Ocean acidification has breached a key ‘planetary boundary’

Acidity in the sea has reached hazardous levels. Even more worryingly, scientists have discovered it crossed this critical threshold 5 years ago, much faster than they had assumed. The authors of a major study led by the UK’s Plymouth Marine Laboratory and the NOAA are sounding a red alert, calling the findings a ‘ticking timebomb’. But what causes ocean acidification? And what does it mean for ecosystems?