When lightning strikes the ground, it heats, melts, and fuses the sand in soils to form glass tubes known as fulgurites. We report here the composition of CO2, CO, and NO contained within the glassy bubbles of a fulgurite from the Libyan Desert. The results show that the fulgurite formed when the ground contained 0.1 wt% organic carbon with a C/N ratio of 10–15 and a δ13C of −13.96‰, compositions similar to those found in the present-day semiarid region of the Sahel, where the vegetation is dominated by C4 plants. Thermoluminescence dating indicates that this fulgurite formed ∼15 k.y. ago. These results imply that the semiarid Sahel (at 17°N) reached at least to 24°N at this time, and demonstrate that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology.