제목: Emit now, mitigate later? Earth system response to overshoots of different magnitude and duration

We have simulated a large set of idealized simulations with the Norwegian Earth System Model (NorESM2) to explore the Earth system response to net-zero and net-negative CO2 emissions. Our simulations reach net-zero emissions after different levels of prior emissions and are complemented by phases of negative emissions such that we obtain scenarios of temperature overshoots of different magnitude and duration. I’ll present results from these simulations focusing on two aspects. First, I show that in our model, which has a strong AMOC decline in response to global warming, high northern latitudes cool substantially relative to peak warming after emissions are phased out. Interestingly, such cooling can be amplified under deliberate CO2 removal (CDR) and results in a temporary undershoot of a targeted temperature level, depending on the timing of CDR relative to the recovery of AMOC. I present evidence that such behavior might be generally found in Earth system models that show a strong AMOC decline in response to anthropogenic forcing. Second, I’ll analyze the centennial scale reversibility of the Earth system after an overshoot. In our model, increased carbon loss from land during an overshoot is compensated by increased ocean uptake such that atmospheric CO2 concentrations are remarkably similar in the overshoot simulations compared to the reference simulation without overshoot. I find that many aspects of the Earth system are indeed reversible except for very large overshoots, which are unfeasible in view of limited CDR capacity.​