Cause of Excess CO2
Since the formation of the earth roughly 4.567 billion years ago, the concentration of CO2 in the atmosphere has shown significant “natural” variability [being as much as twenty times higher in the distant past and some 4-5 times higher when most plants evolved] but in no case has ever been more than an extremely minor constituent .
This is somewhat surprising since all plant life on earth gets roughly half of its entire mass from this trace gas which is currently present at only about one molecule in 2500. As a consequence, if the democrats managed to scrub all the CO2 pollution from the atmosphere, they would kill every living thing on earth. This is because below about 250-150 ppm of CO2, all plants stop growing and die. This is a problem because not only would the food chain collapse starving everybody but also because ALL the free oxygen on earth is produced by plants. And yes Virginia, we and the animals and even the fish need OXYGEN to breathe despite what democrat politicians [full of democrat-science (DS]) try to tell you.
The other thing to note is that historically CO2 shows absolutely no correlation with global temperatures. This should not be surprising because CO2 molecules are less abundant than water molecules by a factor of from 25 to 100 [i.e. H2O is 1-4% and CO2 is currently less than 0.04%]. Also each CO2 molecule has about one eighth the warming effect of an H20 molecule. Also CO2 is both saturated and its infrared spectra is largely masked by identical absorption bands in H20. Also the “feedback” effect of excess CO2 is strongly NEGATIVE and not positive. The net effect is that CO2 amounts to 2-3% of the entire global warming “greenhouse” effect and mankind’s contribution is significantly less than even that small amount.
EQUILIBRIUM CONCENTRATIONS OF CO2
There are two major sources and sinks of CO2 on earth which are roughly in equilibrium with the tiny, small, miniscule, barely detectable amounts of CO2 in the atmosphere, namely the plants on the land and dissolved in the world’s oceans. And the amount of CO2 in the ocean VASTLY outweighs all other sources and for all practical purposes is the sole reservoir determining equilibrium concentrations in the atmosphere.
As the temperature of the earth naturally warms, the oceans release CO2 into the air benefiting everybody. The graph for the solubility of CO2 in water is given below  and is STRONGLY dependent on mean temperature.
The second largest, but minor, source of CO2 is in plants. As plants grow in the spring summer they remove CO2 from the air, and return this in winter when they die and decay. And since the Northern hemisphere is roughly 40% land as compared to the Southern which is only 20% land, the imbalance creates a predictable seasonal fluctuation.
Or when the same data  is plotted at full scale, we can get a better idea of relative contributions as follows ;
More CO2 in the air, because of a warming ocean, means more plant life on earth, perhaps 10-20% over the last century. And because plants prefer the C-12 isotope of carbon over the less abundant C-13 isotope, this tends to obscure any human contribution to excess CO2.
TRANSIENT HALF LIFE OF CO2
Despite wild claims by global warming alarmists in the IPCC that excess CO2 once emitted into the air remains for hundreds of years before equilibrium is again established, actual observations indicate otherwise. From the end of WWII in 1945 to the Nuclear Test Ban Treaty (NTBT) in 1963, nuclear weapons exploded above ground generated a spike in carbon-14 in the atmosphere. Measurements of how fast this rare isotope of carbon in CO2 is naturally removed from the air gives a half life for all isotopes of CO2 of about 6 years .
MANKIND’s CONTRIBUTION TO EXCESS CO2
The current level of CO2 in the atmosphere is at historically extremely low levels [previously in earth’s history was 20 times more abundant]. The current increase is mostly due to equilibrium with dissolved gases in the oceans as the Earth has been warming over the last 3-4 centuries coming out of the Little Ice Age, which MASSIVELY DOMINATES all other sources. And it is the first 200 feet below the sea surface that is well mixed and contributes the largest amount.
But transient and minor fluctuations are due partially to human causes and part to natural ones. Vegetation on earth fluctuates on time scales of weeks to months and is TWO ORDERS of magnitude larger than any putative human contribution. Simple considerations place an upper limit on human contributions to the yearly increase of CO2 in the atmosphere of less than 1/3rd. But more likely, the human contribution to annual CO2 increases is less than 5-10% of the total. 
That IPCC models continue to miss-represent CO2 lifetimes and human contributions from the simplest of theoretical considerations indicates a political agenda rather than a scientific one.