1. No signature of CO2.
The signature of an increased greenhouse effect is a hot spot about 10km up in the atmosphere over the tropics. We have been measuring the atmosphere for decades using radiosondes: weather balloons with thermometers that radio back the temperature as the balloon ascends through the atmosphere. They show no hot spot.
2. No evidence of causality.
There is plenty of evidence that global warming has occurred, and theory suggests that carbon emissions should raise temperatures (though by how much is hotly disputed) but there are no observations by anyone that implicate carbon emissions as a significant cause of the recent global warming.
3. Temperatures are dropping again.
The warming trend ended in 2001, and that the temperature has dropped about 0.6C in the past year (to the temperature of 1980). Land-based temperature readings are corrupted by the "urban heat island" effect: urban areas encroaching on thermometer stations warm the micro-climate around the thermometer, due to vegetation changes, concrete, cars, houses. Satellite data is the only temperature data we can trust, but it only goes back to 1979.
4. Temperature drives CO2 concentrations, not vice versa.
The new ice cores show that in the past six global warmings over the past half a million years, the temperature rises occurred on average 800 years before the accompanying rise in atmospheric carbon. Which says something important about which was cause and which was effect.
Read David Evans full description of why he went from a CO2 climate modeler and policy maker to a skeptic. He warns that when it comes to light that the CO2 scare is bogus, governments will be regarded as either criminally negligent or ideologically stupid. He concludes by saying "The onus should be on those who want to change things to provide evidence for why the changes are necessary."