Consequently, there needs to be a better justification for making climate predictions which assume constant relative humidity. The treatment of clouds as black bodies is only an approximation. In fact, the extinction and absorption properties of cloud drops are quite sensitive to droplet size for the sizes typically found in clouds. It is important to quantify the influence of cloud absorption. The important point is that reflectance can either increase or decrease with increasing droplet concentration depending on the way in which absorption increases with droplet concentration.
Shiptrack experiments help to quantify the effect of air pollution on clouds.
Shiptracks are found to be twice as bright as cloud surrounding the tracks, implying a factor of 8 increase in droplet concentration, and consequently a halving of mean radius in order to conserve liquid water content. The point is that these sensitivities are as big as the direct CO 2 doubling effect, so that our uncertainty about which scenario is most realistic has important implications for our global change predictions. The approximate treatment of precipitation and infrared transmission in clouds together with uncertainties in the effect of a warmer climate on the global aerosol loading introduce potentially significant sources of error into current global change predictions.