Appendix 1

In this appendix, we show that the cloud feedbacks produced by the experiment model are unaffected by model tuning. There are two model changes differentiating our experiment model from the control: (1) We adjust the T ice parameter in the shallow convection scheme to detrain more liquid and less ice in mixed-phase clouds (the T ice change). And (2) We tune the model by adjusting the relative humidity for low cloud formation (the tuning change). The tuning change is necessary in both the slab ocean and fully coupled frameworks to maintain a stable climate. However, the tuning change impacts cloud amount in a given climate state and therefore it seems possible that the tuning change could impact the cloud feedbacks and cloud amount decreases produced by the experiment model.

To determine whether this is the case we use an AMIP (Atmospheric Model Intercomparison Project)-style, or atmosphere only, modeling framework where surface temperatures and sea ice are prescribed to the model in a repeating annual cycle. With this framework, we can run the model with only the T ice change in isolation and not be concerned with global energy balance as the surface temperatures are prescribed. We prescribe sea surface temperatures and sea ice conditions to the AMIP model by taking averages of the monthly mean temperatures and sea ice conditions from 10 years of the SOM1xExp and SOM2xExp runs. We then run two AMIP runs for ten years each; one with pre-industrial CO 2 levels (AMIP1xT ice Only) and one with doubled CO 2 levels. (AMIP2xTi ce Only). Both AMIP runs include the T ice change and omit the tuning change. See Table 4 for a full description of the two AMIP runs.

Comparing the total cloud feedback between the SOM Experiment runs and the AMIP T ice Only runs reveals almost no change in the cloud feedback when the T ice change is made without the tuning change (Fig. 13a). We also see that the change in low cloud fraction due to doubled CO 2 , which is responsible for part of the positive cloud feedback at mid-latitudes, is virtually unchanged between the SOM Experiment runs and AMIP T ice Only runs (Fig. 13b). Though inclusion of the tuning change does result in a lower cloud fraction in the SOM Experiment runs compared with the T ice Only runs (not shown), this is true in both doubled CO 2 and preindustrial runs and the difference between the two remains unchanged. The difference in cloud feedbacks between our experiment and control (Fig. 3) is primarily the result of our changes to shallow convective mixed-phase clouds (the T ice change) and largely unaffected by our model tuning change.

Table 4 Description of atmosphere-only model runs. All runs use the Community Earth System Model with the Community Atmosphere Model, version 5 [CESM(CAM5)] at one-degree horizontal resolution Full size table