The phrase "cometary impact" refers to the collision of a comet, which is a large chunk of ice and dust from space, with another object like a planet or moon. Full definition
Increasing CO2 does increase the greenhouse effect, but there are other factors which determine climate, including solar irradiance, volcanism, albedo, orbital variations, continental drift, mountain building, variations in sea currents, changes in greenhouse gases, even cometary impacts. (legalinsurrection.com)
The determination of the vertical profile of water in the atmospheres of the outer planets and Titan is a diagnostic of its origin (e.g. cometary impact vs. permanent dust deposition), but also of the rate at which water is transported within and removed from the atmosphere (Hartogh et al. 2009). (swsc-journal.org)
Although major impactors have become comparatively rare occurrences during the past 500 million years, the extinction of the Dinosaurs may have been assisted by a large asteroidal or cometary impact about 65 million years ago centered near Puerto Chicxulub, at the tip of Mexico's Yucatan Peninsula. (members.nova.org)