First off, I do believe humans have an effect on global climate for the following reasons: a. clearing of land for agriculture, b. building cities of concrete and asphalt, and c. emissions of CO2 through the burning of fossil fuels. As we all know, the one that is concentrated on is CO2 emissions perhaps since correcting that problem has the most straightforward solution.
The key question is how much effect each of those human activities has, especially CO2 emissions. Right now, CO2 is getting the lion's share of the blame for recent warming and that is based on two prominent pieces of evidence. First, CO2 has been shown in a lab to have a greenhouse gas capability to absorb and emit infrared radiation. Second, scientists have used this first piece of evidence to produce climate models based on fluid dynamic systems. In these models, they have taken past temperatures and CO2 levels and produced future "projections" which predict that increased CO2 levels will result in warmer global temperatures.
These climate models however make many assumptions about the complex interactions of the global climate system. One of the key assumptions they make is that the climate sensitivity is high. That means that putting extra CO2 into the atmosphere will result in a rise in temperature with no natural mechanism to counteract this. Several climate scientists (and I mean climate scientists not journalists or bloggers) have been studying climate sensitivity (which is a large part of climate feedback mechanisms). These scientists claim that the climate sensitivity estimated by the IPCC for the climate models is too high, perhaps much too high.
While it has yet to be proven how sensitive the climate actually is (to CO2 level increases for example), a soon to be published peer-reviewed scientific report will be coming out soon which suggests the estimates used for the model's climate sensitivity is on the order of 6 times too high: http://masterresource.org/?p=4307
To support the claim that IPCC model temperature projections are too high, we can look at the predictions vs the latest temperature trends. As you may know, there are many ways to cherry pick temperature data and pretty well every type of trend has been put forth from 10 year cooling to a reduction in warming to business as usual and everything in between. It's pretty hard to argue though that temperatures are rising exactly as predicted by IPCC models. One the lead IPCC modelers has even come out and said as much and predicted one or two decades of cooling.
So most people who believe in man-made global warming will point to the bulk of scientists and scientific organizations as a place of authority. That's not unusual. However, if you dig a little deeper, it's not at all as clear as it seems on the surface.