I think this deserves its own post before it gets buried in the comments. doorman asserted that

A. CO2 levels have increased from 280-380ppm

B. Human industrial activity only accounts for a very small percentage of total worldwide CO2 emissions.

Point A is true, of course. A very simple calculation and a little googling though shows that the human contribution to the CO2 in the atmosphere is everything but small. What we need:

the weight of the atmosphere on the surface. This is 10.2 tons per square metre. This is the same as the atmospheric pressure.

the surface area of the earth: 510,065,600 km² = 5.100656e+14 m²

the concentration of CO2 by weight: 0.0582% (current value). The concentration per volume is 0.038% (=380 ppm), but CO2 is a bit heavier than air.

First we calculate the total mass of the atmosphere. This is just the surface area multiplied with the weight of the atmosphere: 5.100656e+14 m² * 1.02e+4 kg/m² = 5.202669e+18 kg. To get the mass of the carbon dioxide we multiply this with the fraction that is the carbon dioxide: 5.202669e+18 kg * 0.000582 = 3.027953e+15 kg, or about 3000 gigatons.The excess of CO2 since 1850 is (1 – (280/380)) * 3000 gigatons = 789 gigatons.

We can now compare this with the world carbon dioxide emission rate, which was according to this source 25,028 million tons = 25.028 gigatons in 2003 (compare this with the emission of the volcanoes). So if the rate stayed constant (it does not, it’s increasing), we can accumulate the excess in about 32 years. Of course the emission rate wasn’t as high as it is now beginning in 1850, but you get the idea. It gets also more complicated because a large fraction gets dissolved in the oceans.

The image is from here. Note that the values there are the mass of the pure carbon. To compare with carbon dioxide values, you need to multiply them by about 3.67 – carbon dioxide has one carbon atom with atomic weight 12 and two oxygen atoms with atomic weight 16, and (2*16+12)/12 = 3.67.