As hypervelocity ejecta from the Chicxulub (Yucatán, Mexico) impact fell back to Earth, the surface may have received a deadly dose of thermal radiation sufficient to ignite global wildfires. Using a two-phase fluid flow code, which includes ejecta and air opacities in a radiative transfer calculation, we modeled the atmospheric reentry of spherules arriving at distal sites. The models predict a pulse of thermal radiation at the surface peaking at 5–15 kW/m2, analogous to an oven set on “broil” (~260° C). Previous calculations, which ignored spherule opacity, yielded >10 kW/m2 sustained over >20 min and such an extended pulse is thought to be required for wood ignition. However, the new modeling suggests that fluxes only exceed the solar norm for ~30 min and are only >5 kW/m2 for a few minutes. Previous models failed to consider the self-shielding effect of settling spherules, which block an increasing proportion of downward thermal radiation emitted by the later-arriving spherules. Such self-shielding may have prevented widespread wildfire ignition, although the thermal pulse may have been sufficient to ignite localized fires and kill fauna lacking temporary shelter. An opaque cap of submicron dust in the upper atmosphere could, however, override the self-shielding effect.

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