 At the Geophysical Fluid Dynamics Laboratory Mahlman collaborated with Syukoro Manabe to develop dynamical models of the stratospheric circulation that demonstrated the importance of meanders in the polar jet stream for producing exchange between the polar and subtropical stratosphere.He then worked to extend these models to include the chemistry of nitrous oxide and ozone.While Mahlman was skeptical of early work that suggested that chlorofluorocarbons were responsible for depleting the ozone layer, measurements of high levels of free chlorine in the ozone hole caused him to change his mind in late 1987, and he later was one of the first to sound the alarm about ozone depletion over the Arctic.