Why do you think some people have a difficult time believing that global warming is real?

I'm not so sure that people don't trust scientists in general, but they don't trust scientists in certain areas that disagree with their world view. Climate change and evolution are two examples that come readily to mind.

First, a web site I found informative about climate change. It's even run by real climate scientists. http://www.realclimate.org/

Climate change is happening, as every national and international science organization states that it is. Being that climate scientists follow the same rules of peer review as every other scientist; if you can't trust them then you can't trust any scientist.

Next is the question of whether or not climate change is being caused by humans. So far the science points to human activity. Greenhouse gasses emitted by human activity so far is best cause we have for a rapidly changing climate. Orbital cycles, volcanic activity, solar output, all of these (and more I likely don't know of) have been accounted for. This leaves human activity as the primary driver for climate change.

Carbon dioxide is a greenhouse gas. That is a basic property of carbon dioxide (and other greenhouse gasses). From basic radiative transfer, to increase the concentration of greenhouse gasses in an atmosphere increases the surface temperature. This is basic physics we see on earth, venus, and other planets. (Side note: earth's surface temperture would be about -18.8C [-1.8F] without any greenhouse gasses) So far all the peer reviewed science indicates that human activity is, on average, warming the planet.

So the conclusion that human activity is increasing the average temperature of the earth (global warming/climate change) is not in dispute (among the scientific community, pundits otherwise). The question now is, How bad will it get?

This leads the debate as to whether prevention or adaptaion is the more economical solution. Running with folklore (A ounce of prevention is worth a pound of cure) I'm inclined to side with prevention. I see high costs assosciated with adaption; as well as costs assosciated with prevention. Increased food costs from drought/storms, infrastructure costs from improving our water systems, costs from dealing with port facilities and sea level rise, and so on...