Looking back at that bygone warm period in Earth's history could offer help in forecasting the impact of human-spurred climate change, researchers said of a review of hundreds of studies of ancient climate records published in the journal Science.
Quickly acidifying seawater eats away at coral reefs, which provide habitat for other animals and plants, and makes it harder for mussels and oysters to form protective shells. It can also interfere with small organisms that feed commercial fish like salmon.
The phenomenon has been a top concern of Jane Lubchenco, the head of the US National Oceanic and Atmospheric Administration, who has conducted demonstrations about acidification during hearings in the US Congress.
Oceans get more acidic when more carbon gets into the atmosphere. In pre-industrial times, that occurred periodically in natural pulses of carbon that also pushed up global temperatures, the scientists wrote in the study released on 1 March.
Human activities, including the burning of fossil fuels, have increased the level of atmospheric carbon to 392 parts per million from about 280 parts per million at the start of the industrial revolution. Carbon dioxide is one of several heat-trapping gases that contribute to global warming.
To figure out what ocean acidification might have done in the prehistoric past, 21 researchers from the the United Kingdom, the Netherlands, Germany, Spain and the United States reviewed studies of the geological record going back 300 million years, looking for signs of climate disruption.
Those indications of climate change included mass extinction events, where substantial percentages of living things on Earth died off, such as the giant asteroid strike thought to have killed the dinosaurs some 65 million years ago.
The events that seemed similar to what is happening now included mass extinctions about 252 million and 201 million years ago, as well as the warming period 56 million years in the past.
The researchers reckoned the 5,000-year hot spell 56 million years ago, likely due to factors like massive volcanism, was the closest parallel to current conditions at any time in the 300 million years.
Scratching the surface
To detect that, they looked at a layer of brown mud buried under the Southern Ocean off Antarctica. Sandwiched between layers of white plankton fossils, the brown mud indicated an ocean so acidic that the plankton fossils from that particular 5,000-year period dissolved into muck.
During that span, the amount of carbon in the atmosphere doubled and average temperatures rose by 6 degrees Celsius, the researchers said. The oceans became more acidic by about 0.4 unit on the 14-point pH scale over that 5,000-year period, the researchers said.
That is a fast warm-up and a quick acidification, but it is small compared with what has happened on Earth since the start of the industrial revolution some 150 years ago, study author Bärbel Hönisch of Columbia University's Lamont-Doherty Earth Observatory said by telephone.
During the warming period 56 million years ago, known as the Paleocene-Eocene Thermal Maximum, or PETM, and occurring about 9 million years after the extinction of the dinosaurs, acidification for each century was about .008 unit on the pH scale, Hönisch said.
Back then, many corals went extinct, as did many types of single-celled organisms that lived on the sea floor, which suggests other plants and animals higher on the food chain died out too, researchers said.
By contrast, in the 20th century, oceans acidified by 0.1 unit of pH, and are projected to get more acidic at the rate of 0.2 or 0.3 pH by the year 2100, according to the study.
The UN Intergovernmental Panel on Climate Change projects world temperatures could rise by 1.8 to 4 degrees Celsius this century.
"Given that the rate of change was an order of magnitude smaller [in the PETM] compared to what we're doing today, and still there were these big ecosystem changes, that gives us concern for what is going to happen in the future," Hönisch said.
Those skeptical of human-caused climate change often point to past warming periods caused by natural events as evidence that the current warming trend is not a result of human activities. Hönisch noted that natural causes such as massive volcanism were probably responsible for the PETM.
She said, however, that the rate of warming and acidification was much more gradual then, over the course of five millennia compared with one century.