TCRE, ZEC and ocean heat uptake efficacy response to AMOC

Remaining carbon budgets consistent with limiting global warming below certain temperature thresholds are estimated from the transient climate response to emissions (TCRE) and the zero emissions commitment (ZEC). TCRE is the amount of warming per unit of cumulative carbon dioxide emissions, while ZEC is the amount of warming that would occur following a complete cessation of emissions. IPCC AR 6 (Canadell et al, 2023, Chapter 5) concluded with medium confidence that TCRE is nearly constant with time and independent of the rate of emissions (or emissions pathway) and therefore it is a good predictor of CO2-induced warming after emissions reductions, although some studies (MacDougal 2017; Seshadri 2017) have pointed towards pathway dependence at very high and very low emissions rates. In all studies there is the implicit assumption that the cumulative fraction of carbon taken up by the terrestrial biosphere is constant, and that the climate feedback parameter and ocean heat uptake efficacy do not change in time. Using a suite of emissions-driven Earth system model simulations, we explore the impact of immediately halting CO₂ emissions under different levels of global warming. We show that the climate system undergoes state shifts when AMOC weakens substantially due to forcing and/or internal variability, and only then significant cooling occurs following CO₂ emissions cessation but with considerable consequences for regional climates.  We identify ranges of non-zero likelihood for AMOC collapse and the associated global warming levels, emissions thresholds and a possible mechanism linked to high latitude sea ice transport variability. We demonstrate that TCRE, ZEC and ocean heat uptake efficiency are state dependent and, while fast feedbacks control ZEC when mitigation occurs at lower emissions levels, AMOC weakening becomes the leading driver setting ZEC at higher emissions levels. Even the most ambitious mitigation of climate change would be ineffective if action is delayed and the climate system is too close to a tipping point of the Atlantic Meridional Overturning Circulation, since there would be significant differences in the committed warming.