California's coastal water are twice as acidic as the global average, posing increasing threat to marine life

These changes, for the most part, are fuelled by human-induced pollution.  When carbon dioxide emissions from cars or power plants are absorbed by the oceans, it drives the water to become more acidic. However, natural factors are also contributing to the rising acidic levels, finds the study.


                            California's coastal water are twice as acidic as the global average, posing increasing threat to marine life
Emily Osborne, a marine scientist with NOAA's Ocean Acidification Program, examines a sediment sample taken from the Santa Barbara Basin off California. (NOAA)

The coastal waters off California have raced ahead of the global oceans on one front: ocean acidification, climate change's equally evil twin. The waters here are acidifying twice as fast as the global ocean average, according to a new study.

These changes, for the most part, are fuelled by human-induced pollution. When carbon dioxide emissions from cars or power plants are absorbed by the oceans, it drives the water to become more acidic. However, natural factors are also contributing to the rising acidic levels, finds the study.

This increase in acid levels could threaten marine life, impacting fisheries as California is known for its economically valuable salmon, crabs and shellfish.

Other studies have shown that these effects extend to the most-vulnerable organisms such as mussels and oysters. Rising acidic levels mean these organisms find it difficult to continue building their protective layers: shells and skeleton. In high enough concentrations, carbon dioxide can even cause these shells and skeletons to dissolve entirely.

In this study, the research team tracked changes in ocean acid levels over the last century. Holding records of these changes are shells of microscopic animals called foraminifera.

These colorful spots are tiny foraminifera shells taken from the mud of core samples as seen under a microscope.(NOAA)

Accordingly, the team collected mud samples from the seafloor off Santa Barbara. In these samples, scientists were expecting to find shells of dead foraminifera, which fall onto the bottom of the ocean as they die. Once they land on seabed, these shells get covered by sediment. As time progresses, layers of sediment settle on the shells, forming a vertical record of change. 

The team examined nearly 2,000 shells of these animals and measured how their shells changed over a century. More specifically, they look for changes in thickness.

"By measuring the thickness of the shells, we can provide a very accurate estimate of the ocean's acidity level when the foraminifera were alive," said lead author Emily Osborne, who used this novel technique to produce the longest record yet created of ocean acidification using directly measured marine species. She measured shells within cores that represented deposits dating back to 1895.

According to observation from these records, human-caused carbon dioxide emissions are driving ocean acidification, in addition to natural causes. The natural variation matches a natural warming and cooling cycle -- the Pacific Decadal Oscillation.

"During the cool phases of the Pacific Decadal Oscillation, strengthened winds across the ocean drive carbon dioxide-rich waters upward toward the surface along the West Coast of the U.S.," said Osborne, a scientist with NOAA's Ocean Acidification Program. "It's like a double whammy, increasing ocean acidification in this region of the world."

The study has been published in Nature Geoscience.

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