Atmospheric CO2 levels have increased even more than the IPCC predicted in 2001. If their predictions were correct there should have been an even bigger increase in atmospheric temperatures than they predicted, but over the 10 years since their report was published there has been no increase in those temperatures at all.
Why? What is missing?
The core idea of every climate model the IPCC uses is the idea that for every unit of increase of temperature caused by CO2 there will be a total increase of 3 units caused by water vapor. Disagreement over this assumption is the source of all the disagreements and misunderstandings about climate models.
What did they find when they tried to prove that the assumption about water vapor is correct?
If the assumption about water vapor was correct then it should have been possible to detect a hot spot in the troposphere over the tropics. During the last 4 decades weather balloons and satellites have failed to detect any such spot, not even a trace of it.
This means that the assumption about water vapor is incorrect. This first became clear in the mid '90s, and with Aqua satellite data it is even more clear.
And if you plug a more reasonable figure for the water vapor effect into the climate models you get results that, surprise, match what has happened over the last 10 years and over the previous 100 years as well.
Why haven't the scientists we're all familiar with acknowledged this? I suspect it's because if they did they would lose their funding. There is simply no question that they would. Without the disaster scenario of global warming there would be no interest in the field. They'd sink out of sight into obscurity.