Melting sea ice is acidifying the Arctic Ocean

More carbon in the air means more acid in the seas.
Field work on and ice shelf with RV Xuelong II in 2020. Ruibo Lei; Di Qi Key Laboratory for Polar Science of the Ministry of Natural Resources, Polar Research Institute of China, Shanghai 200136, China

Climate change is changing the chemistry of the Earth’s oceans. The heat from increased carbon dioxide in the atmosphere is melting critical sea ice and causing ocean acidification, or a reduction in the pH of the ocean over an extended period of time.

Over the past 200 years, the pH of surface ocean waters has fallen by 0.1 pH units, a 30 percent increase in the ocean’s acidity. According to NOAA, seawater turns acidic when carbon dioxide (CO2) dissolves into ocean water to form carbonic acid. This weak acid then breaks into hydrogen ions and bicarbonate ions. There is currently more CO2 in the atmosphere due to human activities like burning fossil fuels, so there is more CO2 dissolving into the world’s oceans. As the ocean continues to absorb more carbon dioxide, the pH decreases and the ocean becomes more acidic.

Ocean acidification hurts sea life, particularly clams, oysters, scallops, mussels, and other fish with skeletons and shells made up of calcium carbonate. NOAA has even found evidence that ocean acidification can dissolve the shells of some sea snails.

However, acidification doesn’t occur in the same way across the whole planet—it is happening in some areas more than others. A study published today in the journal Science finds that acidification in the western Arctic Ocean is happening at a rate three to four times higher than in other ocean basins.

“Once that is diluted, its ability to neutralize the carbon dioxide drops,” Wei-Jun Cai, an oceanographer, marine chemist, and author of this paper in an interview, tells Popular Science. “2008 was the first time we really saw this in the Arctic Ocean and I was really shocked that the CO2 was so high in the surface water. Acidification can even make heavy metals in the water more toxic.”

They study suggests that the increase has been driven by the rapid loss of sea ice in the area, which has exposed the ocean water to the atmosphere where it can absorb more carbon dioxide. NASA estimates that the arctic is losing sea ice at a rate of 12.6 percent per decade do to global warming. According to Cai, in the 1980s and 1990s, carbon dioxide levels were lower in the Arctic Ocean and it was understood that ice melt was happening more in the Marginal Sea basin, and it advanced into other Arctic Ocean basins by 2007.

“You now have atmospheric contact in places where there wasn’t any,” Cai explains.

[Related: As oceans grow more acidic, they’re eating away at their protective floors.]

The team collected data from 47 Arctic research cruises from 1994 to 2020 to estimate changes in seawater pH. They additionally tested both the saturation state of the calcium carbonate and the mineral aragonite. These are both accurate measures of ocean acidification. Their results revealed a faster pace of acidification for the western Arctic and a surprising correlation of these two features with the regional decrease in sea ice over the past 26 years.

[Related: Inuit researchers are leading a scientific movement to understand life on the ice.]

The process could continue to intensify over the next few decades if sea ice continues to disappear from the western Arctic.

“This acidification will likely continue for the next two decades until all the ice melts. Other scientists have predicted that this will happen between 2030-2050,” Cai says. “Currently, the ice melt has slowed down compared to the year 2012 where we saw maximum melting, but we are not sure if it will continue to slow down. It could accelerate again.”

The next steps are to continue to dive deeper into this data to better understand acidification. Cai also added that this is a warning for lower latitude areas. The speed of ice melt in the Arctic can only get worse in other places.