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 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. 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 United States, the United Kingdom, the Netherlands, Germany and Spain 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. 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 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 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 Baerbel Hoenisch of Columbia University's Lamont-Doherty Earth Observatory said by telephone.
