Some like to point to cycles when dismissing climate change, brushing off warming as simply being the thing that happens right before cooling. In this view, concern about climate change is akin to the naïve worry that half of schools are performing below average. This is why we need context. We need to know whether an observed change is more like a world premiere or a familiar re-run.

A new paper in Science examines the geologic record for context relating to ocean acidification, a lowering of the pH driven by the increased concentration of carbon dioxide in the atmosphere. The research group (twenty-one scientists from nearly as many different universities) reviewed the evidence from past known or suspected intervals of ocean acidification. The work provides perspective on the current trend as well as the potential consequences. They find that the current rate of ocean acidification puts us on a track that, if continued, would likely be unprecedented in last 300 million years.

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Trends can be difficult to detect in real-world data, and the noisier the data, the tougher the task becomes. A longer time series can help limit the impact of noise, but these can be difficult to come by. Verifying the human alteration of ocean chemistry requires tackling challenges like these.

Ocean acidification entails a decrease in the pH of ocean water as the carbonate that buffers it is consumed. That carbonate does more than just maintain pH, though. Lots of marine organisms, from plankton to mollusks to coral, use it to build shells and skeletons. As the buffer is depleted, the saturation state of carbonate minerals like calcite (and its polymorph aragonite) decreases, making it more difficult for organisms to incorporate them. In most areas of the surface ocean, calcite and aragonite are supersaturated, making it easy for organisms to build shells and skeletons. In undersaturated water, the equilibrium tilts the other way, and dissolution of these structures becomes possible.

Calcite and aragonite saturation states vary regionally and seasonally, so how can we make sure the acidification trend we’re measuring is real and human-caused? One way to look into this question is to take the measurements we have and model the whole ocean to see what natural variation would have looked like before humans started emitting CO₂. A recent study in Nature Climate Change does just that.

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