What will happen id a 40 by 100 km asteroid hits the earth?

Asteroid Bast 1908  - The blast of june 1908
By Nic
@academic2 (7005)
July 5, 2008 7:28am CST
I read some scientiic journal that At 7:17am on 30 June 1908, an immense explosion tore through the forest of central Siberia.It was a small asteroid which was just a few meters long and wide which hit serbia 100 years ago today-it generated energy and fire that was 1000 times much greater than the Atomic bomb dropped in japan in 1945-it flaten 2 mllion trees and its force threw people about like paper! what will happen to our world if 40 by 100 km asteroid hit our planet?
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