In his satirical Devil’s Dictionary, Ambrose Bierce defined the ocean as ‘a body of water occupying about two thirds of a world made for man – who has no gills’. Bierce may have been right to poke fun at human arrogance, but he underestimated the importance of the seas. Averaging almost 3,700 metres (12,000ft) deep, the ocean constitutes around nine tenths of the habitable space on our planet. It plays a commensurate role in the Earth system, not least as an engine – a ‘blue machine’ in the phrase that also titles an excellent book by the physicist and science presenter Helen Czerski – that moves heat around the planet.
The ocean has also been humanity’s get out of jail free card, absorbing more than 90 per cent of the additional heat trapped in the atmosphere by our greenhouse gas emissions. This grace period is, however, probably coming to an end. Worldwide, ocean surface temperatures have been rising faster than predicted, and have broken records almost continuously since March last year. In future, storms and heatwaves are likely to increase in intensity and frequency. Sea levels will rise further. Warmer waters will kill more coral reefs. Oxygen concentrations in the deep ocean will fall, and seawater continue to acidify.
Not everything is a story of catastrophe. When they are protected, ocean ecosystems can rebound remarkably quickly from overfishing, pollution and other human impacts. This is well attested in Helen Scales’s What the Wild Sea Can Be. No less vitally, the seas remain places of astonishing discoveries and wonders. Their past and future are full of potential as well as devastation, as James Bradley documents in Deep Water.
The impacts of noise pollution – above all from shipping, but also from other activities including seismic surveys beneath the seabed for oil and gas – may not be as serious and pervasive in the ocean as those of climate change, but they can be devastating to marine life.
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