Catalogus

Future ocean acidification mainly depends on the continuous ocean uptake of CO₂ from the atmosphere. The trajectory of future atmospheric CO₂ is prescribed in traditional climate projections with Earth system models, leading to a small model spread and apparently low uncertainties for projected acidification, but a large spread in global warming. However, climate policies such as the Paris Agreement define climate targets in terms of global warming levels and as traditional simulations do not converge to a given warming level, they cannot be used to assess uncertainties in projected acidification. Here, we perform climate simulations that converge to given temperature levels using the Adaptive Emission Reduction Algorithm (AERA) with the Earth system model Bern3D-LPX at different setups with different Transient Climate Response to cumulative carbon Emissions (TCRE) and choices between reductions in CO₂ and non-CO₂ forcing agents. With these simulations, we demonstrate that uncertainties in surface ocean acidification are an order of magnitude larger than the usually reported inter-model uncertainties from simulations with prescribed atmospheric CO₂. Uncertainties in acidification at a given stabilized temperature are dominated by TCRE and the choice of emission reductions of non-CO₂ greenhouse gases (GHGs). High TCRE and relatively low reductions of non-CO₂ GHGs, for example, necessitate relatively strong reductions in CO₂ emissions and lead to relatively little ocean acidification at a given temperature level. The results suggest that choices between reducing emissions of CO₂ versus non-CO₂ agents should consider the economic costs and ecosystem damage of ocean acidification.