By Deborah Zabarenko
WASHINGTON (Reuters) - The world's oceans are turning acidic at what could be the fastest pace of any time in the past 300 million years, even more rapidly than during a monster emission of planet-warming carbon 56 million years ago, scientists said on Thursday.
Looking back at this bygone warm period in Earth's history could offer help in forecasting the impact of human-spurred climate change, researchers said.
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 U.S. National Oceanic and Atmospheric Administration, who has conducted demonstrations about acidification during hearings in the U.S. Congress.
Oceans get more acidic when more carbon gets into the atmosphere. In pre-industrial times, this occurred periodically in natural pulses of carbon that also pushed up global temperatures, the scientists wrote in the journal Science.
Human activities, including the burning of fossil fuels, have increased the level of atmospheric carbon from about 280 parts per million at the start of the industrial revolution to 392 parts per million now. Carbon dioxide is one of several heat-trapping gases that contribute to global warming.
The researchers viewed the 5,000-year hot spell 56 million years ago, likely due to factors like massive volcanism, as the closest parallel to current conditions at any time in the 300 million years.
During that span, the amount of carbon in the atmosphere doubled and average temperatures rose by 10.8 degrees F (6 degrees C), 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's a fast warm-up and a quick acidification, but it's small compared to what has happened on Earth since the start of the industrial revolution some 150 years ago, study author Baerbel Hoenisch of Columbia University's Lamont-Doherty Earth Observatory said by telephone.
EXTINCTIONS ON THE SEAFLOOR
During the warming period 56 million years ago, known as the Paleocene-Eocene Thermal Maximum (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, Hoenisch said.
Back then, many corals went extinct, as did many types of single-celled organisms that lived on the sea floor, which suggests that other plants and animals higher on the food chain died out too, researchers said.
By contrast, in the 20th century, oceans acidified by .1 unit of pH, and are projected to get more acidic at the rate of .2 or .3 pH by the year 2100, according to the study.
The U.N. Intergovernmental Panel on Climate Change projects world temperatures could rise by 3.2 to 7 degrees F (1.8 to 4 degrees C) 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," Hoenisch 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. Hoenisch noted that natural causes such as massive volcanism were probably responsible for the PETM.
However, she said the rate of warming and acidification was much more gradual then, over the course of five millennia compared with one century.
Richard Feely, an oceanographer at the U.S. National Oceanic and Atmospheric Administration who was not involved in the study, said looking at this distant past is a good way to foresee the future.
"These studies give you a sense of the timing involved in past ocean acidification events - they did not happen quickly," Feely said in a statement. "The decisions we make over the next few decades could have significant implications on a geologic timescale."
(Editing by Will Dunham)