Now high-resolution eddy-resolving model simulations performed by Nicolas Gruber of ETH Zürich, Switzerland, and colleagues have projected that within the next 30 years the top 60 m of much of the nearshore region will be undersaturated with aragonite throughout the summer. As more carbon dioxide enters the ocean, the water’s pH decreases and carbonate chemistry changes; aragonite is a form of calcium carbonate that is vital for shell building.

“Ecosystems in this region, in particular those organisms and life stages that are sensitive to low-saturation states (and also other related changes in pH etc) will soon be seriously challenged,” Gruber told environmentalresearchweb. “This is bound to lead to changes in ecosystem structure.”

Gruber reckons that these changes will likely occur in the next 20 to 30 years “largely irrespective of whether we follow a low or a high emission pathway”. As he explained: “this is because the inertia in our fossil-fuel emission trajectory will bring us to atmospheric carbon-dioxide levels well above those today, even if we manage to implement relatively strict emission controls in the near future.”

By 2050 the team’s projections show that water with a saturation state of aragonite (Ωarag) greater than 1.5 will have largely disappeared from the California Current System, and more than half of the water will be undersaturated year-round. Values of Ωarag below one indicate that the ocean will dissolve aragonite while measurements greater than one indicate favourable conditions for forming shells and skeletons.

“Having measured ocean acidification for many years off the coast of Los Angeles, I was aware that eastern boundary upwelling systems would be rather prone to ocean acidification but I was not able to quantify the potential evolution into the future,” said Gruber. “The most important insight is how fast and how widespread the nearshore regions of the California Current Systems will become undersaturated with regard to the calcium-carbonate mineral aragonite, i.e., that this system will be crossing an important chemical threshold in the next 20 to 30 years.”

To obtain the projections, Gruber and co-workers from ETH Zürich, Princeton University, US, and the University of Bern, Switzerland, employed a modelling system developed for understanding the dynamics of the carbon cycle. “It was relatively straightforward to set up the runs,” said Gruber. “The challenge was the computational time, since these simulations took about six weeks to be completed. Given the high resolution of the model, which is clearly needed in order to simulate the nearshore evolution properly, the resulting amount of data proved to be another challenge.”

Upwelling regions tend to have a lower pH and lower aragonite saturation state since the waters coming up from the depths are rich in carbon dioxide from the remineralization of organic matter. Global ocean models have so far failed to recognize ocean acidification in Eastern Boundary Upwelling Systems, according to the researchers writing in Sciencexpress, because they do not have enough resolution to resolve the local dynamics responsible for bringing the waters with low pH and aragonite saturation to the surface.

It is hard to predict the impact of decreased carbonate saturation on wildlife but there are some indications that aragonite-secreting organisms such as pteropods or oysters will respond badly. That said, the ecosystems of the California Current System, which are already used to naturally low and variable pH and aragonite conditions, may be less vulnerable than some.

“Our model simulations for the future were conducted without changes in climate, i.e., we assumed that the only property changing into the future was the atmospheric carbon-dioxide concentration,” said Gruber. “Our next step is to investigate the response of the system to changes in both atmospheric carbon dioxide and climate.”

The team is also looking at the impact of ocean acidification on other eastern boundary upwelling regions such as the Canary Current System and the Humboldt Current System.

Gruber and colleagues reported their work in Sciencexpress.