Re: How would weather patterns be affected by the advent of natural disasters?

Hello Thomas,

This is an interesting question that raises a rather alarming possibility - 
that a large asteroid impact would do more than devastate the region where 
it landed but may have global effects on systems like the weather.

The type of asteroid you have described is on the large side but is not the 
largest object that could conceivably impact with earth.  It is also likely 
to be travelling faster than you have imagined - tens of miles per second! 
 The effects of asteroid impacts are very dependant on the size.  Smaller 
objects less than 100m are likely to break up in the atmosphere but still 
with massive local effects.  Impacts on the climate are only likely to 
occur for an asteroid large enough to produce a very large crater and send 
large quantities of dust and gasses into the atmosphere.  Current thinking 
suggests an asteroid of about 2km across and certainly larger than 1km.  Of 
course anything even bigger like a comet 10km across would have even more 
devastating consequences.

In researching this answer the best web site I found was the report of the 
Spaceguard Survey on the NASA web pages.  This site discusses the risks 
associated with different sizes of asteroid and describes the likely 
effects of a large asteroid much better than I can so I have copied the 
relevant section for you:

"Dust thrown up from a very large crater would lead to total darkness over 
the whole Earth, which might persist for several months. Temperatures could 
drop as much as tens of degrees C. Nitric acid, produced from  the burning 
of atmospheric nitrogen in the impact fireball, would acidify  lakes, 
soils, streams, and perhaps the surface layer of the oceans."
 
Spaceguard Survey, Chapter2, 
Spaceguard Report.
 
In addition longer term effects might be cuased due to the water vapour and 
CO2 released into the atmopsphere by the fireball and forest fires etc.  As 
a result additional golbal greenhouse warming might occur whcih would 
further stress plant animal nah humans that had lived through the period of 
dark and global cooling.

Fortunately the likelihood of one of these objects impacting earth soon is 
small.  For global scale effects an asteroid of about 2km across is 
probably required.  Impacts with objects this size are thought to occur 
only every 300,000 - 500,000 years.  Our understanding of the nature of 
these objects and the process of mapping their orbits is now progressing so 
we have a good chance of being able to predict impacts in the future and 
perhaps even nudging dangerously close asteroids away from the earth.

For lots more information about asteroid and comet impacts have a look at 
the NASA pages at:
Comets and Asteriod 
Impact Hazards.

Ivan Gee, Atmospheric Research and Information Centre, Manchester, UK.