evidence: Furthermore, solar cycle has a separate external forcing than CO2. The response to this external forcing is amplified by the climate feedback processes of the Earth system. It is hoped that by examining the solar-cycle response at the Earth’s surface, we can estimate the effect of the climate feedback involved, and hence the climate sensitivity of the Earth system. This is a different approach than some of the current ones, for example, of using the effect of Arctic snow-albedo12, or tropical low clouds13,14 to provide a constraint on the climate sensitivity (ECS in the cited cases). Using a component of the feedback process to constrain ECS or TCR requires an additional assumption that the other feedback components are "unbiased", e.g. not compensating9. This structural uncertainty has so far not been assessed.