Current ocean acidification may be fastest in 300 million years

Oceans get more acidic when more carbon gets into the atmosphere. Human activities, including the burning of fossil fuels, have increased the level of atmospheric carbon dioxide (CO₂) from about 280 parts per million (ppm) at the start of the industrial revolution to 392 ppm now. Carbon dioxide is one of several heat-trapping gases that contribute to global warming.

Researchers at the US National Oceanic and Atmospheric Administration viewed the 5,000-year hot spell 56 million years ago, likely due to factors such as massive volcanism, as the closest parallel to current conditions at any time in the last 300 million years. During that span, the amount of CO₂ in the atmosphere doubled and average temperatures rose by 6 degrees C. The oceans became more acidic by about 0.4 of a unit on the 14-point pH scale over that 5,000-year period, the researchers said.

That represents a rapid rate of warming and acidification, but is small compared to what has happened on Earth since the start of the industrial revolution. During the warming period 56 million years ago, acidification in each century was about 0.008 pH units. 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.

By contrast, in the 20th century, oceans acidified by 0.1 pH unit, and are projected to get more acidic at the rate of 0.2 or 0.3 pH units by the year 2100, according to the study. “Given that the rate of change was an order of magnitude smaller 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,” commented the author of the study, Baerbel Hoenisch of Columbia University.

Source: PlanetArk/Reuters, 2 March 2012