Look it's like this:
There have always been variations, warmings and coolings. But this one is different.
Pretty much every reputable scientific organisation now agrees that human activity is the reason for this.
What we don't know for certain is how bad it will be.
There are a small number of people who are still refusing to accept this - often for political reasons.
Here is the Royal Society's page refuting the arguments most commonly put forward by the skeptics:
http://royalsociety.org/page.asp?id=6229
The Earth's climate is always changing is dealt with in point 1
The idea that it's caused by the Sun is point 6
Etc.
Of course you could believe that thousands of the most capable scientists in the world have all missed these obvious points
Or like youngmafbog choose to believe that actually it's all a gigantic conspiracy between hundreds of thousands of scientists across the world to get more funding.
But I think were rather heavilly into Dan Brown country by then aren't we?