Assessing the remaining carbon budget through the lens of policy-driven acidification and temperature targets

Basing a remaining carbon budget on warming targets is subject to uncertainty due to uncertainty in the relationship between carbon emissions and warming. Framing emissions targets using a warming target therefore may not prevent dangerous change throughout the entire Earth system. Here, we use a climate emulator to constrain a remaining carbon budget that is more representative of the entire Earth system by using a combination of both warming and ocean acidification targets. The warming targets considered are the Paris Agreement targets of 1.5 and 2 °C; the acidification targets are −0.17 and −0.21 pH units, informed by aragonite saturation states where coral growth begins to be compromised. The aim of the dual targets is to prevent not only damage associated with warming, but damage to corals associated with atmospheric carbon and ocean acidification. We find that considering acidification targets in conjunction with warming targets narrows the uncertainty in the remaining carbon budget, especially in situations where the acidification target is more stringent than, or of similar stringency to, the warming target. Considering a strict combination of the two more stringent targets (both targets of 1.5 °C warming and −0.17 acidification must be met), the carbon budget ranges from −74.0 to 129.8PgC. This reduces uncertainty in the carbon budget from by 29% (from 286.2PgC to 203.8PgC). This reduction comes from reducing the high-end estimate of the remaining carbon budget derived from just a warming target. Assuming an emissions rate held constant since 2021 (which is a conservative assumption), the budget towards both targets was either spent by 2019 or will be spent by 2026. Plain language summary The relationship between atmospheric CO2 and warming is uncertain, which means that we do not know precisely how much carbon we have left to emit until we reach the Paris Agreement warming targets of 1.5 and 2 °C. However, the relationship between atmospheric CO2 and ocean acidification is better understood, so by considering targets for acidification rather than warming alone, we can narrow down our estimate of how much emitted carbon is acceptable. Including acidification targets as well as warming targets means that we can directly address the issue of ocean acidification, which poses a threat to corals and the ecosystems reliant on them. By considering acidification and warming targets together, we can lower uncertainty in acceptable carbon emissions by 29%.