The after-effects of a large asteroid or comet impact would be numerous, and the destructive power of the impact would extend well beyond the region of the crater. It is thought that a 200 metre object could destroy a large city.
The first dangerous effect would be the enormous shock wave generated by the impact. This would move outwards very rapidly, resulting in scorching hurricane force winds, strong earthquakes and widespread fires.
Shortly after the shock wave, we would see severe changes in the weather, resulting from the millions of tonnes of dust (land impact) or water vapour (sea impact) released into the atmosphere during impact. Computer simulations suggest that around 1000 times the volume of the asteroid are released directly into the upper atmosphere through a process known as the "chimney effect".
As a result of all this dust and/or water vapour, the atmosphere would become opaque and block out sunlight. This would lead to a fall in the global temperature, and in the worst case prevent any photosynthesis taking place. The dust could remain airborne for many months, if not years. In addition, chemical changes in the atmosphere could lead to an imbalance in the amount of oxygen, carbon dioxide and nitrogen in the air, which are vital to life.