Spurious Late Historical-Era Warming in the CESM2 Large Ensemble and Other CMIP6 Simulations

Abstract:

A notable feature of many CMIP6 climate models is their increased equilibrium climate sensitivity (ECS) relative to their CMIP5 counterparts. Also evident in many models is an elevated rate of warming in the later decades of the historical era, raising the question as to whether this elevated warming is symptomatic of increased sensitivity or whether other mechanisms are at play?

In this work, these questions are explored in large ensembles of the Community Earth System Model Versions 1 and 2. It is found that simulated historical-era warming in CESM1/2 is nearly indistinguishable south of about 30N, with major differences being apparent mainly in the northern extratropics. Associated with this enhanced northern extratropical warming in CESM2 is a co-located abrupt increase in downwelling solar radiation from the mid-1990s through 2014 and a similar abrupt decrease in the transitions to the SSPs after 2014. Using a suite of model sensitivity experiments, we show the radiative increase to arise mainly from details in the CMIP6 forcing and their interactions with clouds rather than from intrinsic differences in model feedbacks in response to warming. Similar effects are also identified in many CMIP6 simulations. Our results therefore highlight the complexity in attributing transient warming solely to model ECS and caution against using late historical era warming in CMIP6 simulations as an observational constraint.