Actually, a couple of flares however massive might fry a few satellites and cause Northern/Southern lights 'way south or north, but will hardly cause global warming.

But one theory I have read within the past couple of years is that, truly, global warming or cooling *is* due to overall fluctuations in the Sun's output of energy.

For one interesting scenario about global warming, read Barnes' Mother of Storms. This isn't the reviews section, but it's a pretty nice book.