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Thursday, March 15, 2012

Ocean acidification: another problem with CO₂ emissions


SINGAPORE — We tend to measure time by the span of a human life, making a century seem like an era and a millennium a mega-stretch of time. In this perspective, a million years is an eternity. So it can be revealing to consider our place in geologic history measured in hundreds of millions of years.

This is what a group of researchers from the United States and four other countries did recently. They reviewed existing evidence on the impact of changes in the atmospheric concentration of carbon dioxide (CO₂), the main global warming gas, from decades of research on fossilized remains and other evidence from Earth's geologic record.

Published earlier this month, their findings reinforce warnings from many climate scientists that the world's oceans, a vital source of fish food protein, may be turning acidic faster today from human CO₂ emissions than they did during four major episodes of animal and plant extinctions in the last 300 million years, when natural surges of CO₂, probably from catastrophic volcanic eruptions or meteor strikes, sent global temperatures soaring.

Of the four episodes, the closest analogue for the present is what happened some 56 million years ago, a period known as the Paleo-Eocene Thermal Maximum (PETM), when oceans changed fast but not nearly as fast as today.

The scientific review suggests that acidification is now happening at least 10 times more quickly than during PETM, raising "the possibility that we are entering an unknown territory of marine ecosystem change." For an ocean-oriented nation like Japan, where fish is such an important part of the national diet, this is a warning that should be taken seriously.

Increasingly acidic oceans and seas harm a range of marine life, from reef and shell-building organisms to the tiny snails favoured by salmon. A scientific study last year found that marine life diversity of coral reefs off Papua New Guinea had declined by as much as 40 percent because of acidification. Coral reefs are an important breeding ground and shelter zone for fish.

"We know that life during past ocean acidification events was not wiped out (and that) new species evolved to replace those that died off," said Barbel Honisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory, who was a lead author of the recent review of the scientific evidence. "But if industrial carbon emissions continue at the current pace, we may lose organisms we care about — coral reefs, oysters, salmon."

By burning fossil fuels and clearing forests for agriculture, industry and cities in ever greater extent, humans are releasing vast amounts of CO₂ into the atmosphere. In the last century, atmospheric CO₂ has risen about 30 percent, to 393 parts per million.

Approximately one quarter of the excess CO₂ is absorbed by land plants and another quarter by the oceans, which cover some 70 percent of Earth's surface. Once dissolved in the ocean, a carbon atom will stay there on average more than 500 years.

The excess CO₂ from the atmosphere reacts with seawater to form carbonic acid. Over time, this mild acid is neutralized by fossilized carbonate shells on the sea floor. But if CO₂ goes into the oceans too quickly, as it is now, it can deplete the carbonate ions that corals, mollusks and some plankton in the seafood chain need for reef and shell-building.

In the early 1990s, scientists used a deep-drilling ship to extract sediment cores from deep in the seabed off Antarctica. They found a layer of mud from the PETM period wedged between thick deposits of white plankton fossils. Because seawater chemistry is partly controlled by temperature, sediments and fossil shells retain a signature of the ambient temperatures under which they formed.

The scientists estimated that in a span of about 5,000 years, a mysterious pulse of CO₂ doubled atmospheric concentrations, pushed average global temperatures up by about 6 degrees Celsius, and dramatically changed ecology as well as the environment.

The result: carbonate plankton shells littering the sea floor dissolved, leaving the brown layer of mud. As many as half of all species of benthic foraminifers, a group of single-celled organisms that live at the bottom of the ocean, became extinct, indicating that organisms higher in the food chain may also have disappeared.

"It's really unusual that you lose more than 5 to 10 percent of species over less than 20,000 years," said Ellen Thomas, a Yale University paleoceanographer who took part in the Antarctic drilling expedition. "It's usually on the order of a few percent over a million years."

Temperature records show that the average sea and land surface temperature has risen by nearly one degree Celsius since the start of the industrial revolution over 200 years ago. At the time of the PETM, the world was already much warmer than it is today.

Still, it is worth pondering one of the potential similarities. The most likely cause of the PETM was the mass release of methane (the main component of natural gas) from sediments on the sea floor, where the gas, as it is now, was frozen and held in place by the cold and intense pressure in a solid form known as methane hydrate. Once in the atmosphere, methane would have quickly oxidized as CO₂.

The PETM did not wipe out life on Earth and it affected different groups of organisms and habitats in different ways. Many species adjusted to warming by expanding their ranges into higher latitudes. Tropical plants did well, perhaps because they were already adapted to warm conditions. Nonetheless, the changes were profound and they took place over what for humans today would be unimaginably long times.

Now that we have become a dominant influence on the way Earth's intricate climate system works and we know from the geologic record what natural surges of CO₂ can do, should we continue to risk overloading the planet with greenhouse gas emissions?

Michael Richardson is a visiting senior research fellow at the Institute of Southeast Asian Studies in Singapore.


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