Underestimated Global Warming Hidden by Internal AMOC Weakening

Abstract:

Improving our knowledge of the Earth’s equilibrium climate sensitivity is necessary to quantify more precisely the amount of greenhouse gases that might be emitted to keep global warming below the 1.5 or 2°C objectives. The observational record over the last century and our best estimates of the radiative forcing suggest a relatively low climate sensitivity, which is difficult to reconcile with the larger sensitivity of the latest (CMIP6) generation of climate models. We propose a new insight here, using an ensemble of 32 historical simulations performed with the IPSL-CM6A-LR model. We observe a rather large range of apparent equilibrium climate sensitivity when this quantity is estimated from the model historical simulations. We show that the ensemble member the most in line with the observations of global mean surface air temperature is characterized by a large AMOC weakening since the 1950s. We also show that this member is consistent with several observed AMOC fingerprints, which gives further confidence in this result. The analysis of the individual simulations against the ensemble mean suggests that a large part of this AMOC weakening is due to internal climate variability. These results thus suggest that internal variability might have considerably dampened the global warming magnitude over the historical era.