Conclusion

The oceans have been buffering climate change by absorbing about a quarter to a third of the CO2 emitted into the atmosphere from anthropogenic sources. This has resulted in the measurable alteration of surface ocean concentrations of CO2, HCO3 , CO3 and pH as well as the reduction of the

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FIGURE 6 Trajectories for surface ocean pH decrease calculated for different atmospheric CO2 concentration profiles leading to stabilisation from 450 to 1000 ppm. From Turley [92].

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FIGURE 6 Trajectories for surface ocean pH decrease calculated for different atmospheric CO2 concentration profiles leading to stabilisation from 450 to 1000 ppm. From Turley [92].

saturation state and shoaling of the saturation horizons of CaCO3 minerals. Since pre-industrial times ocean pH has decreased by a global average of 0.1 and it has been estimated that unmitigated CO2 emissions will cause ocean pH to decrease by as much as 0.4 by the year 2100 and 0.77 by 2300. These will be the most rapid and greatest changes in ocean carbonate chemistry experienced by marine organisms over the past tens of millions of years. Laboratory experiments, field observations of natural CO2-rich seawater 'hot spots' and studies of previous ocean acidification events in Earth's history, indicate that these changes are a threat to the survival of many marine organisms but particularly organisms that use CaCO3 to produce shells, tests and skeletons (e.g. coccolithophores, pteropods, foraminifera, corals, calcareous macroalgae, mussels, oysters, echinoderms and crustacean). The ASH is already shoaling, bringing increasingly corrosive waters to the productive, shallower shelf seas along the western coast of North America and models predict that polar and some sub polar waters will be undersaturated this century while saturation states in tropical surface oceans will be substantially reduced. Recent experiments reveal that other important biological processes (productivity, internal physiology, fertilisation, embryo development, larval settlement and communication) are also vulnerable to future changes in ocean chemistry. There could also be changes to ocean carbon and nutrient cycles but, because of their complexity, it is hard to predict what the implications of the changes to biology will be on marine food webs, ecosystems and the services they provide. However, examination of previous episodes in Earth's history indicates that unmitigated CO2 emissions are likely to result in widespread extinctions. It will take tens of thousands of years for the changes in ocean chemistry to be buffered through neutralisation by calcium carbonate sediments and the level at which ocean pH will eventually stabilise will be lower than it currently is. The only way of reducing the impacts of ocean acidification on a global scale is through urgent and substantial reductions in anthropogenic CO2 emissions. Ocean acidification is a key argument for united global societal action in future climate change negotiations.

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