These reefs are havens for biodiversity in the sea. Many coastal communities make
a living from them. But ifcarbon dioxideemissions continue to rise, then by the
middle of this century 98% of reef habitats will be bathed in water too acid for
the reef to grow.
Among the first to feel the effects will be Australia’s Great Barrier Reef, the world’s
largest living thing.
Chemical oceanographersKen Caldeira and Long Cao report their results in a multi-author
paper in the December 14 issue of Science. They also present them at the annual meeting
of the American Geophysical Union in San Francisco on the same date.
The work is based on computer simulations of ocean chemistry. The scientists looked
at the effects of different levels of carbon dioxide in the atmosphere. The levels
ranged from 280 parts per million to 5000 parts per million.
The first is the level before there were any industries in the world. The second
is much higher than the present level. That is roughly 380 ppm.
“About a third of the carbon dioxide put into the atmosphere is absorbed by the oceans,”
says Caldeira. "This helps slow greenhouse warming. But it is a major pollutant
of the oceans.”
The carbon dioxideabsorbed in the sea produces carbonic acid. This is the acid in
soft drinks that gives them their fizz. What it does in the ocean is make certain
mineralsdissolve more easily in seawater.
This is especially true for aragonite. This is the mineral used by corals and many
other marineorganisms to grow their skeletons.
Before the industrial revolution, almost all warm-water coralreefs were bathed with
open ocean waters which was 3.5 times supersaturated with aragonite, says Cao. "This
means that corals could easily extract it to build reefs.”
But if atmospheric carbon dioxide sticks at 550 ppm, no existing coralreef will
remain in such an environment. The chemical changes will affect some regions sooner
than others. At greatest risk are the Great Barrier reef and the Caribbean Sea.
Levels of carbon dioxide are rising rapidly, Cao added, and even to get them to stick
at 550 ppm would take concerted international efforts. These look very unlikely at
Carbon dioxide’s chemical effects on the ocean are pretty much independent of its
effects on climate. So efforts to slow global warming won't slow the acidification
of the seas, the researchers say. That is unless emissions are reduced.
In fact, the expected chemical changes in the sea may need emissions to be cut even
more than for climate alone.
“These changes come at a time when reefs are already stressed by climate change,
overfishing, and other types of pollution,” says Caldeira. “So unless we take action
soon there is a very real possibility that coralreefs - and everything that depends
on them - will not survive this century.”
Carbonic acid is quite common. Where would you normally find it?
But carbonic acid in the sea changes the chemistry of the ocean. What is one effect
When there is very little carbonic acid in the sea the waters around coral reefs
are -------------- with aragonite.
Because of this the aragonite comes out of the water easily. This makes it easy for
the coral to use it to make what?
But if carbonic acid levels in the water rise, it means aragonite dissolves more
------ in the water.
That means the water isn't so supersaturated with aragonite. So it is ------ for
the coral to get the aragonite out of the water.
At what level of carbon dioxide in the atmosphere did the computer simulations show
that no coral reef in the world would still be in highly supersaturated water?
What can you say about the chances of those levels occurring in the years ahead?
What one action will both slow global warming and slow the acidification of the
What is the worst that can happen to the reefs?
How likely do you think that is?
Explain your thinking for your last answer.
Topics for discussion, research or pupil presentations
Here some of the issues, implications and applications taken directly from the text.
Students should be encouraged to find others themselves.
These reefs are havens for biodiversity in the sea.
Many coastal communities make a living from them.
But if carbon dioxide emission continues to rise, then by the middle of this century
98% of reef habitats will be bathed in water too acid for it to grow.
But if atmospheric carbon dioxide sticks at 550 ppm, no existing coral reef will
be in waters that are so supersaturated.
Levels of carbon dioxide are rising rapidly and even to get them to stick at 550
ppm would take concerted international efforts. These look very unlikely at the moment.
Here are some possible topics for group discussion, research or presentations arising
from these. Students should be encouraged to think up others:
What does 'havens for biodiversity' mean and why is it important?
How exactly do coastal communities make a living from coral reefs?
Explore the science of this, which is not too difficult but is not particularly well
explained in the story. The image at the top of the page should help. Explain the
science to the rest of the class.
Do they look unlikely? Take a look at the latest news from the intergovernmental
panel on climate change in Bali and present what's happening there briefly to the
class. A good place to start is this BBC page, which also has a link to a nice visual demonstration
of the greenhouse effect.