The ocean’s climate-change driven slide towards acidity could be worse than previously realised, scientists have warned.
As humanity burns ever more fossil fuels, much of the resulting carbon dioxide ends up in the ocean, where it shifts marine chemistry in ways that makes seawater less alkaline and more acidic.
It has long been known that this “ocean acidification” has major negative consequences for marine life, especially so-called “calcifying species” such as corals and molluscs that make hard shells. This can devastate fishers who rely on these species (see: Imperilled by ocean acidification).
Ocean acidification is one of nine “planetary boundaries” – limits to systems key to keeping Earth’s environment in a state similar to how it has been for human existence. Previously three of the nine boundaries were not deemed crossed. That may no longer be the case.
Researchers behind a new analysis say that around 60% of deeper waters and 40% of surface waters have crossed the acidification boundary. The boundary is defined as a 20% reduction from the pre-industrial averages of “aragonite saturation state”, a a chemical measure which indicates the degree of acidification.
But they also believe the boundary should be revised from 20% to just 10%. Writing in the journal Global Change Biology, they say this level is “a more stringent and ecologically meaningful target” and that 20% “provides insufficient protection of many crucial ocean habitats beyond the surface waters”.
And here is the rub: that shift would mean the boundary was crossed in the 1980s, with the whole ocean surface having passed it sometime in the 2000s, according to Helen Findlay of the Plymouth Marine Laboratory in the UK and her colleagues.
“Looking across different areas of the world, the polar regions show the biggest changes in ocean acidification at the surface. Meanwhile, in deeper waters, the largest changes are happening in areas just outside the poles and in the upwelling regions along the west coast of North America and near the equator,” says Findlay.
“Most ocean life doesn’t just live at the surface – the waters below are home to many more different types of plants and animals. Since these deeper waters are changing so much, the impacts of ocean acidification could be far worse than we thought.”