The end-Cretaceous mass extinctions, 65 million years ago, profoundly influenced the course of biotic evolution. These extinctions coincided with a major extraterrestrial impact event and massive volcanism in India. Determining the relative importance of each event as a driver of environmental and biotic change across the Cretaceous-Tertiary boundary (KTB) crucially depends on constraining the mass of CO₂ injected into the atmospheric carbon reservoir. Using the inverse relationship between atmospheric CO₂ and the stomatal index of land plant leaves, we reconstruct Late Cretaceous-Early Tertiary atmospheric CO₂ concentration (pCO₂) levels with special emphasis on providing a pCO₂ estimate directly above the KTB. Our record shows stable Late Cretaceous/Early Tertiary background pCO₂ levels of 350-500 ppm by volume, but with a marked increase to at least 2,300 ppm by volume within 10,000 years of the KTB. Numerical simulations with a global biogeochemical carbon cycle model indicate that CO₂ outgassing during the eruption of the Deccan Trap basalts fails to fully account for the inferred pCO₂ increase. Instead, we calculate that the postboundary pCO₂ rise is most consistent with the instantaneous transfer of 4,600 Gt C from the lithic to the atmospheric reservoir by a large extraterrestrial bolide impact. A resultant climatic forcing of +12 W·m² would have been sufficient to warm the Earth's surface by 7.5°C, in the absence of counter forcing by sulfate aerosols. This finding reinforces previous evidence for major climatic warming after the KTB impact and implies that severe and abrupt global warming during the earliest Paleocene was an important factor in biotic extinction at the KTB.