A new study finds strong evidence that an asteroid or comet hit the earth about 13,000 years ago and may have played a role in a dramatic climate shift around that time. A 16 member team from UC Santa Barbara has found nanodiamonds and other artifacts of the theoritical impact which fits like a glove with a sudden cold snap at the end of the most recent ice-age called the Younga Dryas:
(Red Orbit) — The data suggest that a comet or asteroid –– likely a large, previously fragmented body, greater than several hundred meters in diameter –– entered the atmosphere at a relatively shallow angle. The heat at impact burned biomass, melted surface rocks, and caused major environmental disruption. .. The sediment layer identified by the researchers is of the same age as that previously reported at numerous locations throughout North America, Greenland, and Western Europe. The current discovery extends the known range of the nanodiamond-rich layer into Mexico and the tropics.
If you play around with impact simulators, and make some middle of the road assumptions, the consequences of of an object 400 meters in diameter are significant.
Via the Purdue University Earth Effects Program: assuming you are 100 km away, a rocky object 400 meters in diameter arriving at 20 km/s at a 30 degree angle striking sedimentary rock produces a crater almost five km across and 500 meters deep. It delivers 4.26 gigatons of TNT blast and is ten times brighter than the sun (Meaning if you saw it streak overhead or hit on the horizon it would blind you at least temporarily and probably do permanent damage to your retina as well as giving you first degrees burns over any exposed skin). Fragments over an inch in size would rain down for several minutes at least, setting everything on fire, and magnitude 7 quake would be felt for a hundred miles or more from the blast.