Climate Models, known knowns and the rest
Keith Dixon, a modeler at GFDL; "The climate is warming, and we can say why. Looking to the 21st century, all reasonable projections of what humans will be doing suggest that not only will the climate continue to warm, you have a good chance of it accelerating. Those are global-scale issues, and they’re very solid.”
The problem is that warming causes all sorts of changes — in the amount of ice in the Arctic, in the kind of vegetation on land, in ocean currents, in permafrost and cloud cover and more — that in turn can either cause more warming, or cool things off. To model the climate accurately, you have to account for all of these factors. Unfortunately, says James Hurrell, who led the NCAR's most recent effort to upgrade its own climate model, you can't. "Sometimes you don't include processes simply because you don't understand them well enough," he says. "Sometimes it's because they haven't even been discovered yet."
So really, really good about forecasting apart from the stuff we can't forecast or the stuff we don't know much about or the stuff we don't even know we don't know about; all the stuff that could make a big difference or not, we don't know, but apart from that they are really good.