thumb|250px|right|Artistic impression of an asteroid slamming into tropical, shallow seas of the sulfur-rich [[Yucatán Peninsula in what is today Southeast Mexico. The aftermath of this immense asteroid collision, which occurred approximately 66 million years ago, is believed to have caused the mass extinction of non-avian dinosaurs and many other species on Earth. The impact spewed hundreds of billions of tons of sulfur into the atmosphere, producing a worldwide blackout and freezing temperatures which persisted for at least a decade.]]

An impact winter is a hypothesized period of prolonged cold climate due to an impact event upon the Earth's planetary surface by a large asteroid or comet. If such meteors were to survive atmospheric entry and successfully impact land or a shallow body of water, it would eject an enormous amount of debris, dust, ash and other particulate materials into Earth's atmosphere, blocking off irradiance from the Sun. This would lead to global dimming and cause the climatic temperature to decrease drastically. If an asteroid or comet with the diameter of about or more were to hit in a deep body of water or explode mid-air before hitting the surface, there would still be an enormous amount of debris released into the atmosphere.

It has been proposed that the rapid climate shifts associated with impact winters could cause severe floral disruptions and subsequently cascading collapse of food chains, leading to mass extinctions of the Earth's biosphere that wipe out many of the world's extant species. An example of such an event would be the Cretaceous–Paleogene extinction event, which probably involved a worldwide impact winter caused by the diameter Chicxulub impactor, and led to extinction of nearly all tetrapods weighing more than , which included all non-avian dinosaurs, pterosaurs and Mesozoic marine reptiles.

Possibility of impact

Each year, the Earth is hit by diameter meteoroids that deliver an explosion above the surface with the power equivalent of one kiloton of TNT. The Earth is hit every day by a meteor less than in diameter that disintegrates before reaching the surface. The meteors that do make it to the surface tend to strike unpopulated areas and cause no harm. A human is more likely to die in a fire, flood, or other natural disaster than to die because of an asteroid or comet impact. though when first discovered the probability was 0.3%.

Necessary impact factors

The Earth experiences a never-ending barrage of cosmic debris. Small particles burn up as they enter the atmosphere and are visible as meteors. Many of them go unnoticed by the average person even though not all of them burn up before they hit the Earth's surface. Those that strike the surface are known as meteorites. Although extinction events happen very rarely, large projectiles can do severe damage. That is over six billion times larger than the atomic bomb yield (16 kilotons, 67 TJ) that was dropped on Hiroshima during WW2. This impactor excavated the Chicxulub crater that is in diameter. With an object this size, dust and debris would still be ejected into the atmosphere even if it hit the ocean, which is only deep. This is controversially purported to have occurred following the Toba eruption.

These pulverized rock particles would remain in the atmosphere until dry deposition and due to their size, they would also act as cloud condensation nuclei and would be washed out by wet deposition/precipitation, but even then, about 15% of the sun's radiation might not reach the surface. After the first 20 days, the land temperature might drop quickly, by about 13°C. After about a year, the temperature could rebound by about 6°C, but by this time about one-third of the Northern Hemisphere might be covered in ice. This volcanism could alone therefore create a volcanic winter, irrespective of the other impact effects.

Multiple firestorms

In combination with the initial debris ejected into the atmosphere, if the impactor is extremely large ( or more), like at the Cretaceous–Paleogene extinction event (estimated ), there might be the ignition of multiple fire storms, possibly with a global reach into every dense and therefore firestorm-prone forest. These wood fires might release enough amounts of water vapor, ash, soot, tar and carbon dioxide into the atmosphere to perturb the climate on their own and cause the pulverized rock dust cloud blocking the sun to last longer. Alternatively it could cause it to last for a much shorter time, as there would be more water vapor for the rocky aerosol particles to form cloud condensation nuclei. If it causes the dust cloud to last longer, it would prolong the Earth's cooling time, possibly causing thicker ice sheets to form.

Impact on humans

thumb|right|Artist's impression of the [[Toba eruption, circa 74,000 years ago. Some scientists believe this eruption led to a population collapse and subsequent genetic bottleneck in humans.]]

An impact winter would have a devastating effect on humans, as well as the other species on Earth. With the sun's radiation being severely diminished, the first species to die would be plants and animals who survive through the process of photosynthesis. This lack of food would ultimately lead to other mass extinctions of other animals that are higher up on the food chain and possibly kill up to 25% of the human population. These factors would make life on Earth, for humans, extremely difficult.

Agriculture

With the Earth's atmosphere full of dust and other material, radiation from the sun would be refracted and scattered back into space and absorbed by this debris. The first effect on the Earth, after the blast wave and potential multiple fire storms, would be the death of most, if not all, of the photosynthetic life forms on Earth. Those in the ocean that survive would possibly become dormant until the sun came out again.

Economics

The cost to clean up after an asteroid or comet impact would cost billions to trillions of dollars, depending on the location impacted. or 96.5% of an estimated total of about 920.

See also

  • Asteroids in fiction
  • Comets in fiction

References

  • Geotimes story on the Chicxulub impact and the possibility of an impact winter
  • NASA's Near Earth Object Program