CO2

(from http://www.esrl.noaa.gov/gmd/ccgg/trends/)

The red dashed line is the actual CO2 measurement with careful sampling and some averaging to account for local fluctuations on Mauna Loa. (the really raw data is shown here) The black line has the seasonal cycle removed. The seasonal cycle is caused mainly by northern hemisphere vegetation. There is much more land mass in the northern hemisphere and in spring the vegetation grows quickly causing CO2 to drop. In the fall the CO2 rises rapidly as much of the CO2 is released as the leaves oxidize, in a matter of days to a month. The remaining leaf litter oxidizes more slowly, but the litter from previous seasons provides a more steady source of CO2.

The other cause of the seasonal rise and fall is warming of the southern hemisphere's oceans as the earth gets closer to the sun (early Jan) and the SH is tilted towards the sun.

What if there were no man made CO2? In that case CO2 would rise naturally, but at a rate much smaller than what is observed. The end of the Little Ice Age resulted in a natural rise of about 0.5C which, by warming the oceans, would increase atmospheric CO2 by about 5-10 ppm over a century or so (in contrast to the observed 2-3 ppm rise per year).