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Stanford, CA: 14-Dec-2007 14:00 Eastern US Time

Coral reefs in acid oceans

Carbon emissions from human activities are not just heating up the world. They are changing the ocean’s chemistry. This could soon be fatal to coral reefs.

 

These reefs are havens for biodiversity in the sea. Many coastal communities make a living from them. But if carbon dioxide emissions 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.

 

This is the main conclusion of calculations by scientists at the Carnegie Institution’s Department of Global Ecology

 

Among the first to feel the effects will be Australia’s Great Barrier Reef, the world’s largest living thing.

 

Chemical oceanographers Ken 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.

 

Carbon dioxide levels in the atmosphere are rising fast because of increasing emissions  from human activities. The main one is the burning of fossil fuels.

 

“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 dioxide absorbed 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 minerals dissolve more easily in seawater.

 

This is especially true for aragonite. This is the mineral used by corals and many other marine organisms to grow their skeletons.

 

Before the industrial revolution, almost all warm-water coral reefs 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 coral reef 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 the moment.

 

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 coral reefs - and everything that depends on them - will not survive this century.”

 

Hyperlink highlights

Don’t miss the supersaturated video from PBS. It’s cool, instructive and very entertaining.

 

More help with words

average  

conference  

dissolved  

elements  

fossilised  

greenhouse gas  

journal  

models  

solution  

variety  

 

What's it all about?

1. Carbon emissions are heating up the world, but they are also doing something to the oceans. What is that?
2. Scientists have been looking at the effects of this on coral reefs. These effects are very worrying for at least two reasons. State one of them.
3. What effect does carbon dioxide in the ocean have on the water?
4. What is the world's largest living thing?
5. The oceanographers have presented their results in two different ways. State one of them.
6. What kind of science did the researchers do - experiment, calculations or a mixture of both?
7. What does "parts per million" mean?
8. What was the level of carbon dioxide in the atmosphere before the industrial revolution?
9. What is the level now?
10. Use these two figures to work out how much, as a percentage, carbon dioxide levels have risen since the industrial revolution. [Mouseover for Hint.]
11. Carbon dioxide absorbed by the oceans has two effects. One is good. What is that?
12. One is not good at all. What is that?
13. Carbonic acid is quite common. Where would you normally find it?
14. But carbonic acid in the sea changes the chemistry of the ocean. What is one effect of this?
15. When there is very little carbonic acid in the sea the waters around coral reefs are -------------- with aragonite.
16. Because of this the aragonite comes out of the water easily. This makes it easy for the coral to use it to make what?
17. But if carbonic acid levels in the water rise, it means aragonite dissolves more ------ in the water.
18. That means the water isn't so supersaturated with aragonite. So it is ------ for the coral to get the aragonite out of the water.
19. 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?
20. What can you say about the chances of those levels occurring in the years ahead?
21. What one action will both slow global warming and slow the acidification of the oceans?
22. What is the worst that can happen to the reefs?
23. How likely do you think that is?
24. 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.

 

1. These reefs are havens for biodiversity in the sea.
2. Many coastal communities make a living from them.
3. 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.
4. But if atmospheric carbon dioxide sticks at 550 ppm, no existing coral reef will be in waters that are so supersaturated.
5. 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:

 

1. What does 'havens for biodiversity' mean and why is it important?
2. How exactly do coastal communities make a living from coral reefs?
3. So what?
4. 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.
5. 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.

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