Whack-a-Mole

Remember the arcade game?  Moles pop up in a field of holes and you club them down with a mallet? 

Unfortunately, our planet is engaged in cosmic game of whack-a-mole.  How bad is it?  I've undertaken realistic scientific simulations using a common sledge hammer and a hamster and I have to say it's not pretty.  In fact, it's so "not pretty" that decorum prevents me from posting pictures of it.

But seriously folks:

Small asteroids - five to 10 metres in diameter - enter the Earth's atmosphere about once a year, but normally explode before impact. Larger minor planets - of about 1km in size - strike every 500,000 years.

I was initially impressed by the "meteor impact killed off the dinosaur" thing until I learned that the Jurassic dinosaur populations showed a lot of signs of trouble well before the KT boundary, and that many dinosaurs have been found after the KT impact.  So, while I doubt the mass extinction link in that case, it's pretty darned obvious that a large impact would make any of your other troubles look like a really sweet day.  On a long enough time scale a big impact is inevitable.  The Telegraph (UK) has an interesting article on how our increased efforts to map of the number of asteroids has drastically increased the number we're aware of.  It has been said that the total number of scientists doing this work wouldn't even staff the average sized MacDonald's, so it's impressive results. 

Watch the video.  Do it in full screen, HD mode.  After you read the UK link explaining what it all means.