The Community Earth System Model (CESM) and Energy Exascale Earth System Model (E3SM) are two climate models that have been developed recently in the United States.  When recent versions of these models were being finalized, it was discovered that their equilibrium climate sensitivities (ECS) are surprisingly large: about 5.3 K.  This lies outside the traditional range of 1.5 to 4.5 K.

The reasons for these large ECS values are being investigated.  We will discuss two possible reasons: a cloud-phase feedback in the extratropical storm tracks (Tan et al. 2016) and subtropical low-cloud feedbacks.  

This sort of surprise, after years of model development, raises a general question: How do we know if a climate model is trustworthy?  Trust may require more than seemingly sound physics, an accurate representation of present-day climate, or even an accurate reproduction of 20th century global warming.