Approximately 65 million years ago, or so the current theory holds, an asteroid plummeted to Earth, likely just off the Yucatan peninsula in Mexico. The K-T (Cretaceous-Tertiary) mass extinction event was responsible for the extinction of the non-avian dinosaurs, along with many species of other reptiles, plants, and invertebrates.
The object that hit the earth was at least 17 km in diameter, and created a crater over 180 km across. Travelling at a likely speed of 22 kilometers per second, it would have hit with a force of 480 million Megatons of TNT. Compare this to the biggest nuclear weapon ever detonated, the 50 Megaton yield Tsar Bomba. The volcanic eruption of Krakatau, in comparison was “only” 200 Megatons, and yet the sound from it was heard 5000 km away.
Impacts like the K-T event are very rare, with a calculated frequency of about once every 500 million years. However, even a strike from a much smaller object can cause a great deal of damage, not just to human works and lives, but to the environment as well. A 5 km diameter asteroid is likely to hit about every 26 million years, with a force of about 9.4 million Megatons. From 500 kilometers away, such an impact would produce a visible fireball many times larger than the sun, and cause third degree burns.
But even that sort of impact is rare. A 500 meter object is likely to strike every 15,000 years, while a 50 meter object should strike the earth at intervals of less than a thousand.
The Torino Scale was created to measure the threat level of an asteroid, from level 0 (No threat) to Level 10 (catastrophic collision certain). The highest level the Torino scale has ever gotten was level 4, in 2004, when the asteroid 99942 Apophis was calculated to have a greater than 1% chance of hitting the Earth in 2036. More calculations revised the threat level downwards, and it currently is not considered to be a threat.
99942 Apophis is a 270 meter diameter chunk of rock. If it were to hit, the environmental effects would be severe, though likely short-lived. While local damage at the impact zone would be serious, the real concern is the amount of dust and other ejected material the impact would throw into the atmosphere. This could disrupt weather patterns for several years, cooling things off and damaging crops across the globe. Recovery would be on the order of a few years before the dust was cleared out and things returned to normal.
Larger objects would have correspondingly greater effects, including serious earthquakes, the risk of tsunamis, damage from atmospheric shockwaves, and even the risk of firestorms from the heat of the impact. Even 1000 km away from the dinosaur killer, an observer would have still suffered 3rd degree burns over much of their body, from a fireball that would have appeared 38 times larger than the sun.
Asteroid impacts are notable in our list of possible environmental disasters in that they are one of a comparative few that are completely beyond human control, or recourse. Aside from a few wild-eyed ideas, and the movies not withstanding, there is little or nothing we could do to divert an asteroid or comet.
Our best bet is to watch for them, to prepare, and try to determine how to survive in their aftermath.