EDIT
Here is what the Royal Society says about the eight most common claims by skeptics:
1. Climate change is nothing to do with humans: "Levels of CO2 in the atmosphere are now 35 per cent greater than they have been for 650,000 years. From the radioactivity and chemical composition of the gas we know that this is mainly due to the burning of fossil fuels, as well as the production of cement and the widespread burning of the world's forests."
2. CO2 makes up only a small part of the atmosphere and so cannot be responsible for global warming: "Even in tiny concentrations
has a large influence on our climate . . . . Before industrialization CO2 made up about 0.03 per cent of the atmosphere or 280 ppm. Today, due to human influence it is about 380 ppm. Even these tiny quantities have resulted in an increase in global temperatures."
3. Rises in levels of CO2 in the atmosphere are the result of global warming, not the other way around: "The recent steep increase in the level of CO2 -- some 30 per cent in the last 100 years -- is not the result of natural factors. This is because, by chemical analysis, we can tell that the majority of this CO2 has come from the burning of fossil fuels . . . CO2 from human sources is almost certainly responsible for most of the warming over the last 50 years. There is much evidence that backs up this explanation and none that conflicts with it."
4. Observation of temperatures taken by weather balloons and satellites do not support the theory of global warming: "Discrepancies have been found to be related to problems with how the data was gathered and analysed and have now largely been resolved."
EDIT
http://www.canada.com/vancouversun/news/editorial/story.html?id=3bfd5526-9e7e-4666-aaa4-dba75fd4fc6a