Holes in the sky

Nature has a useful overview of The Real Holes in Climate Science. In 2007, the IPCC highlighted 54 key uncertainties:

...such [uncertainties] do not undermine the fundamental conclusion that humans are warming the climate, which is based on the extreme rate of the twentieth-century temperature changes and the inability of climate models to simulate such warming without including the role of greenhouse-gas pollution. The uncertainties do, however, hamper efforts to plan for the future. And unlike the myths regularly trotted out by climate-change denialists..., some of the outstanding problems may mean that future changes could be worse than currently projected...

An accompanying editorial says:

...Perhaps the most important lesson is that researchers must be frank about their uncertainties and gaps in understanding — but without conveying the message that nothing is known or knowable...

On 'future changes worse than currently projected,' see this.