Author Response

“The altitude of a cloud plays a cardinal role because clouds at different heights (i.e., temperatures) exert different feedbacks. High-altitude clouds absorb infrared radiation that comes from the lower atmosphere and radiate like blackbodies. Since the temperature contrast between elevated clouds and lower atmosphere is high, the clouds have a positive contribution on the local net energy balance. At the same time, their albedo is small because they are, in most cases, comparably thin. Thus, they can warm the atmosphere more than they cool it, exerting a positive feedback. Conversely, low-altitude clouds are strongly reflecting objects owing to their high optical density but they ineffectually shield infrared radiation emitted by atmospheric gases to the outer space. The reason is that the temperature of low clouds is closer to the temperature of the ground and the local net energy balance is close to zero. These clouds in turn can cool the climate system more than they warm it and thus exert a negative feedback. Loeb et al., 2012 observed this mechanism in a study that focused on clouds in the tropical belt. However, this situation may change when considering the Pacific Northeast over a decadal time window. Evidence of a positive feedback by low-level clouds has already been demonstrated (Clement et al., 2009). It is therefore likely that no general description is possible on a global scale and regional studies should be conducted instead.”