Comets & Meteorites
The Earth's atmosphere protects us from the impacts of comets and meteorites, by vaporising all or most of the incoming material before reaching the Earth's surface. Scientists believe however, that every once in a while a celestial body of significant size collides with the Earth, causing untold destruction and an ensuing global climate change.
The comet/meteorite impact theory of climate change has been considered to account for the extinction of the dinosaurs 65 million years ago. Although critics of this theory suggest volcanism offers a better explanation for the mass extinction, evidence does exist which seems to support the case for a meteoritic impact which occurred at this time in Earth History. A proposed impact site that has been dated at 65 millions years can be found on the tip of the Yucatan Peninsula in the Gulf of Mexico, Central America. In addition, a thin layer of clay deposited at this time at many localities around the Earth contains the rare element iridium. Although released in volcanic lava originating from deep within the Earth's interior, the impact theory indicates that the required iridium could come from a celestial object 10km across. An impact from a body this size would have released huge quantities of vaporised material into the atmosphere, blocking out the Sun and causing an initial "impact winter". Debris containing the iridium would gradually return to Earth forming the clay deposit. After this there would have been an increase in temperatures caused by the large amounts of carbon dioxide released by a spread of global fires. In addition, chemical reactions taking place in the atmosphere between pollutants would result in the formation of globally distributed acid rains.
Luckily such impacts only occur rarely, perhaps every few million or tens of millions of years. Although smaller objects hit the Earth more frequently, they have much less impact. Nevertheless, it is almost certain that another large comet or meteorite will at some time in the future strike the planet with potential consequences for the global climate and for life on Earth.