Global warming is caused by a lack of pirates. That's all there is too it. More pirates = less global warming. This
scientific graph proves it.
Okay, time to be serious (even if the FSM is the coolest thing ever). Climate change is something that naturally occurs. Over time, global temperatures will rise and fall like a sine graph. When temperatures are extremely low, we enter into an ice age. We're currently entering into one of the high temperature periods.
However, this is not to say that humans have no effect. Since the Industrial Revolution, the amount of carbon dioxide and other greenhouse gases in the atmosphere has steadily increased, corresponding to a steady increase in global temperatures not associated with the natural trend. The assumption, therefore, is that humans have, in a small way, changed the natural cycle. I like to think of it like adding a drop of water to a full glass. The one small drop may not compare at all to the amount of water in the glass, but it will still make the glass overflow.
In the global warming context, the drop is the CO2 humans have contributed to the atmosphere. The glass is the natural balance, and the water in the glass is the naturally-produced CO2 (from volcanoes, the ocean, decaying biomatter, etc.). The small human contribution, which in no way compares to the huge amount of naturally-occurring CO2, is still enough to screw up the balance, making the climate change worse than it would be otherwise.
The natural cycle is still occurring, but average temperatures are higher than they were before the Industrial Revolution. Think about it: the old ice ages covered much of the Northern Hemisphere with ice, but the most recent ice age (the Little Ice Age) was only enough to make things a bit colder than usual. My conjecture is that the increased temperature since the Industrial Revolution is what caused the Little Ice Age to be so, well, little.
In short, global warming is a naturally-occurring phenomenon that humans have made worse, if only by a small amount. I still prefer the pirate explanation, though.
The more you know, the less you don't know.