Ocean acidification as a result of absorption of CO2 from the atmosphere is believed to have potentially detrimental impacts on ecosystems and biogeochemical cycles in the ocean, affecting both its productivity and its ability to act as a “buffer” to climate change.
A combination of simple and complex coupled-carbon General Circulation Model (GCM) have been used to examine the projected changes in ocean surface pH from a range of non-mitigation and mitigation policy global emissions scenarios produced in Work Stream 1 of AVOID.
The traceability of results between the simple model and GCM is demonstrated, indicating that the simple model has useful skill in projecting global mean ocean surface pH, allowing a rapid assessment of projected ocean acidification resulting from a wide number of proposed mitigation policy options.
A “pattern scaling” approach is also used to combine the spatial patterns of acidification from complex models with the simple model results to produce a mapping of projected acidification for different atmospheric CO2 concentrations. In combination with models of ecosystem or fisheries impacts from different level of acidification these will provide a useful assessment of projected impacts of ocean acidification on ecosystem productivity and the oceans ability to absorb CO2 from the atmosphere. This will enable a quantification of the impacts of acidification that can be avoided through mitigation to inform the policy process.
Maximum global mean acidification by 2100 is influenced by both peak year of emissions in mitigation scenarios and plausible post-peak reduction rates, but long-term acidification (out to 2500) is, as with global mean temperature, largely determined by the long-term emissions minimum that is reached.
Global mean acidification since the pre-industrial era (1750) is projected to double from its current level by 2100 under aggressive mitigation with peak emissions in 2016 and a post peak emissions reduction of 5% per year to a low value long-term minimum emissions (6 GtCO2e/yr). Under an IPCC SRES A1B BAU scenario, acidification is projected to increase four-fold by 2100 compared to pre-industrial values.