One century and 2,000 shells later, scientists are saying California’s coastal waters are acidifying twice as fast as the global average. The changes do not bode well for the region’s salmon, crab, and shellfish supply, note NOAA researchers in Nature Geoscience.

The team turned to microscopic animals called foraminifera (forams for short), whose shells are sensitive to growing levels of acidity in our planet’s waters. The tiny shell-builders record ambient seawater conditions in the thickness, size, and chemistry of their shells, making them valuable tools for paleoceanographers to understand how the oceans have changed in the geologic past. 

They are also a key sign of a more global problem: greenhouse gases. As the oceans absorb carbon dioxide, they become more acidic (lower pH). The single-celled creatures only live for a month or so before their shells "rain down" on the ocean floor and are blanketed in sediment, providing a timestamp of acidity levels through the years. 

"It was amazing to actually see how something as simple as these tiny microscopic shells were being impacted by ocean acidification," lead author Emily Osborne, a scientist with NOAA, told IFLScience. "There was such a clear change in the shell wall thickness of the planktonic foraminifera that I examined from around the year 1900 compared to those that I looked at around the year 2000."

Relative to 1900, foraminifera shells in California's coastal waters have become 20 percent thinner and 7 percent larger. Osborne said she "had anticipated to see some change" but "the magnitude of change was impressive." 

"Another neat thing about this study, apart from elegantly showing the trend in anthropogenic ocean acidification, is that it allows us to put some sort of number on the amount that changes in ocean pH can lead to changes in the amount of CaCO3 being delivered to the seafloor... and will help us to incorporate this feedback into predictive models," added Michael Henehan, an isotope geochemist at the Helmholtz Centre potsdam who was not involved in the study. 

Researchers have spent more than a decade studying foram shells as a proxy tool to estimate pH levels in the past. Like many other marine calcifiers, forams build their shells by using dissolved ions in the seawater around them. They precipitate their shells primarily via a combination of dissolved calcium and dissolved carbonate ions. 

"While CO₂ emissions are resulting in overall higher concentrations of dissolved inorganic carbon in seawater, there is a relative shift in the species of carbon so that there is overall less carbonate ions and more bicarbonate ions," said Osborne. "So, with fewer carbonate ions in seawater to be used by foraminifera as building blocks for their shells, the shell walls of these organisms are thinner."

"When we started analyzing the data, we were also surprised the see the relationship that emerged between the seawater acidity (estimated by shell thickness) and Pacific Decadal Oscillation (PDO)."

Pacific Decadal Oscillation is a cycle of natural warming and cooling in the northeast and tropical Pacific Ocean. Over the century, the foraminifera shells changed in tandem with the PDO. During phases of PDO when upwelling is generally stronger, higher acidity conditions persist for 20-30 years, and vice versa.

"This means that in the future, we can expect that ocean acidification along the West Coast will be both intensified and alleviated by this natural variability that is associated with PDO in this system," added Osborne.

Since the dawn of the industrial era, Earth’s oceans have absorbed 27 percent of global carbon dioxide emissions, rising from 280 parts per million to more than 400 parts per million. In regions like California, where there’s coastal upwelling, acidic water is being driven to the surface at a faster rate. The acidification could impact California’s fisheries, which account for around 10 percent of the nation's seafood production. 

"There have been ocean acidification events in the past," said Henehan. "Around 56 million years ago, at the 'Paleocene-Eocene Thermal Maximum’ or PETM, the ocean’s pH dropped by about 0.3 pH units, we think because of a sharp episode of volcanism and methane release."

This drop in pH took place over thousands to tens of thousands of years, giving organisms a bit of time to adapt. On the other end of the scale, the asteroid event at the K-Pg boundary likely resulted in a sudden acidification event due to the raining of sulphuric acid.

"In this case, because we’re talking about a much more sudden event, there was massive extinction amongst the calcium carbonate-producing plankton in the oceans, and the Earth’s carbon cycle took hundreds of thousands of years to recover," said Henehan. "The key question for today, I guess, is where our current rates of human-induced acidification are going to fall within that spectrum, and what effect will acidification have on ocean ecosystems by the end of this century?"