Understanding mechanisms of the Zero Emission Commitment using MIROC-ES2L

Zero Emissions Commitment (ZEC), defined as a change in global-mean surface temperature expected to occur after net-zero CO₂ emissions, is an important factor for estimating future climate and mitigation policies.

While the carbon budget arguments predict ZEC to be zero, it actually varies between slight positive and negative values in Earth system models (ESMs) and therefore uncertainty remains. Previous studies have shown that ZEC tends to be more positive with a greater amount of cumulative CO₂ emissions, but the underlying mechanisms are not yet understood well.

To clarify them, we performed an idealized global warming experiments using MIROC-ES2L, one of the CMIP6 ESMs. The experiments consist of the so-called flat10 run (with 10PgC emission) for 1000 years and zero-emission runs branched off at the time points when global-mean surface temperature reaches different values between 2 and 8°C in flat10.

We identified that the sign and value of ZEC in MIROC-ES2L depend on the global warming level when net-zero CO₂ emission is achieved. Specifically, GSAT tends to decrease when emissions are stopped at lower warming levels, whereas it increases when emissions are stopped at higher warming levels. This behavior arises from the state dependence of the ocean heat uptake weakening and change in the effective radiative forcing associated with the carbon uptake. Using the global energy budgets, we could estimate ZEC in the equilibrium state, which was similar to the ZEC in the first 200 years after net-zero CO₂ emissions.