not sure about global warming but an asteroid that could destroy totally a big city is likely to hit Earth in the next 100 years (not a specific asteroid, but just talking about statistics)
http://www.gfy.com/showthread.php?t=...light=asteroid
Quote:
How often is Earth hit by an asteroid?
More frequently than you may think. An incident like Tunguska occurs approximately once per every hundred years. Smaller ones, but easily big enough to take out a large city, occur at least 3 times per century, for example Brazil in 1930. While most of the recent recorded impacts have happened in places that were barely inhabited, we won't always be that lucky. If the Tunguska event had happened over the city of New York it would have been nearly leveled.
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and for the record, such small objects are NOT monitored, so this could happen at any time