A recent paper by Cox et al. introduces Ψ, “a theoretically informed metric of global temperature variability”, which scales with equilibrium climate sensitivity (ECS) across 16 general circulation models (GCMs). Cox et al. report that Ψ provides a strong constraint on ECS, ruling out both high and low values. Our analysis shows that this constraint is sensitive to the GCMs considered, primarily reflects the forced climate response rather than climate variability and does not narrow the uncertainty in ECS. It is therefore premature to rule out the possibility of large ECS values.