I believe it is true as an environmental engineer engaged in atmospheric modeling. Atmospheric modeling is a field in which the standard scientific method seems to be working well, that is, there is a large benefit to researchers who are right and/or can prove others wrong. This means that there is a lot of effort going into improving models that are already quite accurate, to the limits of the data you input. For example, the 1990 model of climate change does quite well if you give it better data, and at least correctly predicts the temperature trend with bad data. http://www.huffingtonpost.com/2012/12/10/1990-ipcc-report_n_2270453.html Similar to comments below, the IPCC is an enormous body, and I find invalidating their arguments to require an implausible conspiracy theory. You can look up the executive summary for the various reports at your leisure, they are quite readable.

This is one of those things you should probably just take on authority, like relativity or the standard model of particle physics. That is to say, it's an exceedingly complex topic in practice, and any argument stated for either side which can readily be understood is likely to be wrong. You have two or three options: study the field long enough to know what's going on, or trust the people who have already done so. (The third option, 'form an opinion without having any idea what's going on', is also commonly taken.) In short: I believe it's happening because this is what scientists tell me, and it's not worth putting in the time required to understand the field well enough that I could trust my opinion over theirs.