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.
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