The decay of Martian global (great) dust storms is investigated. One-dimensional (vertical, static atmospheric) and two-dimensional (latitude-height, steady state circulation) simulations carried out with an aerosol transport-microphysical model indicate that atmospheric motions play a significant role in the observed decay of global dust storms. Spacecraft observations (Mariner 9, Viking) of the 1971 and the two 1977 planet-encircling dust storms have provided suggestions about some characterisric of storm decay. Specifically, the dust particle size distribution is inferred to have remained essentially unchanged for particles with radiii between 1 and 10 μm during decay of the 1971 storm, and surface visible opacity declined quasi-exponentially with time in northern mid-latitudes during the decay of the two 1977 storms. The results from this investigation indicate that two- and three-dimensional dynamical processes play a significant role in the observed deay features of Martian global dust storms. The most important processes are the lofting of dust by vertical motions in the dust source region of the southern hemisphere subtropics and a continuing advective resupply of atmospheric dust into the dust sink regions of the northern hemisphere. Our work has implications for Viking data analyses and future Mars observer observations and requires that the particle size distribution be treated as a time and latitude dependent quantity. ¿American Geophysical Union 1990