The Simple Cloud-Resolving E3SM Atmosphere Model (SCREAM) is E3SM's new km-scale global atmosphere model. It was built from scratch in C++ using the Kokkos performance portability library to enable efficient simulation on all of DOE's exascale computers. Using all nodes of the world's fastest supercomputer (Frontier), it can complete 1.25 simulated years in a single wall day. This extreme resolution leads to improved representation of clouds and storms, though some biases remain. SCREAM has a high climate sensitivity at 3 km horizontal resolution and much weaker sensitivity at 12 km grid spacing, which poses interesting questions about the need for extreme resolution in capturing climate change.