I think that is the biggest problem when it comes to the consistently overly conservative reports on the effects of climate change (global warming). The IPCC probably does do a very good job of reporting the science, but scientists (due in part to our inherently conservative bent and in part to things like peer review, and the very fuzzy nature of projections) are likely to shy away from more dire positions...at least within journals, and maybe meetings.
Non-scientists, however, simply don't understand scientists. Scientists want to keep the uncertainty in measurements and predictions as low as possible--or, another way of putting it, to have as high a confidence as possible. But that focus can be problematic when new data doesn't match existing. There can be an urge to dismiss outlying data, to assume that if something is significantly different than what others have found, it is likely to be wrong. Prediction in particular is a very difficult area to deal with, especially when those predictions relate to complex systems and long time scales. Very small changes in initial settings can lead to very large changes in the results. This isn't normally wrong. Typically, the odds are that if your data is
different from all existing data, you messed something up.
The political particulars of global warming impose another constraint--one that causes a systematic push towards more conservative reporting. No one wants to get it wrong on the high side and give fodder to the anti-science, anti-gloabal warming crowd (not that they really need anything since they tend to make up crap even without). (Also, no one wants to be chicken little or the boy who cried wolf.)
So things are worse than most predictions. Al Gore was right, but he's fat and claimed he invented the internet so no one should listen to him. On the other hand the world may end in a couple weeks here when the long count runs out and we won't have to worry about the inevitable humanitarian crisis to come after all.