The Arctic is suffering so many consequences related to climate change, it's hard to know where to begin anymore. It's warming more rapidly than almost any other part of the planet; its glaciers are melting and its sea ice is retreating; and its most iconic wildlife, including polar bears and walruses, are suffering.
But that's not all – a new study, just out Monday in the journal Nature Climate Change, indicates that the Arctic Ocean is also becoming more acidic, another consequence caused by greenhouse gases in the atmosphere. It's a process that occurs when carbon dioxide dissolves out of the air and into the sea, lowering the water's pH in the process.
Scientists believe acidification is occurring at varying rates all over the world. But this week's study gives researchers renewed cause to worry about the Arctic, suggesting that a large – and increasing – swath of the ocean may have reached a level that's dangerous for some marine organisms.
The new research focuses on the water concentrations of a mineral called aragonite, which is a form of calcium carbonate, a chemical compound that plankton, shellfish and even deep-sea corals use to build their hard outer shells. When ocean water becomes more acidic, chemical reactions occur that impede the formation of calcium carbonate and lower its concentration in the water, which can be a major threat for these marine animals.
These aragonite levels are a "very important parameter" which can be an indicator of how much carbon dioxide is dissolving into the sea, according to Liqi Chen, a scientist with China's State Oceanic Administration and a co-author on the new study.
By analyzing data collected from the ocean during expeditions between 1994 and 2010, the scientists have found that some parts of the western Arctic Ocean are undersaturated with aragonite – in other words, their concentrations are lower than they could be. And these areas have expanded more than sixfold since the 1990s.
In 1994, the data suggested that about 5 percent of the water between 70 and 90 degrees north latitude was undersaturated with aragonite. Most of this undersaturated area occurred above a depth of 125 meters, or about 400 feet, and stopped below 80 degrees north latitude. But additional surveys, conducted in 1998, 2005, 2008 and 2010, show that these undersaturated areas have crept farther north and extended deeper into the ocean in the years since.
By 2010, the undersaturated area had expanded from 5 percent to about 31 percent of the water column, the study suggests. And the scientists found undersaturation at up to 250 meters deep and in locations above 85 degrees North latitude. In other words, the acidifying zone had expanded both northward into the Arctic Circle and deeper into the water. The researchers point out that the levels of aragonite in these areas are below the point scientists believe is a threat to marine organisms.
The continued emission of greenhouse gases into the atmosphere is an obvious reason this expansion is happening. But there are a number of other factors that may be helping it along, the researchers point out.
For one thing, carbon dioxide tends to dissolve more easily in cold water. As the Arctic continues to warm, melting both from floating sea ice and from glaciers on the Greenland ice sheet provides an influx of cold, fresh water to the ocean, which may make acidification easier. Additionally, the less sea ice there is on the surface of the ocean, the more liquid water is exposed to carbon dioxide in the atmosphere.
Recent research has also shown that Pacific Ocean water is increasingly intruding into the Arctic. The researchers note that this Pacific water has certain chemical properties – for instance, lower salt content and a higher level of dissolved, carbon-rich organic material – that may be contributing to the increase in acidification. The reasons for this are complex, Chen told The Washington Post, and have to do with recent changes in northern ocean currents.
Pacific water naturally streams into the Arctic by way of the Bering Strait and then flows eastward through the Beaufort Sea, around Greenland and back out again. But in recent years, the researchers note, there's been an increase in the flow coming through the Bering Strait and a decrease in the eastward current that flows through the Beaufort Sea, thus causing an accumulation of Pacific water in certain parts of the Arctic, particularly the Chukchi Sea. The reasons for this phenomenon are unclear, but the researchers suggest that it may have something to do with recent changes in wind patterns that are preventing the Pacific water from flowing through as it used to.
The researchers also point to recent changes in a swirling ocean current north of Greenland known as the Beaufort Gyre. This current is capable of swirling in either a clockwise or counterclockwise direction, and historically it's tended to switch back and forth between the two regimes every five to seven years. Over the past two decades, though, the researchers note that the gyre has settled into a mostly clockwise pattern, which has caused the current to strengthen, expand and carry Pacific water to a greater and deeper area of the sea.
With all these conditions in mind, the researchers decided to conduct a model simulation to see how Arctic acidification might progress in the future.
"Models indicate that sea ice will continue to decrease and the prediction is that the Arctic Ocean may be ice-free in the summer by 2030," they write. If this occurs, their projections suggest that the entire surface of the Arctic Ocean, up to about 30 meters deep, may be undersaturated with aragonite within two decades. And given the rate of expansion they've observed since 1994, they suggest that the entire western Arctic Ocean – up to 250 meters deep – could also become undersaturated within a few decades.
Some of these trends may depend on future changes in Arctic currents, such as the Beaufort Gyre. But these uncertainties aside, the study highlights the interconnected nature of climate consequences in the Arctic – the way that greenhouse gas emissions, rising temperatures, ice melt and ocean acidification are all linked and help to reinforce one another. And it points to yet another example of a climate effect that's not just a concern for the future, but is already an issue – and a growing one – today.