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Seas without shells

Remember Silent Spring? It was in 1962 that Rachel Carson's book alerted the world to the problems of the insecticide DDT in the food chain. Birds of prey were particularly vulnerable, with their eggshells becoming so thin they could no longer contain growing embryos. The threat of springtime with no birdsong catapulted the world into a new awareness of ecology and conservation.

Forty-seven years on, a new threat is looming, this time in the sea. Once again the busy rhythm of people is causing an ecological crisis. Not as complicated as modelling global warming, not as simple as banning a pesticide, our newest planetary drama is called ocean acidification. It happens because of the connections between air, water, and shells.

We know that human activities, particularly the burning of coal, oil, petrol and wood, have for the past 200 years increased the amount of carbon dioxide, or CO2, in the atmosphere. While these molecules float around in the air, they act like a blanket keeping Earth warm and eventually changing the whole climate. The warming effects of CO2 have been less than they could have been, however, because about a third of CO2 from the air gets mopped up by the oceans.

What's good for global climate change, however, is bad for the sea. When you add CO2 to sea water, it becomes more acid. And that means that the carbonate ion, CO3, gets scarcer. That might seem like no big deal, but many marine plants and animals use carbonate, along with calcium, for constructing protection and structure.

Clams, snails, urchins, corals, some algae, and many plankton all use calcium carbonate (CaCO3) to build their shells.

Marine ecologists have only just begun to investigate the potential problems that a more acid ocean might pose to creatures in the sea. What they have found so far is alarming. Tiny plankton, zillions of which form part of the basis of the marine food chain, are usually protected by a robust and complex ball of carbonate.

But when you grow them in more acid conditions, these little shells become thinner and more frail. Even more alarming, experiments with corals show that under acid conditions, some do not make a skeleton. They sit there like a jelly glob with no sign of the complex architecture that makes coral reefs so diverse and so attractive to tourists - and to fish.

This isn't just a problem for squishy marine critters. Marine aquaculture and multimillion-dollar fisheries such as mussel farming are likely to be affected.

Tourism to coral reefs is another multimillion-dollar industry, and some economies are wholly reliant on it. There is even the suggestion that a more acid ocean could be more corrosive and thus affect shipping and ports.

The sea is growing more acid by the day. Early estimates suggested that acidity could go up 30 per cent by the end of this century. Now scientists are warning that, in the Southern Ocean, we could be seeing measurable changes within a few decades. The effects of what we have already pumped into the air are probably irreversible. There are no practical solutions or cures - no antacid for the sea's indigestion. The only thing we can do is to slow it down.

Luckily, we already want to reduce carbon emissions and know we need to stop the invisible clouds of CO2 rising into the air. We already have mechanisms in place to change how we live and travel. Ocean acidification provides another, and perhaps a more urgent, reason for continuing on this path as fast as possible.

We still have birds of prey. That is because people cared, listened and took action. Ordinary gardeners stopped using DDT, and eventually governments also responded. Now you can't buy DDT and you can't spray it around.

Geologists, who specialise in the long- term view, are beginning to call the present time period the anthropocene epoch.

They mean that the activities of humans are so pervasive that they will be the dominant signal in the geologic record of our time. So far it appears that the anthropocene will be renowned for its great extinction event - a period in which Earth became so unhealthy that hundreds of species of animals and plants ceased to be. Given that acidification is to be added to the effects of coastal pollution, ongoing development, sedimentation and over-fishing, it is not surprising that our coastal ecosystems are set to crash.

We can choose to make a difference. Just don't drive. Turn off the power. Think about all those millions of plankton making their complex and perfect skeletons. Think about that exhaust, puffing out the back of every car, each little bit of CO2 heading into the air, into the sea, a little drop of poison for our planet. Each of us can make small differences. Think about what you could do, today, to save just one plankton, just one coral. Because a sea without shells is like springtime without birds.

* Associate Professor Abigail M Smith is a geochemist in the Marine Science Department at the University of Otago.

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