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Rising levels of carbon dioxide in the atmosphere are a major contributor to climate change, and now a new study has confirmed that atmospheric CO2 is also affecting the ocean chemistry, potentially threatening marine life.

Montana State University scientist Robert Dore has been taking samples of water in the Pacific Ocean for almost two decades.

"We're sailing out of Honolulu harbor. We're in the harbor right now and just about to break away from the dock."

I reached Prof. Dore on board the research vessel Kilo Moana, about to leave for a point in the Pacific known as Station Aloha, where he has been studying the ocean water since the late 1980s.

"We've been going to the same spot in the Pacific Ocean, and we've been measuring a whole suite of different chemical, biological, physical measurements to try and characterize long-term change in the open ocean environment. And one of the key things that we measure is CO2 levels. And we've been able to document this progressive invasion of atmospheric CO2 into the ocean."

Scientists expected that as atmospheric CO2 increased, more and more of the carbon dioxide would be absorbed into the ocean, affecting the chemical balance of the seawater, with a potentially harmful impact on shellfish and coral in particular.

"As carbon dioxide dissolves in the water, or seawater in this case, it forms a weak acid, carbonic acid," Dore explains. "And therefore, as the concentration of CO2 in the atmosphere goes up and that exchanges with the surface seawater, it drives the pH down, makes it more acidic."

The seawater samples Dore and his colleagues have analyzed confirm what the theory predicts.

"The pH of the ocean out here has been decreasing. And it has been decreasing at pretty much the rate that we'd expect from the physics and the chemistry."

The lower pH levels - measuring increased acidification - varied seasonally and also from year to year. It also varied with depth.

The effect was particular striking at about 250 meters down, and again at 500 meters. Dore and his colleagues came up with two possible explanations. It could be that surface water picked up CO2 and then moved to those depths. Or there could be a biological explanation.

"So you can think of it as algae growing at the surface, taking up CO2, [they] die, sink, bacteria eat them. CO2 comes back out at some deeper depth. It's just a way of actually transporting the signal [measured CO2] to depth more quickly than just mixing."

Dore's laboratory measurements are not just equations and charts in his paper. He says they also have a real-world impact.

"It's important to realize that acidification of the oceans is really happening. And it can have negative impacts on a whole variety of marine life from fisheries to coral reefs. It's potentially catastrophic."

Montana State University environmental scientist John Dore's paper appears this week in the Proceedings of the National Academy of Sciences.

carbon dioxide: 二氧化碳

atmospheric: 大氣中的

algae: 海藻

coral reef: 珊瑚礁