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. [...] In our recent publications67,68 we estimated the solar-cycle induced changes in NO2 using ground-based observations, WACCM, and our own 1-D photochemical model. The changes are largely in the stratosphere, near the 1-hPa level, with an exception during rare nonlinear events such as tropopause folding in the mid-latitude68. Thus, the overall solar-cycle (or GCR) induced changes in the tropospheric NOx is much less than other variabilities, e.g. anthropogenic pollution, and therefore we do not focus on its climate impact in the current study. [...] Tung, K. K. & Camp, C. D. Solar-cycle warming of the earth’s surface in NCEP and ERA-40 data, a linear discriminant analysis. J. Geophys. Res.113, D05114 (2008).