Headline-making firestorms in southeast Australia in 2003, responsible for at least 500 destroyed buildings and four lost lives, culminated with pyro-cumulonimbus (pyroCb) "eruptions" that ravaged Canberra on 18 January. Here we reveal that in their 3-hour lifetime, the Canberra pyroCbs also produced a stratospheric smoke injection that perturbed the hemispheric background analogous to the theorized "nuclear winter." We use an unprecedented array of data to analyze the Canberra pyroCbs' distinctive stratospheric impact, microphysics, energetics, and surface manifestations-including suppressed precipitation, an F2 tornado, and black hail.