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Most meteors and asteroids burn up in the earth's atmosphere incinerating all or most of the incoming material before reaching the Earth's surface.
When a very large body collides with the Earth it can cause huge destruction and change global climate.
The comet/meteorite impact in Siberia theory of climate change has been considered to account for the extinction of the dinosaurs 65 million years ago. And another impact site dated at 65 millions years can be found on the tip of the Yucatan Peninsula in the Gulf of Mexico, Central America. Evidence, such as a thin layer of clay deposited at this time all over the globe contains the rare element iridium found only in meteors. An impact from an object 10km across could:
release huge quantities of vaporized material into the atmosphere, blocking out the Sun;
Cause a shock wave for hundreds of miles
Produce firestorms for thousands of miles
Rain debris containing the iridium coating plants
Create 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.
References:
Biggest extinction in history caused by climate-changing meteor, University of Western Australia
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