Perhaps you had better go retake chemistry. While there are other biproducts, combustion is mostly Hydrocarbon + O2 = CO2 + H20 (yes, your car makes water. So do you, chemically you do the same thing just a molecule at a time)
This cycle has been going on for millions of years. In addition, dead plants are only trapped when the conditions are right under the ground. Most of the time though when they decay the C02 enters the atmosphere again.
Thus over millions of years bogs and things have ever so slowly reduced the total carbon available to the biosphere. It stays trapped under the ground as oil, coal, gas, ect and would still be there if it hadn't be dug or pumped out (or released in a volcano. It is believed that the lions share of the CO2 emmissions the Earth is guilty for). Yes, at one time the earth's ecosystem thrived in a much higher C02 level environment, but most existing plants and animals are no longer suitable.
Because the change (adding carbon to the system) is happening over a period of centuries instead of tens of millenia, evolution cannot possibly keep up. This means that eventually adding too much carbon may cause negative effects.
Fortunatly, for manufacturing desired items (everything made of plastic and other compounds) there is far more than sufficient carbon in the ecosystem to make anything conceivable with current world economies. The only thing a constant supply of hydrocarbons is needed for is cheap energy. With sufficient investment, however, more than adequate energy is available with existing nuclear technology and possibly other sources for nearly any imaginable use. Cutting the jugular on middle eastern oil is quite doable, in fact stopping the import of all oil is possible with sufficient investment.
However, for vast quantities of resources and labor have gone into the construction of our existing oil infrastructure. Trillions of dollars. This means that any conceivable change, even if it is cheaper, could take many decades to pay for itself, tying up vast quantities of resources that might go to other things. This is the real reason why it may prove to be cheaper just to get more oil to drive the machine.
While no-one says this, for less than a trillion dollars the entire middle east could be seized militarily. There is probably ten times that dollar value in oil there. This would be a far more profitable investment than converting the entire country to windmills and hydrogen power, for instance.
It could even be better for many of the people in these nations. Much of the resources being paid to them is diverted into the hands of a few. In addition, these few didn't earn it : the vast majority of the work to even get the oil was supplied by foreign oil companies.
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