Scientists find link between fast-melting Arctic ice and ocean acidification

An international team of researchers have sounded new alarm bells about the changing chemistry of the western region of the Arctic Ocean after discovering acidity levels increasing three to four times faster than ocean waters elsewhere.

The team, which includes University of Delaware marine chemistry expert Wei-Jun Cai, also identified a strong correlation between the accelerated rate of melting ice in the region and the rate of ocean acidification, a perilous combination that threatens the survival of plants, shellfish, coral reefs and other marine life and biological processes throughout the planet's ecosystem.

The new study, published on Thursday, Sept. 30 in Science, the flagship journal of the American Association for the Advancement of Science, is the first analysis of Arctic acidification that includes data from more than two decades, spanning the period from 1994 to 2020.

Scientists have predicted that by 2050 -- if not sooner -- Arctic sea ice in this region will no longer survive the increasingly warm summer seasons. As a result of this sea-ice retreat each summer, the ocean's chemistry will grow more acidic, with no persistent ice cover to slow or otherwise mitigate the advance.

That creates life-threatening problems for the enormously diverse population of sea creatures, plants and other living things that depend on a healthy ocean for survival. Crabs, for example, live in a crusty shell built from the calcium carbonate prevalent in ocean water. Polar bears rely on healthy fish populations for food, fish and sea birds rely on plankton and plants, and seafood is a key element of many humans' diets.

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

First, a quick refresher course on pH levels, which indicate 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, 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 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 on 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 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 the 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."

Read more at Science Daily

Rivers speeding up Arctic ice melt at alarming rate

Irina Panyushkina grew up in Siberia, near the Arctic Circle. She was raised on stories of explorers trudging through seas of ice to reach the North Pole.

Now, she is a climate scientist and associate research professor of dendrochronology in the University of Arizona Laboratory of Tree-Ring Research. And she is trying to understand how a warming world is transforming the place she once called home.

Someday, the Arctic Ocean may no longer host ice, since the northern regions of the world are warming are faster than the rest -- a trend scientists refer to as Arctic amplification. As Arctic ice melts, new opportunities and challenges for humans will arise, researchers say.

Freshwater flowing into the Arctic Ocean from the continent is thought to exacerbate Arctic amplification, but the extent of its impact isn't fully understood. New research led by Panyushkina measures how the flow of the Yenisei River -- the largest freshwater river that flows into the Arctic Ocean -- has changed over the last few hundred years, and describes the impact freshwater has had on the Arctic.

Previous studies have attributed recent changes in wintertime freshwater flow into the Arctic to warming air temperature, seasonal precipitation changes or snowpack. But more recent research, including Panyushkina's study, suggests that the primary driver is actually degradation of permafrost -- or frozen ground -- as well as forest fires across southern Siberia.

Panyushkina's research, funded by the National Science Foundation Polar Office, is published in the journal Environmental Research Letters.

What trees can tell us

Data collected by instruments at the upper reaches of the Yenisei River in Tuva, in southern Siberia, only goes back so far. To overcome this, Panyushkina and her team used tree-ring data to double the number of years' worth of the stream flow data they had, allowing them to look back 300 years.

Stream flow, or the amount of water that moves through a certain area of a river over time, can be inferred by measuring changing tree-ring thickness over the years. Measurements of stream flow over specific seasons can even be teased out of the data.

Annual stream flow information is commonly used by water managers to reveal the average changes in stream flow trends. But Panyushkina and her team did something novel when they decided to also investigate winter stream flow specifically.

"We found an unprecedented increase in the winter flow rate over the last 25 years," Panyushkina said. This winter flow rate is nearly 80% above the average seen over approximately 100 years.

"In contrast, annual flow fluctuated normally during the 300-year period, with only a 7% increase over the last 25 years," Panyushkina said.

The winter stream flow data revealed the role of permafrost melt on Arctic ice.

Since ice covers rivers during winter in Siberia, the team's stream flow measurements only captured information about river waterthat originated underground rather thanfrom the sky. That includes water from thawing permafrost, as well as water from sub-permafrost aquifers, as permafrost loss leads to an increased exchange of water between the river and aquifers. These two sources of groundwater are warm compared to the frigid air above, and when they eventually flow into the Arctic Ocean, they melt the ice.

An uncertain future

Forest fires are also thought to be a driver of Arctic ice melt.

"We know the frequency and intensity of forest fires in Siberia have been increasing," Panyushkina said. "When fires happen in forests with permafrost, there is deep thawing under the fire event, and the affected area often doesn't recover for up to 60 years. When we have large-scale fires and long-burning fires and more frequent fires, we're maybe hitting the critical point when permafrost degradation cannot return to normal. Forest fires are also another process that increases connectivity between aquifers and stream flow."

The combined effects of permafrost degradation and fires are very strong at the Yenisei River basin, with more fresh water and heat flowing into the Arctic Ocean in recent decades, according to the study. In turn, melting sea ice also exacerbates global warming.

"Research interest in the region is booming because the surface temperature is warming much faster here than anywhere else in the world," Panyushkina said. "It's a hot spot for climate research, and because I grew up there and understand how the system works, it's a natural topic of study for me. I'm also very interested in knowing the impact of an ice-free Arctic on the surrounding landscape. Humans have never seen an ice-free Arctic before, ever. My mind still cannot comprehend how the Arctic Ocean can be free of ice."

By the middle of the century, changing sea ice conditions are expected to lead to greater navigability for open water-vessels crossing the Arctic. A future trans-Arctic shipping route called the Supra Polar Route will link the Atlantic and Pacific Oceans through the Arctic, potentially paving the way for more trans-Arctic commerce.

There is a need to quantify the Arctic amplification impacts to manage and regulate Arctic seas of the future, Panyushkina said.

"This strong prospect of the global trade fleet entering the Arctic opens the Pandora's box of near-future geopolitical and environmental issues and reinforces the urgency for a new regulatory framework by international organizations to ensure adequate environmental protections and vessel safety standards," she said.

Read more at Science Daily