The issue with most global warming 'evidence' is that it comes from intentionally manipulated charts. Any chart starting in the 1800s is about as close to a lie as you can get. Almost every one of them starts in the 1800s.
Why should we care? Well, from about 1350 to 1850 there was a period known as the 'little ice age'. This was a historically cold period. As shown in the chart below, the temperature of the earth has fluctuated many times in the last 5,000 years.
If you change the focus and the length of your observation, you can make the charts say whatever you want. If my dog died Tuesday, and the neighbor mowed his yard Tuesday, I could easily make a chart showing that his mowing coincided with my dog dying. No reasonable person would accept the conclusion that his mowing caused my dog to die though(unless he was a mutilated bloody chopped up corpse). If you made a chart that showed the entire month, and you saw he mowed his yard 4 times that month then the correlation no longer exists. This is how AGW proponents intentionally fudge charts to show that mankind is the cause of global warming. This also is how they show CO2 causing global warming.
The chart below shows CO2 concentrations in correlation or lack thereof with temperature. Notice that it correlates about as well as two lines drawn by a 2 year old.
Note these two charts. The first is CO2 and the second is Temperature. They appear to correlate.
However, when you zoom out and look at the history of the earth you come up with a difference picture, the one from above that looks like a little kid drew on it. I think the most telling thing that all the charts show is that CO2 rises AFTER the temperature rises. Higher temperatures come before the higher CO2 all through history.
We know in the 1800s, BEFORE the industrial revolution was starting that the temperatures were already rising, and had little to nothing to do with mankind. We must then accept that the increase was at least partially natural. Even AGW proponents agree that up to 1940 it was mostly a natural warming of the earth. Given that we really don't understand what causes the hot and cold periods we so often see perfectly, is it so hard to believe that a significant portion of our current temperature is just the natural fluctuation of the earth?
Look at the blue part of the chart, note the 'current temperature' mark. Note the fairly even spacing between spikes of heat? It gets 4-5 degrees hotter than it is right now every 100k years give or take. It just so happens that we are around 100k years from the last heat snap. If we expect the pattern to repeat itself then we are looking at a plateau temperature very soon, a thousand years or so of even temperatures, and then a 10 or 15 degree drop. I hope it is obvious to everyone that the temperature of the earth is always changing rapidly and never remains the same.