Abstract Polar amplification is an established scientific fact which has been associated with the surface albedo feedback and with heat and moisture transport from the Equator to the Poles. In this paper we unify a two-box climate model, which allows for heat and moisture transport from the southern region to the northern region, with an economic model of welfare optimization. Our main contribution is to show that by ignoring spatial heat and moisture transport and the resulting polar amplification, the regulator may overestimate or underestimate the tax on greenhouse gas emissions. The direction of bias depends on the relations between marginal damages from temperature increase in each region. We also determine the welfare cost when a regulator mistakenly ignores polar amplification. Numerical simulations confirm our theoretical results, while ballpark estimates indicate that the tax bias could be as high as 20%, while welfare cost could reach 2% of global effective steady-state consumption.