Glossary

**Extinction** is the complete and permanent disappearance of a biological species, occurring when no living individuals of that species remain anywhere on Earth. Species become extinct due to a range of environmental and evolutionary factors, including habitat fragmentation, climate change, natural disasters, overexploitation, interspecific competition, genetic inbreeding, and declining reproductive success. An estimated 99% of all species that have ever existed are now extinct. Under normal conditions, species disappear at a low, continuous rate of roughly one to five species per year across the entire fossil record, a process termed **background extinction**. Periodically, however, extinction rates spike dramatically during **mass extinction** events, in which a substantial proportion of Earth's biodiversity — typically 75% or more of species — is lost within a geologically brief interval. These catastrophic events, driven by asteroid impacts, large-scale volcanism, rapid climate shifts, and other global-scale perturbations, fundamentally restructure ecosystems and open ecological niches for surviving lineages, thereby shaping the trajectory of evolution.

An impact winter is a hypothesized period of prolonged global cooling and darkness triggered by the injection of massive quantities of dust, sulfate aerosols, and soot into the stratosphere following the collision of a large asteroid or comet with Earth. In the specific context of the Cretaceous–Paleogene (K–Pg) mass extinction approximately 66 million years ago, the Chicxulub impactor—an asteroid roughly 10–12 km in diameter—struck the Yucatán carbonate platform in present-day Mexico, ejecting enormous volumes of fine silicate dust from pulverized bedrock, sulfate aerosols from vaporized anhydrite target rock, and soot from both the combustion of sedimentary organic carbon within the crater and subsequent global wildfires. These atmospheric contaminants partially to almost completely blocked incoming solar radiation, reducing surface sunlight to levels insufficient for photosynthesis. The resulting impact winter produced severe global surface cooling—modeled estimates range from approximately 15 °C to over 26 °C below pre-impact temperatures—and persisted on timescales of years to decades. The collapse of photosynthesis disrupted both marine and terrestrial food webs at every trophic level, making the impact winter the primary proximate killing mechanism in the K–Pg mass extinction that eliminated approximately 75% of all species, including all non-avian dinosaurs.