This is the question... But what if the answer was not what we thought it was, or what we have been led to believe?
We know as absolute scientific fact that dirt and other debris in the atmosphere causes global cooling. Volcanoes, meteor strikes, etc, etc, etc, have shown us this through out history. Only in recent days has the thought that more pollution means higher temperatures...
Anyway, contrary to popular belief, the earth is much cleaner now than it was say 20 years ago. With all of that soot and dirt and yuck that has been removed, it has been documented that much more sunlight is getting to the surface of the earth than in previous years since the 18th century, before the industrial revolution.
A scientist, Christian Ruckstuhl of the Institute for Atmospheric and Climate Science in Switzerland, observed that warming over Europe was much more drastic than anthropomorphic global warming should be. What else could have caused this drastic warming trend over the last 30 years? He found that as we have cleaned up the atmosphere, more sunlight has gotten through, and, consequentially, the earth has warmed significantly.
Interesting, huh? The earth went through a significant cooling trend in the "Little Ice Age" due to very low solar activity and high volcanic activity. More junk from the volcanoes, and less solar radiation, made for a cold earth. After the 1850s though, the sun started to really pour out the radiation, peaking in 1998, and we really started to pour junk in to the atmosphere. Could it be that because of the junk we put in to the atmosphere, that we reduced a significant warming trend? And now that we are cleaning things up, we are experiencing the tail end of that warming trend?
It could be that global warming is indeed man made, but not because of greenhouse gasses, because of the environmental movement. How is that for irony?
Interesting thoughts, and a very good scientific read... if you can stay awake.