Hopes that future efforts to extract excess carbon dioxide from the atmosphere could spare the planet the worst impacts of climate change have been dimmed by a new study finding the acidification of oceans could take centuries to reverse.

The world's oceans have already become about 30 per cent more acidic since pre-industrial times as seas absorb about one quarter or more of the excess carbon dioxide, triggering a chemical reaction. Combined with heat stress caused by warming waters, the rising acidity levels are already affecting complex ecosystems from plankton to shellfish and corals.

A delicate balance: Many species at risk from rising acidification of the seas. Credit:Gary Cranitch

Researchers at Germany's Potsdam Institute for Climate Impact Research examined the prospects for massive geo-engineering efforts to remove CO₂ from the atmosphere and say it would take centuries for the oceans to become less acidic.

Assuming carbon dioxide removal could be ramped up to 90 gigatonnes a year – or about twice current annual emissions – oceans would not be brought back to pre-1750 levels until at least 2700, the researchers said in a paper published this week in Nature Climate Change journal.