Possibility for strong northern hemisphere high-latitude cooling under zero and negative emissions.

The Atlantic meridional overturning circulation (AMOC) transports large amount of heat northwards, where this heat contributes to warm the atmosphere. It is well established that an AMOC collapse, or strong reduction would substantially cool the northern high latitudes. New Earth system model (ESM) simulations indicate the possibility that such cooling, due to the large inertia of the climate system, could also happen when CO₂ emissions are phased out after AMOC has been weakened by preceding climate change. Our simulations show a temporally and spatially strongly variable zero emission commitment (ZEC), which is first negative (up to -0.4K) and then turns positive (up to 0.5K) as AMOC recovers. Regionally, the strongest cooling-warming pattern is found in the northern high latitudes north of 40°N, where the amplitude of this cycle can exceed several degrees. The mechanism of this cooling can be understood by the fact that the radiative forcing of CO₂ is reduced when emissions become zero while the Earth system is in a state of reduced northward heat transport. We further show that this northern high-latitude cooling could be amplified under deliberate CO₂ removal and result in a temporary undershoot of a targeted temperature level. Such “stronger than intended” cooling north of 40°N under net negative emissions is a robust feature of Earth system models that show a strong AMOC decline in response to warming, both in idealized simulations but also in an overshoot scenario created for CMIP6 by an integrated assessment model. The future fate of ocean circulation is highly uncertain in ESM projections, and the amplified northern high latitude cooling under zero or net negative emissions is a feature of ESMs with a high sensitivity of AMOC to warming. Nevertheless, or results underscore (i) the importance of understanding the physical climate system’s response to phasing out emissions to avoid surprises, (ii) the importance of coordinated experimentation with ESMs such as CMIP6 ZECMIP and CDRMIP, and finally (iii) the importance of assessing zero and net negative emissions beyond simple metrics like global mean surface temperature.