About 66 million years ago, a large fraction of the planet's species, including the non-avian dinosaurs, vanished when an asteroid hit the Yucatan Peninsula, likely triggering the largest short-term climate change in geologic history. Yet there have been no modern simulations of this climate change. We outline the initial conditions needed for such global climate simulations. There is much unknown about the aftermath of the impact. We discuss uncertainties and suggest new observations.