A conservative calculation of specific impulse for CO2
"If it disagrees with experiment, it is wrong. In that simple statement is the key to science. It does not make any difference how beautiful your guess is, it does not make any difference how smart you are, who made the guess, or what his name is — if it disagrees with experiment, it is wrong."-- Richard Feynman, 1965
Co2 has actually increased in atmosphere from about 320 parts per million in 1960 to about 4200 parts per million now. Its not that humans put lots of it out compared to the carbon cycle mechanisms in nature, its just that we are tipping the balance toward a higher equilibrium. Where the climate change alarmists are wrong is that the marginal effect of the co2 increaee toward warming is not significant. Its effect on helping plants grow and helping feed the world is though. The extra co2 is a net benefit.