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
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
in the media, however, it doesn't seem that way. So exactly how
likely is it that corals can survive climate change?
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.
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.
Corals at Molokini Crater, Hawaii. Source: Google
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
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,
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.
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
too much of a good thing (too many partners).
Scientists have also identified an internal mechanism allowing
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
Synergies at work
A lot of research and media coverage focuses on climate changes'
negative effect, but these climate impacts aren't operating in
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,
"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
Corals at the Apo Islands, Phillipines. Source: Google
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
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
Dr Joerg Wiedenmann from the University of Southampton
"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,
"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
Updated: 15/10/12 Sentence added to reflect new research on
algal partners and ocean acidification - with hyperlink to PDF
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.