David Archer, Associate Professor in Geophysical Sciences, says oceanic surface waters are absorbing more carbon dioxide from the atmosphere, making those waters more acidic and endangering coral reefs.

Coral reefs endangered says study author Archer

Coral reefs will become a casualty of the industrialized world’s growing carbon dioxide emissions by the middle of the next century, according to a study published in the Friday, April 2 issue of the journal Science.

Oceanic surface waters will become more acidic as they absorb carbon dioxide in increasing amounts from the atmosphere, say the study’s co-authors, including Chicago’s David Archer. This rising acidity will in turn significantly interfere with the growth of coral.

“It’s an irrevocable thing that we’re doing to the planet,” said Archer, Associate Professor in Geophysical Sciences. “Seventy-five percent of carbon dioxide has a lifetime of hundreds of years in the atmosphere. But a smaller fraction, seven or eight percent, has a lifetime of hundreds of thousands of years.”

The Science article’s six co-authors, led by Joan Kleypas of the National Center for Atmospheric Research in Boulder, Colo., document the first direct detrimental effect of rising carbon dioxide. In 1700, the concentration of carbon dioxide in the atmosphere was approximately 280 parts per million. Today, it’s nearly 370 and rising as much as two parts per million per year.

“It’s almost going to double from the pre-industrial value some time in the next century. That’s almost unavoidable,” Archer said.

Fossil-fuel combustion and deforestation both have contributed to rising atmospheric carbon dioxide. “There was a long, broad rise of carbon dioxide throughout the 19th century, which predates the use of coal and other fossil fuels,” Archer said. “They call it ‘the pioneer effect,’ for when the New World forests were cut down.”

This increase in carbon dioxide adds to the greenhouse effect, which traps heat in the atmosphere, possibly leading to global warming. Warming trends during the last 10 years, whether from increased carbon dioxide emissions or natural climatic variation, have already caused bleaching of coral reefs.

Corals normally live in symbiosis with algal plants, but warm temperatures upset the relationship. “For some reason that nobody really quite understands, when corals get stressed they spit out the plants,” Archer said. When that happens, corals lose their color, changing from green or brown to white. “This effect of carbon dioxide acidifying the ocean is on top of this already well-known bleaching effect,” Archer said.

The findings are based on a series of studies showing that acidity interferes with the growth of coral reefs. Another of the Science article co-authors, Chris Langdon of Columbia University’s Lahmont-Doherty Earth Observatory, conducted one of these studies in Biosphere 2 near Tucson, Ariz. Biosphere 2 is an enclosed glass and metal-frame structure that houses miniature ecosystems, including a 900,000-gallon research ocean.

Coral reefs are mostly made of calcium carbonate. When carbon dioxide dissolves in water, it makes carbonic acid, which causes calcium carbonate to deteriorate. Langdon and his colleagues observed a slowdown in coral reef growth when Biosphere 2 contained higher levels of carbon dioxide.

A key element of the Science study was Archer’s model of carbon movement from the ocean surface to the deepest sea floor. The model takes into account such factors as fluid dynamics, current velocity, water temperature, salinity and chemistry.

“It’s like the models they use to predict weather in the atmosphere, only this is down in the ocean,” said Archer, whose work is supported by the David and Lucille Packard Foundation and the Petroleum Research Fund.

Archer’s model produced carbon dioxide levels that, over decades, rise at the ocean surface where coral reefs grow. These areas are warmer than other parts of the ocean and therefore more buoyant and in more continuous contact with the atmosphere, he explained.

Similar models are used to determine how much carbon is going into the ocean today. The burning of fossil fuels and deforestation combined release approximately seven gigatons of carbon into the atmosphere each year, three of which remain in the atmosphere. Models predict the ocean takes up another two gigatons. Scientists suspect the remaining two gigatons might be fertilizing terrestrial plants and soil in the Northern Hemisphere.“The fate of that two gigatons is still rather mysterious,” Archer said.