Monday Apr 2 09:28 AEST

Overloaded oceans

Friday March 30, 2007
(Getty Images)
By Tony Haymet
Director, Scripps Institution of Oceanography
University of California

Ocean acidification is a real and potentially devastating consequence of climate change that demands immediate attention.

An excess of fossil fuel-derived carbon dioxide is causing profound changes in ocean chemistry. These changes are some of the most pressing problems presented by the burning of fossil fuels, but they get little attention.

A small but growing number of scientists are turning their thoughts to the oceans and their findings demand that we take action.

The science is simple.

The carbon dioxide (CO2) generated by human activities is released into the atmosphere. Some remains there, the rest is taken up by land vegetation or makes it way into the oceans.

Although life in the oceans does take up carbon dioxide, its role in the uptake of this additional CO2 is, as yet, small.

As the amount of CO2 taken up by the ocean increases, it causes an increase in ocean acidity, and the ocean’s natural ability to counter this acidity increase is being compromised.

We have already experienced an increase in ocean acidity of nearly 30 percent compared to pre-industrial times and a doubling has been predicted by 2100.

Increasing the amount of CO2 in the oceans causes an increase in hydrogen carbonate ions, HCO3-, but a decrease in carbonate CO22- which organisms need to make calcium carbonate shells and other structures.

The predicted acidity increase will have unknown consequences for marine life and ecosystems such as coral reefs, tiny marine organisms called pteropods, and fish larvae to name but three.

Each of these organisms plays a fundamental role in local ecosystems and the food web, therefore amplifying the effects of their forced changes.

Take for example the pteropod, a free-swimming snail that lives near the surface of the ocean. It is a key food source for a number of fish and marine mammals. As ocean acidity rises, pteropods experience a double threat. Not only does the water corrode their shells, it may inhibit their ability to build shells in the first place, leaving them without an adequate protective layer.

The effects of their decreasing numbers will ripple through the food web and could eventually impact humans as the fish we eat becomes scarcer because of the decrease in their food supply.

Coral reefs are also in danger due to the corrosive nature of increasing ocean acidity. Representing the most diverse marine ecosystem, coral reefs are crucial to the health and perpetuation of marine biodiversity.

Yet these vital aquatic resources are under siege by a number of environmental stressors including temperature increases, local pollution, coastal erosion, overfishing, and, of course, ocean acidification.

Ocean acidification decreases the calcification rate for corals and other organisms as the carbonate ion concentration is reduced. This is a chronic issue that could eventually lead to the demise of many calcifying marine species, including all coral reefs, even Australia’s Great Barrier Reef, the world’s largest biological structure.

What effect these changes in ocean chemistry will have are still to be seen, but impacts on entire ecosystems and thus to economically vital fisheries are certainly possible.

Atmospheric CO2 has already been linked with water issues in California. The state was recently approached by an environmental group attempting to use the federal Clean Water Act to regulate atmospheric CO2, claiming the gas does harm to the cycle of life in the ocean. Making clear the effect CO2 has on the ocean, requests like this help draw attention to the air-sea-land interconnectivity of the Earth.

The record from the distant past gives added cause for concern. For a period more than 55 million years ago, ocean acidity was much higher than today, and shell-forming organisms, and those coming after them in the food chain, vanished.

Yet the rate of change occurring 55 million years ago is certain to have been much slower than the rate today. It is believed that the speed at which we are altering the current acidity of the ocean has not been seen before, and therefore we may not be able to anticipate the changes based on historical data.

We must not let the ambiguity of exact implications stop action on this problem. Ocean acidity, rather than temperature warming, may determine the upper limit of atmospheric CO2 that Earth can safely tolerate.

Even if emissions stopped today, we will still see an acidification effect in the oceans for a century or more due to the slow cycle of deep-ocean circulation and the long life of CO2 in the atmosphere.

We cannot stop studying this problem just as we are beginning to appreciate its full magnitude. We must begin today to increase focus on this vital area of research.

We call today for a Keeling Curve for the oceans. The Keeling Curve depicts the measurement of CO2 in the atmosphere since 1957 and definitively links the observed rise to the increased burning of fossil fuels.

We also call for a determination of the biological consequences of increasing ocean acidity and its effects on the ocean food web.

Such measurements would build on the WOCE/JGOFS surveys of the 1990s, and would provide a comprehensive measurement of ocean acidity throughout the oceans. As we celebrate the 50th anniversary of the groundbreaking Keeling Curve, now is the time to promote the same long-term measurement and response program for CO2 in the oceans.

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