To the casual observer the problem could seem easy to solve. If some advanced form of carbon capture and storage were available, removing excess carbon dioxide from the atmosphere could be as straightforward as having a bit of a tidy-up and hiding away all the unwanted rubbish out of harm’s way. But ocean acidification means that would be like trying to vacuum up pet hair (or carbon dioxide) from the living room when it had actually integrated itself into the fabric of the carpet (or dissolved into the ocean) and begun to harm the carpet’s ecosystems.

You’d then have two problems – the untidiness of the room, representing climate change, and the hidden damage to the carpet, or ocean acidification. While in the case of a carpet, that ecosystem probably consists mainly of dust mites and bacteria, the oceans are rich with valuable biodiversity.

As an aside, to push this housework analogy to its already-overextended limits, you could say that a geoengineering solution that cuts the amount of sunlight reaching Earth would be like dimming the living room lights so that you can’t see the rubbish – it could help mask the climate change problem but does nothing to stop the oceans from acidifying.

So why does acidification cause problems for ocean-dwellers? Many marine organisms use carbonate shells to protect themselves and a lowering of the water’s pH makes those shells more likely to dissolve and harder to form. Since pre-industrial times, the average surface ocean pH has gone down by around 0.1 units, while lab studies have shown that a decrease in seawater pH of 0.2 to 0.3 units can stop or slow calcification in marine organisms such as corals, foraminifera and calcareous plankton.

Acting on acid
With that in mind, Richard Zeebe of the University of Hawaii, James Zachos of the University of California at Santa Cruz, Ken Caldeira of the Carnegie Institution, US, and Toby Tyrrell of Southampton University, UK, recently called for updates to environmental regulations for pH changes in seawater to take this latest research into account. The US Environmental Protection Agency criteria, for example, were created in 1976 and allow for a pH change of 0.2 units. Writing in a perspective in Science, the researchers say that “once new ranges of tolerable pH are adopted, carbon-dioxide-emission targets must be established to meet those requirements in terms of future seawater-chemistry changes”. They reckon this is likely to favour lower emission targets.

As the scientists stress in their article, nobody knows just how much pH change many marine organisms can tolerate, or the effects of acidification on non-calcareous organisms such as viruses and bacteria, or on food webs as a whole. As a result of acidification, coral reefs could lose their structural stability, which would affect whole reef communities as well as removing protection for neighbouring coastlines. And damage to shellfish could harm commercial aquaculture production.

So the team is calling for more experimental data and field observations of how calcification rates respond to elevated carbon dioxide, and how well different groups of organisms can adapt. “The key is to understand the response of the functional groups that drive marine biogeochemical cycles,” they write, adding that few studies have looked at foraminifera, which play a big role in the production and deposition of calcium carbonate in the ocean.

There’s one very small piece of good news – unlike climate models, say the team, the projections for ocean chemistry are largely model-independent on a timescale of a few centuries. That’s because the chemistry of carbon dioxide in seawater is well understood and the levels in the surface ocean closely track atmospheric concentrations. So it’s relatively easy to predict the effects on ocean pH of different volumes and timescales of carbon dioxide emissions. For example, if emissions could be reduced after 2050 and capped at 1500 petagrams of carbon, surface ocean pH would decline by around 0.35 compared to preindustrial levels. That’s still likely to lead to a drop reductions in coral calcification, however.

According to Anthony Richardson of the University of Queensland, Australia, and Elvira Poloczanska, of CSIRO, ocean ecosystems may be suffering from the temperature effects of climate change more than those on land. As the oceans have acidification to contend with as well, it’s vital that carbon emissions are addressed comprehensively and soon.