On the penultimate week of the planetary boundaries series, we come to the other CO2 problem. Everyone knows about climate change, which we looked at last time. Less known is that atmospheric climate change has a marine counterpart, an “evil twin” as Mark Lynas puts it: ocean acidification.
Since the beginning of the industrial era, the burning of fossil fuels has released half a trillion tonnes of CO2. It has to go somewhere, and much of it has gone into the oceans. Seawater absorbs CO2, and as CO2 dissolves in water it forms carbonic acid. The build-up of carbonic acid changes the pH value of the surface water, making it more acidic. The sea is now 30% more acidic than it was at the end of the eighteenth century.
Even if you’ve never come across the issue before, you can probably begin to guess at the consequences for marine life. Increased acidity is going to be bad for alkaline substances. Coral reefs are made entirely of calcium carbonate, as are the shells of sea creatures, and calcium carbonate is alkaline. The more acidic the seawater becomes, the harder it is for corals to grow new layers, or for sea creatures to form shells. Eventually, it could become so acidic that it actually begins to dissolve coral reefs entirely.
Coral reefs might not sound particularly important to us landlubbers, but 25% of marine species depend on them. Losing coral reefs would mean a catastrophic loss of biodiversity in the oceans. Since they serve as nurseries for many species of fish, there are consequences for global fisheries. An estimated 500 million people earn a living or catch their food on coral reefs, with island nations like the Philippines or Indonesia particularly dependent on them. It’s impossible to put a true economic value on the world’s coral reefs, but one attempt valued them at $375 billion a year.
There are other effects too, although no one is entirely sure what ocean acidification could mean long term. It could benefit certain kinds of algae and seaweed, choking out other life and exacerbating the dead zones that I mentioned in the post on nitrogen. Seabirds, seals and whales could lose their food sources and become increasingly endangered. Plankton numbers could fall, further reducing the sea’s CO2 absorption capacities and accelerating global warming.
Perhaps one of the more obvious signs would be the loss of coccolithophores, the tiny shelled organisms that float in seawater and disperse sunlight. Without them, seawater would darken and the colourful turquoise waters of tropical beaches would be a thing of the past. Nobody’s entirely sure. We’re in new territory here, a planet-sized experiment in changing the chemistry of the sea.
The good news is that if we act to stop climate change, we’d stop ocean acidification at the same time. Two global crises would be averted for the price of one. We know what that entails – a huge shift away from fossil fuels and towards renewable energy, scaling back our ecological footprints and re-configuring our economy away from relentless consumption.
It is possible to set a clear boundary for ocean acidification, but it’s rather technical for those of us that aren’t marine biologists. “As a first estimate,” say the report authors, “we propose a planetary boundary where oceanic aragonite saturation state is maintained at 80% or higher of the average global pre-industrial surface seawater Ωarag of 3.44.” At the risk of oversimplifying, aragonite saturation is a way of measuring how well marine creatures will be able to form calcium carbonate structures. If it falls to 1, they can’t form them at all, and anything below 2.75 seriously affects their ability to grow and thrive. We’re currently at 2.9.
Boundary: Ocean acidification
Safe limit: 2.75 aragonite saturation
Status: within the boundary, just