Is Ocean Acidification Causing the Arctic To Melt?

The discovery highlights a dual danger to the survival of plants, shellfish, coral reefs, other marine species, and the climate.

After finding that the western Arctic Ocean’s acidity levels are rising three to four times faster than other ocean waters, an international team of scientists has sounded new alarm bells about the changing chemistry of the ocean.

The team, which includes Wei-Jun Cai of the University of Delaware, found a strong correlation between the rate of ocean acidification and the accelerated rate of ice melting in the region. This is a dangerous combination that puts the survival of plants, shellfish, coral reefs, other marine life, and other biological processes throughout the planet’s ecosystem at risk.

The new study, published in the prestigious journal Science, is the first to analyze Arctic acidification data covering more than two decades, from 1994 to 2020.

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Researchers, including the University of Delaware’s Zhangxian Ouyang, traveled aboard the icebreaker R/V Xue Long into an active melting zone in the Arctic Ocean to get samples for analysis. Credit: Zhangxian Ouyang, Wei-Jun Cai, and Liza Wright-Fairbanks/ University of Delaware

Arctic sea ice in this region is expected to disappear by 2050, if not sooner due to the region’s increasingly warm summers. Without a persistent ice cover to slow or otherwise mitigate the advance, the ocean’s chemistry will become more acidic as a consequence of this sea-ice retreat each summer.

This poses serious risks to the extremely diversified population of marine animals, plants, and other living things that rely on a healthy ocean for existence. Crabs, for example, live in a crusty shell made of calcium carbonate, which is abundant in ocean water. Polar bears depend on healthy fish populations for food, fish and sea birds rely on plankton and plants, and seafood is an important part of many people’s diets.

That makes the acidification of these distant waters a big deal for many of the planet’s inhabitants.

Scientists collect samples on the ice in the Arctic. Credit: Zhangxian Ouyang, Wei-Jun Cai, and Liza Wright-Fairbanks/ University of Delaware

First, a quick refresher course on pH levels, which indicates how acidic or alkaline a given liquid is. Any liquid that contains water can be characterized by its pH level, which ranges from 0 to 14, with pure water considered neutral with a pH of 7. All levels lower than 7 are acidic, and all levels greater than 7 are basic or alkaline, with each full step representing a tenfold difference in the hydrogen ion concentration. Examples on the acidic side include battery acid, which checks in at 0 pH, gastric acid (1), black coffee (5), and milk (6.5). Tilting toward basic are blood (7.4), baking soda (9.5), ammonia (11), and drain cleaner (14). Seawater is normally alkaline, with a pH value of around 8.1.

Cai, the Mary A.S. Lighthipe Professor in the School of Marine Science and Policy in UD’s College of Earth, Ocean, and Environment, has published significant research on the changing chemistry of the planet’s oceans and this month completed a cruise from Nova Scotia to Florida, serving as the chief scientist among 27 aboard the research vessel. The work, supported by the National Oceanic and Atmospheric Administration (NOAA), includes four areas of study: The East Coast, the Gulf of Mexico, the Pacific Coast, and the Alaska/Arctic region.

The new study in Science included UD postdoctoral researcher Zhangxian Ouyang, who participated in a recent voyage to collect data in the Chukchi Sea and Canada Basin in the Arctic Ocean.

The first author of the publication was Di Qi, who works with Chinese research institutes in Xiamen and Qingdao. Also collaborating on this publication were scientists from Seattle, Sweden, Russia, and six other Chinese research sites.

“You can’t just go by yourself,” Cai said. “This international collaboration is very important for collecting long-term data over a large area in the remote ocean. In recent years, we have also collaborated with Japanese scientists as accessing the Arctic water was even harder in the past three years due to COVID-19. And we always have European scientists participating.”

Cai said he and Qi both were baffled when they first reviewed the Arctic data together during a conference in Shanghai. The acidity of the water was increasing three to four times faster than in ocean waters elsewhere.

That was stunning indeed. But why was it happening?

Cai soon identified a prime suspect: the increased melt of sea ice during the Arctic’s summer season.

Historically, the Arctic’s sea ice has melted in shallow marginal regions during the summer seasons. That started to change in the 1980s, Cai said, but waxed and waned periodically. In the past 15 years, the ice melt has accelerated, advancing into the deep basin in the north.

For a while, scientists thought the melting ice could provide a promising “carbon sink,” where carbon dioxide from the atmosphere would be sucked into the cold, carbon-hungry waters that had been hidden under the ice. That cold water would hold more carbon dioxide than warmer waters could and might help to offset the effects of increased carbon dioxide elsewhere in the atmosphere.

When Cai first studied the Arctic Ocean in 2008, he saw that the ice had melted beyond the Chukchi Sea in the northwest corner of the region, all the way to the Canada Basin — far beyond its typical range. He and his collaborators found that the fresh meltwater did not mix into deeper waters, which would have diluted the carbon dioxide. Instead, the surface water soaked up the carbon dioxide until it reached about the same levels as in the atmosphere and then stopped collecting it. They reported this result in a paper in Science in 2010.

That would also change the pH level of the Arctic waters, they knew, reducing the alkaline levels of the seawater and reducing its ability to resist acidification. But how much? And how soon? It took them another decade to collect enough data to derive a sound conclusion on the long-term acidification trend.

Analyzing data gathered from 1994 to 2020 – the first time such a long-term perspective was possible — Cai, Qi, and their collaborators found an extraordinary increase in acidification and a strong correlation with the increasing rate of melting ice.

They point to sea-ice melt as the key mechanism to explain this rapid pH decrease because it changes the physics and chemistry of the surface water in three primary ways:

• The water under the sea ice, which had a deficit of carbon dioxide, now is exposed to atmospheric carbon dioxide and can take up carbon dioxide freely.
• The seawater mixed with meltwater is light and cannot mix easily into deeper waters, which means the carbon dioxide taken from the atmosphere is concentrated at the surface.
• The meltwater dilutes the carbonate ion concentration in the seawater, weakening its ability to neutralize the carbon dioxide into bicarbonate and rapidly decreasing ocean pH.

Cai said more research is required to further refine the above mechanism and better predict future changes, but the data so far show again the far-reaching ripple effects of climate change.

“If all of the multiple-year ice is replaced by first-year ice, then there will be lower alkalinity and lower buffer capacity and acidification continues,” he said. “By 2050, we think all of the ice will be gone in the summer. Some papers predict that will happen by 2030. And if we follow the current trend for 20 more years, the summer acidification will be really, really strong.”

No one knows exactly what that will do to the creatures and plants and other living things that depend on healthy ocean waters.

“How will this affect the biology there?” Cai asked. “That is why this is important.”

Reference: “Climate change drives rapid decadal acidification in the Arctic Ocean from 1994 to 2020” by Di Qi, Zhangxian Ouyang, Liqi Chen, Yingxu Wu, Ruibo Lei, Baoshan Chen, Richard A. Feely, Leif G. Anderson, Wenli Zhong, Hongmei Lin, Alexander Polukhin, Yixing Zhang, Yongli Zhang, Haibo Bi, Xinyu Lin, Yiming Luo, Yanpei Zhuang, Jianfeng He, Jianfang Chen and Wei-Jun Cai, 29 September 2022, Science.
DOI: 10.1126/science.abo0383