Oceans turned more acidic during a period of great warming some 56 million years ago, leading to an extinction of bottom-dwelling marine species known as foraminifera, a scenario that could be repeated as a result of human-induced global warming today, only much more quickly.
To better understand what could happen in the near future, geochemist Bärbel Hönisch of the Lamont-Doherty Earth Observatory and a few colleagues studied ocean acidity events over the past 250 million years to confirm whether acidification took place during periods of global warming. The conditions of one period in particular – the Paleocene-Eocene Thermal Maximum — were similar to the worst-case scenarios scientists project for future climate change.
“The earlier changes affected the biology, so we’d expect that would happen today,” Hönisch said. She doesn’t like to speculate on precisely what the impact might be; some creatures may disappear, others may adapt.
“I don’t think it will destroy the earth — the earth will cope with us,” she said. “We may not like what happens.” Hönisch gave a talk on her research today at the fall meeting of the American Geophysical Union in San Francisco, as part of a broader discussion there of findings related to ocean acidification.
Increased amounts of carbon dioxide in the atmosphere coming from human sources, such as the burning of fossil fuels, have increased the amount of carbon absorbed by the oceans. That in turn is lowering the pH – and acidity – of seawaters. The changing sea chemistry is tied to a rapid decline of corals around the world and could affect many other organisms that rely on the right balance of calcium carbonate in the water to form their protective shells.
There have been several major extinctions through the course of time, and the researchers looked for evidence for elevated levels of carbon dioxide, global warming and increased saturation of calcium carbonate in seawater – all of which are happening today. The period Hönisch and colleagues honed in on happened during the Paleocene-Eocene Thermal Maximum, when temperatures rose 4-9 degrees C across the globe.
They looked at the shells of the tiny foraminifera that lived on both the surface and the sea bottom, found in sediment cores dug up from different oceans. The shells contain two boron isotopes; the ratio between them varies depending on the water’s acidity. By measuring the ratio, she could determine the pH.
What they found was that over 5,000 years, the pH dropped by about .4, meaning it became more acidic.
Over the past 100 years, she noted, ocean pH dropped by .1, and some estimate it will drop .3 more by end of century – an equivalent change over a much shorter time.
“What we have today is much faster than anything we’ve seen in the past,” Hönisch said.
The work grew out of a workshop on ocean acidification held on Catalina Island in California last year. She worked with colleagues Andy Ridgwell of the University of Bristol, Jim Zachos of the University of California- Santa Cruz, and Ellen Thomas at Yale University.
Hönisch, also an assistant professor in the Columbia University Department of Earth and Environmental Sciences, said she wants to do more experiments to refine the pH estimates, study cores from other sites and measure how fast the changes in sea chemistry happened.
In a paper published in 2009, Hönisch found that seawater is more acidic now than it has been for at least two million years, and that carbon dioxide levels are also currently at their highest since then.