They’ve been mitigating human-induced climate change for centuries. Day after day, year after year they’ve been locking away large amounts of the extra greenhouse gas humankind has been belching into the atmosphere since the industrial revolution. They are the Sinks. In payment for this great service we have systematically attacked them with chainsaws, ploughs and concrete.

Of the 7 billion tonnes of carbon released by our burning of fossil fuels every year, around half is captured and locked away by vegetation, soils and the oceans. The scenes of deforestation and land-use change that represent our damage to the land-based carbon sink are all too familiar, but the great ocean carbon sink is by no means invulnerable.

In the journal Nature today Behrenfield et al. report that rising sea temperatures result in a reduction in the growth rate of phytoplankton, the microscopic plants that are key to carbon dioxide drawdown by the world’s oceans. As the waters warm up, the surface waters become increasingly stratified (layered), with supplies of nutrients quickly becoming depleted. Without these nutrients the phytoplankton starve and the carbon dioxide they would have taken up instead stays in the atmosphere, to drive further global warming, and so yet more stratification of the oceans.

The Earth’s sinks for greenhouse gases have given humankind a vital window in which to take concerted action to reduce our greenhouse emissions. With erosion of the land carbon sink through land-use change and suffocation of the oceanic sink by a blanket of warm water this window is starting to close.