It's particularly bad in the far north: The rising carbon dioxide levels in the atmosphere are leading to significantly more intense acidification in the western Arctic Ocean than in other marine regions, researchers report. The strong trend is due to the loss of sea ice in the region, according to data analysis from 1994 to 2020. Apparently, the increasingly exposed water absorbs the acidifying carbon dioxide particularly intensively. The researchers fear that this could affect the ecosystems in the Arctic Ocean in complex ways.
The principle is familiar from the soda bottle – dissolved carbon dioxide makes water acidic. The same applies on a global scale: CO₂ from the atmosphere passes into the surface water of the oceans and causes the pH value to drop. The more gas there is in the air, the more intense both effects are. In this way, the world's oceans swallowed large amounts of the man-made greenhouse gas, but their water became more acidic in a problematic way. Studies in recent years have shown that falling pH levels can harm marine life. Above all, organisms are affected whose structures consist of calcium compounds. In more acidic water, mussels, corals or tiny crabs and diatoms have problems building their structures.
Arctic waters are particularly affected
It is already known that the extent and rate of increase in ocean acidification differs significantly between the world's marine regions. This has to do with local conditions. An international research team is now focusing on the waters of the Arctic. As part of their study, they evaluated water analysis data collected on a total of 47 research trips between 1994 and 2020 in the western Arctic Ocean. They made it possible to estimate the changes in seawater pH and the saturation state of the calcium carbonate mineral aragonite. These are the two central parameters when studying ocean acidification.
As the team reports, they found a very pronounced acidification trend in the case of the western Arctic Ocean. On average, acidity is increasing there at a rate three to four times higher than in other ocean regions, the scientists write. According to them, the study results also reflect what this peculiarity is due to: the increasing loss of sea ice cover as a result of climate change. This is reflected in a correlation between changes in acidity in seawater and fluctuations in annual ice cover over the study period.
Ice melt factor
As the researchers explain, in the past the Arctic sea ice mostly only melted in marginal areas during the summer season. This began to change significantly from the 1980s. While the magnitude is fluctuating, there has been a strong trend of ice melt, particularly over the past 15 years, reaching ever-larger parts of the Arctic Ocean. In the analysis results of the long-term observation, the connection between ice melt and acidification is now becoming apparent, the scientists report.
As their models suggest, three factors play a role in the particularly intensive acidification in Arctic water as the ice melts: The surface water freed from the cover has a high absorption capacity for atmospheric carbon dioxide and can therefore absorb more of the gas. In addition, the meltwater diluted the carbonate ion concentration in seawater, weakening its ability to neutralize carbon dioxide into bicarbonate. As a result, the pH value drops more, the researchers explain. In addition, the seawater mixed with meltwater is comparatively light, so it stays on the surface and therefore hardly mixes with deep water. This results in a comparatively low distribution of the dissolved carbon dioxide and its acidifying effect.
According to the team, more research is now needed to further investigate the mechanisms. But the data so far are already highlighting the complex and far-reaching effects of climate change. If sea ice in the western Arctic continues to shrink, acidification could continue and intensify over the next few decades, the researchers point out. "When all of the multiyear ice is replaced by one-year ice, alkalinity and buffering capacity decrease and acidification continues," says senior author Wei-Jun Cai of the University of Delaware. “We believe that by 2050 all summer ice will be gone. Some reports predict this as early as 2030. And if we continue the current trend for another 20 years, summer acidification will be very strong. But how will that affect the biology there?” asks Cai. So far it has not been possible to say exactly, but critical effects for living beings that are adapted to certain water values are to be expected, and thus far-reaching ecological consequences.
Source: University of Delaware, Article: Science, doi: Science, doi: 10.1126/science.abo0383