A hotter atmosphere has several effects on the sea. First, it means higher average temperatures for surface waters. One consequence for coral reefs is that the symbiosis between the corals and algae that constitute a living reef is breaking down. As temperatures rise, the algae leave or are expelled, the corals take on a bleached, white appearance and may then die.

Hotter water, slimier slime

Warming also has consequences for ice: it melts. Melting sea ice affects ecosystems and currents. It does not affect sea levels, because floating ice is already displacing water of a weight equal to its own. But melting glaciers and ice sheets on land are bringing quantities of fresh water into the sea, whose level has been rising at an average of nearly 2 millimetres a year for over 40 years, and the pace is getting faster. Recent studies suggest that the sea level may well rise by a total of 80 centimetres this century, though the figure could plausibly be as much as 2 metres.

The burning over the past 100 years or so of fossil fuels that took half a billion years to form has suddenly, in geological terms, put an enormous amount of carbon dioxide into the atmosphere. About a third of this CO2 is taken up by the sea, where it forms carbonic acid. The plants and animals that have evolved over time to thrive in slightly alkaline surface waters — their pH is around 8.3 —are now having to adapt to a 30% increase in the acidity of their surroundings. Some will no doubt flourish, but if the trend continues, as it will for at least some decades, clams, mussels, conches and all creatures that grow shells made of calcium carbonate will struggle. So will corals, especially those whose skeletons are composed of aragonite, a particularly unstable form of calcium carbonate.

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