What's the future for corals under climate change projections?

  • 05 Oct 2012, 10:00
  • Freya Roberts

Source: Google Earth

Scientists widely agree on the fundamental mechanisms by which climate change will affect the world's corals. Rising sea surface temperatures and increasingly acidic oceans make it difficult for corals to grow and survive. But how well corals are able to adapt under these conditions is something that divides scientific opinion.

While most believe corals' future under climate change looks pretty gloomy, some hold optimism that corals can adapt - and their hopes aren't entirely unfounded. From alarming reports in the media, however, it doesn't seem that way. So exactly how likely is it that corals can survive climate change?

Temperature rise

The oceans, much like the atmosphere, are warming in response to manmade greenhouse gas emissions. This warming threatens temperature sensitive species like corals, which respond by bleaching.

Bleaching occurs because the warming water stresses tiny algae living inside corals. These algae give corals their bright colour. Above certain temperatures, or when change happens too quickly, the algae are expelled from the corals, leaving a whitish skeleton behind.

Recent modelling suggests current efforts to keep warming to two degrees Celsius won't be sufficient to protect corals from more severe and frequent bleaching events. If the thermal tolerance of corals doesn't change, they risk large-scale degradation.

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Corals at Molokini Crater, Hawaii. Source: Google Earth

But some research suggests corals may be able to adapt to warming seas. Coral bleaching isn't necessarily permanent, and under the right conditions, corals recover. Certain research suggests species that do recover are better placed to face future temperature change, as are those already accustomed to a wide temperature range.

Corals themselves are resistant, but the colourful algae they house may also help them to adapt. Each relies on the other to survive, so if the algae can adjust quickly, they may help corals survive warmer temperatures - although they too face a struggle adapting. Other research suggests that partnering with many types of algae might also help corals adapt.

Since evolution occurs over generations, some studies suggest coral species with shorter lifespans are most likely to survive. At the same time, however, long-lived species have been shown to be resilient to warming.

Staying put isn't the only option for corals. In recent years, species near Japan have migrated polewards in response to warming temperatures.

Ocean acidification

Increasing ocean acidification also threatens corals. To balance increasing concentrations in the atmosphere, oceans absorb carbon dioxide. But as the carbon dioxide dissolves, it forms an acid. The process also uses up the chemical building blocks that corals need to build their calcium skeleton, hampering their ability to grow.

It's hard to quantify the impact this may have. Overall, modelling suggests that corals will suffer a net decline under high emissions scenarios of climate change. Early research suggested that corals faced imminent decline due to ocean acidification, but more recent research says corals' response to acidity is changing. For example, in one study, some cold-water corals were shown to be less sensitive to pH change when it occurs slowly.

The corals algal partners can have a role in adapting to ocean acidification too. Having a bit of diversity when it comes to algae can be a good thing, but new research indicates that it's possible to have too much of a good thing (too many partners).

Scientists have also identified an internal mechanism allowing corals to buffer rising acidity, and to continue building skeletons. So although migration isn't really an option responding to ocean acidification, there is some evidence corals may be able to adapt in situ.

Synergies at work

A lot of research and media coverage focuses on climate changes' negative effect, but these climate impacts aren't operating in isolation.

Combatting temperature change and acidification at the same time is complicated enough. But there are very immediate human threats already putting pressure on corals. These pressures interact with each other, making it difficult for corals to adapt specifically to the climate threats.

Overfishing, tourism, transport and pollution, for example, long predate the industrial era and for many corals, climate change may not be the greatest threat to their survival.

Professor Terry Hughes from James Cook University, Australia, explains:

"Coral decline won't just start in the 21st century, and it won't just be caused by climate change. Climate change impacts are pre-dated by centuries of overfishing and pollution, so climate change compounds them, not the other way round."

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Corals at the Apo Islands, Phillipines. Source: Google Earth

Taking action

Even though some research suggests corals have some capacity to adapt, scientists by and large agree corals face severe decline in the future. Evolution and adaptation are possible, but this depends on the rate of environmental change. Whether or not humans should interfere with these processes is something that divides scientists.

Some suggest the best chance for corals to survive climate change is through assisted migration. This essentially means digging up and moving corals from one area to another, much further afield.

But the idea has met with a lot of criticism. Corals have poor chances of survival in habitats they are not readily adapted to, and there's a risk that other species and viruses might also be transplanted. Instead, many scientists say the focus should be on protecting reefs to improve their resilience.

Dr Joerg Wiedenmann from the University of Southampton explains:

"This means addressing other negative impacts such as nutrient run-off (pollution), overfishing or damaging coastal construction"

And Professor John Pandolfi agrees with these sentiments, saying:

"It's been shown time and time again now that the best thing we can do to climate-proof our reefs it to manage them for the local threats that all of them are under"

The outlook's bleak - but not hopeless

The future for corals over the 21st century certainly looks challenging. Researchers predict that corals will continue to decline due to a variety of manmade factors, of which climate change is just one. Overfishing, pollution and industry already threaten these species, and rising temperatures and ocean acidification will almost certainly compound this. These combined effects might spell the end for a lot of coral species.

It's unlikely corals will be wiped out altogether, though. The range of species that currently make up reef communities will probably change. There will be winners and losers, but while some species may adapt, it looks like many more will decline.

At the moment, scientific evidence suggests the future for corals looks pretty bleak - but not yet hopeless. A certain amount of warming is already in the pipeline, and action is needed pretty urgently to try to avoid the worst case scenarios of coral decline. Scientists say the best way to tackle this is to make corals more resilient - by addressing the other human factors at play. 


Updated: 15/10/12 Sentence added to reflect new research on algal partners and ocean acidification - with hyperlink to PDF (free).

The images for this blog are taken from Google Maps, in conjunction with Caitlin Sea View Survey. They've made panoramas of some of the world great reef colonies freely available to view online, to enable scientists to study how oceans and thier corals are changing over time.

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