SW might point out that a series of recurrent major droughts in the 10th and 11th centuries CE seemed to have taken down the Classic Maya civilization.  A bit later, a similar climatic pattern in North America probably whacked the proto-urban Anasazi culture and their contemporaries in the American southwest in the 12th - 13th centuries.  

Such things can and do happen on long-term time scales on this variable ol' planet of ours, and SW is correct to suggest that a prudent society would take steps to deal with them.  While I agree with SW on that point, I don't agree that the solution is all of us going back to live like the Amish.  (I rather like 21st C. tech, don't you?)   :)  And, having lived in Arizona for a few years (where it is d****d hot in summer), I know that agriculture certainly is not only possible, but profitable, under higher temperatures than we have in Oregon.

Build a pipeline to bring in Canadian lakewater instead of tar sands oil, for starters.  Problem solved.