Global warming deniers and 9/11 "truthers" are equally full of crap. In fact, they're almost identical in their whole style of argument and their standards of evidence that they'd be nearly indistinguishable if they didn't have separate pet issues. In both cases, you've got yourself a narrative that the believer (or non-believer, in the case of global warming) subscribes to and must not deviate from. Also, they both have a face of their enemy (Bush/Cheney for the truthers, Al Gore for the deniers). Lastly, they both cling to talking points that sound like slam-dunk arguments, but if they took the time to read up on anything that's not espousing their predetermined viewpoint, they'd see just how full of crap they are.
I can just imagine a global warming denier reading this and saying, "No way! I'm nothing like those truthers! Those people are crazy!" Of course, the truthers no doubt are cringing at the comparison to them and the climate change deniers. I suppose that there might even be some people who deny both global warming and that Muslim extremists attacked us on 9/11. Those people are doubly mad, I guess.
I don't feel like going over every single argument and every single piece of evidence, but allow me to be at least a bit more specific. With your deniers, they like to pull out the old "They said there'd be global cooling in the 1970s!" This makes them feel good because since we're not living in an ice age, it just goes to show that these climate scientists were wrong once, so they could very well be wrong again. Well, guess what? That whole talking point is disingenuous at best, a complete lie at worst. There was never the scientific consensus over an imminent cool-down like there is over global warming. (And don't get me started on the whole thing about how scientists are supposedly still debating this issue. If you allow anybody with a BS into the argument, then you might have something. As for climatologists, the ones who know what they're talking about, last I read they were 97% on board with us humans causing the current warming trends.)
With your truthers, they like to pull out the whole thing about how engineers are saying that there is no way the plane explosions could have brought down the twin towers. Well, if they did some research, they'd see that there are some engineers who were able to explain it just fine. Also, a truther recently told me that it was remarkable that there weren't any signs of damage from the plane's wing at the Pentagon. You know what I did? I looked it up. Turns out, there was evidence of it.
I should also point out that when it comes to 9/11, you've got a lot of people who know about as much as I do about engineering and physics (which isn't very much) having really strong opinions as to exactly what a plane crash is and what it should do. With climate change, you've got people who don't understand the difference between their local weather and average global temperatures.
I leave you with a couple of sources. The first is a long list of answers to common claims of global warming deniers. Before you comment with a long-since debunked claim about how it's not true, I suggest you consult it. It's currently standing at 120 debunked myths. The second is Wikipedia's article on 9/11 conspiracies. Yeah, I know, Wikipedia is unreliable. Shoot, I told my seniors that they'd get a zero on their essays if they cited it. However, I also told them that Wikipedia is a great starting point for doing some research, and it's in that spirit that I recommend this particular article. What's great is not just its thoroughness, but the number of links to further information and evidence that the truthers will find to be most inconvenient.
I end with a quote from Sherlock Holmes. He said it in the recent movie, but it's also from the excellent short story, "A Scandal in Bohemia". "It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts."