Another effect that could result from global warming would be the north Atlantic Currrent turning off. At present this brings enormous volumes of warm water from the Carribean north to western Europe warming our climate considerably. If the current stops flowing its going to get a lot colder in the UK, an ironic effect of global warming, see the attachment for more detail and a map.

See:
http://www.nasa.gov/centers/goddard/news/topstory/2004/0415gyre.html