That the capacity of global models to predict the future can be well tested by their capacity to reconstruct past events is generally agreed, as is the definition of normal winter as the numerical equivalent of >5x103 degree-days (with the degrees in Fahrenheit). One-dimensional radiative-convective models of the interaction of radiation, aerosols and the atmosphere have recently been used to predict cooling of about 104 degree (F)days in response to the injection of some lOOx 10 12 g of smoke, much of which might arise from some 105 to 106 km2 of wildfires, themselves caused by a major nuclear exchange. More recent three-dimensional interactive global climate models, however, predict only some 102 degree-days of cooling from between 20 to 180x 10 12 g of realistic 'grey' smoke even in the worst case, at midsummer. The new models assume smoke injection on a continental scale followed by global transport. Temperature drops are from 6 to 18°C, associated with optical depths between l and 3, but decay to ambient conditions in between two weeks and a month. These results agree in large part with the observed outcome of a real smoke injection event of similar magnitude and greater persistence-the great Siberian fire in July and August, 1915.