PARIS – Scientists unveiled the first conclusive evidence Wednesday that growing ocean acidity caused by global warming is already stifling growth of vital coral reefs.
The decline of shallow-water corals, home to a quarter of the ocean’s species and a lifeline for a billion people, has long been in evidence.
Earlier studies had shown that the rate at which living coral reefs calcify, or accumulate mass, had dropped by about 40 percent in just over 30 years.
Up to now, however, it was not possible to tease out the impact of acidification from other threats such as pollution, overfishing and warming water.
The world’s oceans are 26 percent more acidic today than at the start of the Industrial Revolution, when mankind started massively burning fossil fuels which give off harmful carbon dioxide (CO₂).
About a quarter of the CO₂ is absorbed by the oceans, changing their chemical composition, and making the water more acidic and corrosive to corals and shellfish.
“Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth,” said Rebecca Albright, a researcher at the Carnegie Institution for Science in Stanford, California.
“This is no longer a fear for the future. It is the reality of today.”
The findings were published in the science journal Nature.
Albright and colleague Ken Caldeira led experiments on natural reefs off the coast of Australia’s One Tree Island, in the southern Great Barrier Reef.
Manipulating the chemistry of the seawater flowing over the flat reef, the researchers restored its pH — the balance between alkalinity and acidity — to what it would have been without climate change.
As suspected, the corals became better able to build themselves up.
“By turning back time in this way, they demonstrate that — all things being equal — net coral-reef calcification would have been around 7 percent higher than at present,” Janice Lough, a scientist at the Australian Institute of Marine Science, noted in a comment, also published in Nature.
The novelty of the experiment, she said, is that it “restored the ocean chemistry of a natural reef to that of pre-industrial times, thus factoring out other potentially confounding factors, such as temperature.”
Some researchers have proposed artificially reducing the acidity of ocean water around coral reefs — a form of geoengineering — as a means of preserving shallow marine ecosystems.
But even if the experiments underlying the study did exactly that, implementing such a scheme on the required scale would be nigh impossible, the authors caution.
“The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions,” Caldeira said.
“If we don’t take action on this issue very rapidly, coral reefs — and everything that depends on them, including both wildlife and local communities — will not survive into the next century.”
In a separate study released Wednesday, researchers said the oceans are rising at a faster rate than any time in the past 2,800 years, and might even have fallen without the influence of human-driven climate change.
Sea levels rose globally by about 5.5 inches (14 centimeters) from 1900 to 2000, said the study led by Rutgers University, published in Proceedings of the National Academy of Sciences.
In the absence of global warming, the change in sea level would have been far less — ranging between a 1.2-inch (3-centimeter) drop in the last century, to a rise of about 2.8 inches (7 centimeters).
“The 20th century rise was extraordinary in the context of the last three millennia — and the rise over the last two decades has been even faster,” said lead author Robert Kopp, an associate professor in Rutgers University’s Department of Earth and Planetary Sciences.
The study also predicted that global sea level will rise by 1.7 to 4.3 feet (50 to 130 centimeters) in the 21st century if the world continues to rely heavily upon fossil fuels.
Even if fossil fuels were phased out, the seas would likely mount between 0.8 and 2 feet by century’s end, it said.
The average global temperature today is about 1.8 degrees Fahrenheit (1 degree Celsius) higher than it was in the late 19th century.
The Rutgers-led study — with co-authors from Harvard University, the Woods Hole Oceanographic Institution and the Potsdam Institute for Climate Impact Research in Germany — was based on a database that included records from 24 locations around the world, and 66 tide gauge records from the last 300 years.
Scientists say the planet is incredibly sensitive to small changes in temperature, with today’s average climate about 1.8 degrees Fahrenheit (1 degree Celsius) warmer than it was in the 19th century.
“During the past millennia sea-level has never risen nearly as fast as during the last century,” said co-author Stefan Rahmstorf, co-Chair of the Potsdam-Institute for Climate Impact Research’s (PIK) research domain Earth System Analysis.
“The new sea-level data confirm once again just how unusual the age of modern global warming due to our greenhouse gas emissions is — and they demonstrate that one of the most dangerous impacts of global warming, rising seas, is well under way.”
A second report issued to Monday by Climate Central found that without global warming, more than half of the 8,000 coastal nuisance floods seen in the United States since 1950 would not have occurred.