Actually one of the biggest problems for the AGW proponents is the rise in CO2 is too linear
As you can see, The rise has been steady at about +1.5 ppmv per year (15 ppmv per decade)
However, if you look at the world's CO2 output
the emissions have of course been increasing
So the question is why is the rise in CO2 levels per year staying linear? As emissions increased you should see the rate of the increase (the Slope) in CO2 levels per year in the atmosphere also increase.
For example, if pumping out 4000 million tons of CO2 1970 caused the rise of 1.5 ppmv atmospheric CO2, then you would expect that in the year 2000 when we pumped out 7000 million tons of CO2, the atmospheric level should have rose about 26 ppmv that year. But they didn't, it's been holding steady at +1.5ppmv
Here are the actual numbers on the change per year
From Baring Head in New Zealand where the results are nearly the same as Manua Kea they have a nice plot
as you can see there just isn't any trend or correlation between human emissions and rise in CO2. If humans were responsible, the trend would be every year having a bigger rise than the previous one, but instead the rise goes up & down randomly with no tread at all
The reason is obvious, 4000 million tons or 7000 million tons put out by man are both insignificant compared to natural emissions and the rise in CO2 has to be coming from somewhere else. Even 7000 million tons is barely a blip on the seasonal differences.