The issue focused on missing carbon dioxide levels (they found no negative impacts from emissions but something very significant). So when they looked at how much carbon dioxide had been emitted from industrial processes (wheels, drilling and mining), they found there was only one negative impact on carbon dioxide levels: rates of warming. They looked at where each stage of CO 2 synthesis took place and found that where coal plants used natural gas (direct coal burning), human emissions increased by 15%. How many natural gas plants did there use? By