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Coral bleaching could be the norm by the 2050s

  • 24 Feb 2013, 20:00
  • Freya Roberts and Roz Pidcock

Scientists know that corals are sensitive to climate change. But corals in some parts of the world will be better off than others, says new research. Those near the equator are likely to suffer soonest, while those off the coast of Madagascar and around French Polynesia might find temporary refuge for a few extra years. But the long term outlook is still bleak.

Coral bleaching

Coral reefs are one of the richest ecosystems on earth, but they are also the most sensitive to climate change.

Like the atmosphere, the oceans are warming. Rising water temperatures stress the corals, to the point that they are forced expel the tiny colourful algae living inside them that they need to thrive - known as zooxanthellae.

This causes the corals to lose their colour - an effect known as bleaching. A coral can recover from  a single bleaching event, but persistently high temperatures can kill off whole reefs.

But the oceans are unlikely to warm at the same speed. So corals in different parts of the world probably won't all respond in the same way either.

A new study in Nature Climate Change has used the most recent generation of IPCC models and scenarios for global temperature rise to calculate how long corals in different locations might survive.

Modelling the oceans

The team worked out how quickly the world's oceans might warm for the IPCC's four different scenarios of climate change - known as Representative Concentration Pathways (RCPs). In each scenario, greenhouse gas emissions evolve over the next century in a slightly different way.

The scientists divided up the world's coral habitats into 1,707 equally sized boxes and then ran the new generation of climate models several times for each of the scenarios to see how the corals responded to temperature rise. For each location, they looked for the year in which coral bleaching started to happen every year.

First and worst

Rather than bleaching happening at the same rate everywhere, a pattern emerged. Corals living closest to the equator were the first to bleach annually: some parts of northwestern Australia, west Papua New Guinea and the Central Pacific Islands started to bleach every year a full 15 years before the global average.

Corals further away from the equator did not start to bleach on a regular basis until quite a few years later. Corals living in parts of the Great Barrier Reef, the west Indian Ocean towards Madagascar, and the waters around French Polynesia resisted bleaching longer than most. In some places that meant an extra 15 years.

For example, the map below shows a scenario where future greenhouse gas emissions are medium-high. Corals in red and yellow, largely seen close to the equator, are the ones the model predicts will bleach soonest. Further out are the corals predicted to resist bleaching for an extra five to 15 years - shown in blue - or even longer - shown in green.

RCP6

Legend RCP6

Source: Adapted from van Hooidonk et al.

This step-by-step pattern of bleaching, spreading out from the equator, is clear under medium and high emissions scenarios. The pattern, the authors say, is down to the unequal way oceans will be affected by climate change. 

The equatorial Pacific is projected to warm faster than oceans in higher latitudes, as less cold water upwells or as trade winds weaken the outflow of warm waters.

Safe havens

There is some evidence that corals might be able to adapt over time to become more tolerant to warmer water. But it's not clear whether a reprieve of 15 years is long enough for that to happen. Lead author Ruben van Hooidonk told Carbon Brief:

"The answer is that we don't know. Certainly we can't expect evolution to work on those time scales on corals. But we have seen that some corals can change the type of algae from a more temperature sensitive to a more resilient type after bleaching."

But, he added:

"There is an upper temperature limit, and we do not know if corals can adapt in time."

Looking further ahead, the team's modelling suggests even corals that resist bleaching the longest are unlikely to be unaffected by the end of the century, and under the highest emissions scenario all corals will begin to bleach every year by 2056. In lower emissions scenarios, that date can be pushed back - but only to the mid 2070's.

The future?

Research like this highlights the importance of measures to reduce greenhouse gas emissions. But the study's authors say there's another reason for looking at the spatial patterns in coral bleaching. By identifying areas where corals will be worst hit and areas where survival chances are greater, efforts to conserve corals can be targeted, they say.

This study doesn't necessarily mean the end of all coral life. Modelling like this does have to make some assumptions about corals ability to adapt, and certain species could defy the norm.

That said, it does paint a pretty bleak picture. The only future where not all of the world's corals bleach annually is a low emissions future where emissions peak soon and then fall. And right now, that's not looking too likely.

van Hooidonk, R. et al., (2013) Temporary refugia for coral reefs in a warming world. Nature Climate Change, DOI: 10.1038/nclimate1829

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