well, Stephen Schneider, one of the main proponents of the global warming theory, in the 1970's warned of an imminent Ice Age. When he was asked about later this he shrugged it off by saying he was wrong.

as far as Wolfgangs last point about not being able to predict the weather more than a few days ahead and people's willingness to trust theories a century ahead, (Chaos theory, or sensitivity to initial conditions makes it difficult to predict what will happen past 6days or so because there are so many variables, on the other hand,
long term trends may not be as difficult, as we know that temperature has gradually increased over the last century, and atmospheric co2 has increased as well.