Washington, Feb 17 : Scientists are studying 55 million year old global warming events to find clues to what the future might hold if carbon dioxide emissions from human activities continue at the same rate.
For the research work, a team at the University of California, Santa Cruz, US, is studying the PETM (Paleocene-Eocene Thermal Maximum).
PETM refers to an extreme period of global warming 55 million years ago.
According to James Zachos, professor of Earth and planetary sciences at the University of California, "All the evidence points to a massive release of carbon at the PETM, and if you compare it with the projections for anthropogenic carbon emissions, it's roughly the same amount of carbon."
"The difference is the rate at which it was released. We're on track to do in a few hundred years what may have taken a few thousand years back then," he added.
Zachos and his collaborators have been studying marine sediments deposited on the deep ocean floor during the PETM and recovered in sediment cores by the International Ocean Drilling Program.
The ocean has the capacity to absorb huge amounts of carbon dioxide from the atmosphere. But as carbon dioxide dissolves in the ocean, it makes the water more acidic. That, in turn, could make life more difficult for corals and other marine organisms that build shells and skeletons out of calcium carbonate.
Technically, the "acidification" is a lowering of the pH of ocean water, moving it closer to the acidic range of the pH scale, although it remains slightly alkaline.
Lowering the pH affects the chemical equilibrium of the ocean with respect to calcium carbonate, reducing the concentration of carbonate ions and making it harder for organisms to build and maintain structures of calcium carbonate.
Corals and some other marine organisms use a form of calcium carbonate called aragonite, which dissolves first, while many others build shells of a more resistant form called calcite. "As the carbonate concentration starts to decrease, it becomes harder for some organisms to build their shells," said Zachos.
"If you lower the carbonate concentration enough, corals and eventually even calcite shells start to dissolve," he explained.
The effect of ocean acidification on the chemistry of calcium carbonate is reflected in the sediment cores from the PETM.
Marine sediments are typically rich in calcium carbonate from the shells of marine organisms that sink to the seafloor after they die. Sediments deposited at the start of the PETM, however, show an abrupt transition from carbonate-rich ooze to a dark-red clay layer in which the carbonate shells are completely gone.
"We are adding all this carbon dioxide in less than one mixing cycle. That's important for how the ocean buffers itself, and it means the carbonate concentration in surface waters will get low enough to affect corals and other organisms, assuming emissions continue on the current trajectory," said Zachos.