible, yet that’s what some scientists predict. Diver friends talk pragmatically about the impending tragedy, they say they ‘want to see the Great Barrier Reef while it’s still there’.

Each year the impacts of global heating become more apparent and average temperatures keep inching upwards. In Australia, the last decade was nearly one degree warmer than the pre-1950s average.

The oceans though are taking on most of the extra heat – and absorbing most of the carbon dioxide. This double whammy means life in the ocean is fighting a battle on two fronts. 

As well as this, according to scientists from the University of California, marine animals are more vulnerable to rising temperatures than land animals.

This is possibly due to marine life being accustomed to highly stable temperatures (water temperatures don’t fluctuate day-to-day as air temperatures do) and land-dwelling animals can make use of coping strategies that aren’t available to marine life, such as finding cooler, shaded areas to shelter. 

Ocean Heating

More than 90% of the warming over the past 50 years has occurred in the ocean.

In Australia one of the most talked-about impacts of rising water temperatures are the mass bleaching events in the Great Barrier Reef, of which there have been three in the past five years. 

As water temperatures rise, the microscopic algae that gives corals their colour leave, and if temperatures don’t recede the algae may never return and the corals will die – more than 50% of coral on the Great Barrier Reef has already died.

The situation is similar for reefs across the world, and has huge consequences, not just for disappointed divers, but far more importantly for the considerable percentage of the population who rely on the reefs for tourism, food (from fish the reefs support), and the protection reefs provide to coastlines from storms.

Ocean Acidification

It turns out seawater is quite good at absorbing CO₂, so about 30% of the carbon dioxide that’s released into the air ends up in the ocean.

This causes acidification. The CO₂ absorbed by the sea increases the acidity of the water, which reduces the abundance of carbonate ions in the water. Carbonate ions are crucial for building coral skeletons and shells, so this makes it harder for coral and shellfish to grow. 

Increased acidity can create problems for other animals as well, such as confusing some species of fish and making it harder for them to sense predators.

Ocean temperatures are more stable than those on land, making it harder for marine species to cope with rising average temperatures.

What does this all mean for the ocean and coral reefs?

Professor John Pandolfi from the University of Queensland specialises in historical changes to reefs and has observed dramatic changes to Australian reefs firsthand. 

‘I first dove on the Great Barrier Reef in 1987 and since then there’s been a lot of change in terms of coral cover, abundance of fish and sharks, and changes in water quality,’ he says.

John says that until a few years ago he was primarily focused on local factors such as overfishing, pollution, and runoff from land due to land-use changes. 

‘I used to personally think, ‘Oh, I know climate change is a global threat to reefs, but we have all these other impacts that are already substantially damaging the reefs.’ And it has been quite a change for me over the past few years to see all this damage being vastly compounded by the devastating impacts from climate change that’s getting worse and worse,’ he says.

Professor John Pandolfi sampling corals at Lizard Island, Great Barrier Reef, years prior to the recent major bleaching events. | Photo by Ruth Reef

Beyond the very visible damage to coral reefs, sea life across all marine species and marine ecosystems are also struggling to survive rising temperatures. 

‘The number of species that are moving and shifting their ranges to escape the heat is absolutely astounding,’ says John.

Won’t species just adapt?

Some people (including certain members of my family after they’ve been reading Andrew Bolt) argue that it’s too late to do anything about climate change, and say that life on Earth will just adapt to warmer temperatures. But while some studies have been undertaken to assess the ability of species to adapt to climate change, the results so far aren’t especially conclusive or encouraging.

Regarding marine species specifically, John says, ‘Adaptation is also going on, but we [don’t have much] of a handle on that.’ 

He says there appears to be some evidence of species that have survived better than others during the Great Barrier Reef’s multiple bleaching events, and ‘the ones that do well increase their abundance leading to a change in the structure and the abundance pattern of species within the community’. John, however, is cautious about drawing any concrete conclusions about what this means for adaptation. 

It’s also apparent that even if some species manage to adapt and thrive in warmer water, this will still spell massive changes to underwater ecosystems.

Professor Pandolfi and his team surveying the fossil reefs of the Houtman Abrolhos Islands, Western Australia, to compare the effects of climate change in modern and fossil reef communities. | Photo by Marie Kospartov

What does the future hold?

While many people – myself included – might want to know what to expect in the next few years, John points out that precise predictions are hard to make. 

‘The science isn’t exact, it’s not a crystal ball. If you’d asked me [what was about to happen] in 2015, I don’t think anyone would have told you we were likely to lose half the Great Barrier Reef’s corals in five years,’ John says. 

‘We’ve had this increase in global warming effects, but it’s not linear – many biological responses to climate change are not linear. We pass thresholds and see massive changes. I’m very surprised by what’s happened in the last five years and wouldn’t like to say what will happen in the next five,’ he says. 

Is there anything we can do about our warming oceans?

The positive side is that John sees a lot that can be done to protect the reefs. 

‘Reefs that are less affected by threats unrelated to climate will be less impacted by climate change,’ he says. ‘There are local pressures on reefs like water quality, land-use change, over-harvesting, pollution, and if we do something about those our reefs will have a better chance of having a less severe response to thermal stress where it occurs.’ 

Read more: A Guide To Reef & Ocean Safe Sunscreen

By protecting reefs from other external threats we will be giving them the best chance at surviving and adapting to the impacts of climate change. | Photo by Marie Kospartov

However John says that the Great Barrier Reef, like reefs across the world, ‘has already undergone considerable degradation.’

‘We know what to do to manage reefs locally – there’s no rocket science involved with that… but to mitigate damage to reefs globally in response to climate change the two major impediments are political will and financial challenges, which are almost the same thing,’ John says. 

‘First and foremost people have to use their vote if they’re concerned about reefs and climate change. That’s the most important weapon people have in a democracy.’

Feature image by @huntingforparadise