Flying over thin ice

Kwok and Untersteiner reply: As discussed in our article, an average heat imbalance at the surface on the order of 1 W · m^–2 is required to explain the observed thinning of the Arctic sea ice cover during the past three decades. And the level of scientific understanding of the geophysical processes responsible for delivering that heat to the sea ice cover remains low. Comparatively, the current best estimate for the globally averaged radiative forcing by persistent linear aircraft contrails¹ from 2000 to 2005 is +0.010 W · m^–2, where a positive forcing leads to mean surface warming and a negative forcing to cooling. Even though uncertainties are involved in determining global values of contrail forcing, that level of forcing, if distributed uniformly over the globe, is unlikely to account for the observed thinning of the Arctic ice cover.