There are many lines of evidence which clearly show that the atmospheric CO2 increase is caused by humans. The clearest of these is simple accounting - humans are emitting CO2 at a rate twice as fast as the atmospheric increase (natural sinks are absorbing the other half). There is no question whatsoever that the CO2 increase is human-caused. This is settled science.
What is CO2?
Carbon dioxide is a naturally occurring substance composed of one atom of carbon (C) and two atoms of oxygen (O2). Its chemical formula is CO2. CO2 is gas under normal conditions.