As concerns over climate change grow there are increasing numbers of geo-engineering solutions proposed. However, they often do not take into account or resolve the issue of ocean acidification (e.g. addition of sulphur dioxide into the stratosphere to deflect some of the sun's energy or ocean pumps of deep water rich in nutrients to increase productivity and drawdown CO2) nor do they look at potential deleterious impacts on the marine environment (adding quicklime to the oceans to soak up CO2, iron or urea fertilisation to increase ocean productivity and drawdown CO2).

Currently, expert opinion is that the only method of reducing the impacts of ocean acidification on a global scale is through urgent and substantial reductions in anthropogenic CO2 emissions [7,15,29]. A threshold of no more than a 0.2 pH decrease has been recommended to avoid aragonite undersatura-tion in surface waters [93]. In terms of atmospheric CO2 concentration this would be just above the 450 ppm stabilisation scenario (Fig. 6). However some polar waters would experience aragonite undersaturation even at this stabilisation level.

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