We have simulated the evolution of an optically thick dust cloud in the Earth’s atmosphere and have also calculated the effects such a dust cloud would have both on the amount of visible light reaching the surface and on the temperature at the Earth’s surface. The dust cloud simulations utilize a sophisticated 1-D model of aerosol physics. We find that large quantities of dust remain in the atmosphere for periods of only 3 to 6 months. This duration is fixed by the physical processes of coagulation, which cause micron-sized particles to quickly form and sedimentation that swiftly removes the micron-size particles from the atmosphere. The duration of the event is nearly independent of the initial altitude, initial particle size, initial mass, atmospheric vertical diffusive mixing rate, or rainout rate. The duration depends weakly upon the particle density and the probability that colliding particles stick together to form a larger particle. The duration is also limited by the rate at which the debris spreads from the initial impact site. The dust must be uniformly spread over a large fraction of the Earth within a few weeks or the duration of the event will be less than 2 months. We used a doubling code to calculate the visible radiative transfer in these dust clouds. We find that light levels are too low for vision for 1 to 6 months and too low for photosynthesis for 2 months to 1 year. Calculations of the surface temperature show that the oceans cool by only a few degrees owing to their large heat capacity. However, continental surface temperatures drop below freezing for approximately twice as long as sub-photosynthetic light levels persist. We speculate briefly upon several other effects that might occur after the dust clears due to widespread snowfields, enhanced H 2 O amounts, or chemical changes in the atmosphere. We also speculate that low light levels would cause a collapse of the marine food chain and oceanic extinctions. Cold temperatures over the continents and low light levels would prevent some animals from finding food and would cause continental extinctions.