A massive asteroid impact would create long-term changes in the atmosphere and climate of the planet. Upon impact, vaporized dirt and rock would fill the atmosphere, blocking sunlight and creating a state of near-permanent darkness and winter-like conditions. After the particles in the atmosphere settle, an increase in greenhouse gases from impact fires and released stores in the crater area could actually increase temperatures substantially due to an increased greenhouse effect. The severity and length of each of these phases would depend on the size of the impact, with a one or two kilometer-wide asteroid causing enough damage to trigger worldwide disturbances for years.