Let's not forget that the Artic Ice cap is almost gone, Greenland is melting at an ever accelerating rate(it's albedo rate will change), Antartica is calving huge ice sheets, glaciers and permafrost are disappearing...on and on.
The reason is really a combination of factors, some of which we may not be fully aware of, most natural(like the Milankovitch Cycle and others); BUT some manmade. We need to react sooner than later and quibble about how big a role "we" are playing in global "melting" later. I call it melting because that is the most visible effect and will also be the first.
Milankovitch cycles - Wikipedia, the free encyclopedia
IMHO...I think we are already too late and Greenland's ice sheet will melt. This will flood the North Atlantic with cold and fresh(low salinity) water. In turn this catastrophe will SHUT DOWN the Thermohaline Circulation in our world's oceans and cause a climate NIGHTMARE. This circulation is part of the Gulf Stream and used to be called the Atlantic Conveyor; the salinity drop and cold water will disrupt this global current.
What's funny is that this will cause another Ice Age; implying that global warming itself actually triggers an Ice Age. Don't be too skeptical...I'll post links..LOL.
Thermohaline circulation - Wikipedia, the free encyclopedia
Shutdown of thermohaline circulation - Wikipedia, the free encyclopedia
Greenland ice sheet - Wikipedia, the free encyclopedia
Greenland Ice Sheet Is Melting Faster, Study Says
Penn State Live - Greenland and Antarctic ice sheet melting, rate unknown
Global Warming or the Big Freeze
Younger Dryas - Wikipedia, the free encyclopedia
http://www.youtube.com/watch?v=2LR0iZSI9HI
http://www.youtube.com/watch?v=Lrn14Jd-aWI
http://www.youtube.com/watch?v=Fty3zniY5P8
http://www.youtube.com/watch?v=QeNaBga8wgg
http://www.youtube.com/watch?v=3gv7MKCfcLw
WHEW...LOL..We'll see???