Impact Winter

An impact winter is a period of prolonged cold weather caused by the impact on the Earth surface or atmosphere of a large asteroid, comet or gamma-ray burst. If such an impact occurred on land or the floor of a shallow sea, it could cause large amounts of dust or ash to be thrown into the Earth's atmosphere, blocking the Sun's light and dramatically lowering the amount of sunlight reaching the Earth's surface. Impact winter is one of the mechanisms proposed for extinction events, such as the asteroid impact at Chicxulub in Mexico which supposedly led to the extinction of the dinosaurs.

Depending on the size of the object, and the location and angle at which it hits the earth, material can be thrown into the atmosphere by two mechanisms:

1. The impact could eject large amounts of regolith (and perhaps shattered bedrock) into the atmosphere
2. The impact could produce a global firestorm or strike a heavily forested area, throwing up large amounts of smoke and ash into the atmosphere.

The latter scenario is the more dangerous, as the lighter particles from the fire would take weeks or months to fall back to earth, and could be distributed by jet streams around the world, making the cooling a global event.

The mechanism of impact winter is very similar to that which occurs after nuclear war, leading to nuclear winter. Volcanoes also eject large amounts of opaque material into the higher parts of the atmosphere, with large explosions such as the 1991 explosion of Mount Pinatubo and the much larger 1783 Laki eruption, having measurable effects on the world's climate. The simultaneous eruption of a number of large volcanoes, a catastrophic volcanic winter scenario, is another proposed mechanism for extinction events.