Greater acidification in summer threatens Arctic Ocean

Over the past 200 years, our planet’s oceans have absorbed more than a quarter of all anthropogenic carbon dioxide from the atmosphere. As a result, their acidity has increased by nearly 30 percent since the beginning of the Industrial Revolution. In this regard, the water’s pH value isn’t constant; it varies both seasonally and regionally. The lowest values naturally occur in winter. 

But that could soon change, since they could be shifted to the summer by climate change, as an international team including experts from the Alfred Wegener Institute recently demonstrated. If this comes to pass, it could have far-reaching consequences for life in the ocean, as they report in the journal Nature.

The biological activity of marine organisms normally reaches its zenith in summer, as the season is normally characterized by optimal conditions for survival, finding food and reproducing. 

However, climate change is now threatening this status quo by shifting the point at which the pH values are lowest from winter to summer, as experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and France’s Laboratory for Sciences of Climate and Environment (LSCE, part of the CEA); LOCEAN – Laboratory of Oceanography and Climatology; and Institute Pierre-Simon Laplace (IPSL) recently determined. 

In a new study, they conclude that summer acidification could intensify by up to 25 percent by the end of the century. Some organisms living in the Arctic Ocean would be severely affected by this change, reducing their ability to cope with more intensive summertime warming.

This seasonal shift is due to the more intense rise in CO2 in warmer water. In summer, atmospheric temperatures rise in the Arctic, more sea ice melts, and the surface water grows warmer. This warming process becomes so intense that the acidification of the seawater increases much more rapidly and can no longer be compensated for by the photosynthesis of marine algae. 

“These new findings spell trouble for some types of Arctic fish, like the polar cod, which are already threatened by climate change,” says co-author Hans-Otto Pörtner, a biologist and climate researcher at the AWI. “The projected high temperatures will push Arctic fauna to their thermal limits or even beyond, particularly with regard to life stages in which they’re more fragile.” 

First author James Orr from the LSCE and IPSL adds: “Who would have thought that climate change could shift the time of maximum acidification by six months, when studies on seasonal biological rhythms only projected shifts of up to roughly a month?” 

“The fascinating thing about this study is the fact that the chemical winters will actually become chemical summers,” says Lester Kwiatkowski, a co-author from LOCEAN and the IPSL.

In their study, the experts analyzed simulations of 27 Earth system models and prepared future climate scenarios. While doing so, for the first time they assessed the potential for seasonal shifts in acidification, including all related variables. 

Why? Because the degree of acidification isn’t determined by just one factor; it’s a complex interplay of physical and biological processes, influenced by the more intense warming of surface water in summer. These changes were more pronounced in scenarios with moderate and high greenhouse-gas emissions and were far milder in those with lower emissions. In the researchers’ eyes, this represents a glimmer of hope that key elements of the Arctic Ocean’s ecosystem can be preserved if mean global warming can be kept below 2°C.

Photo credit: Flemming Dahlke. A school of polar cod in the laboratory tank.

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