During combustion reactions, CO2 is formed. During photosynthesis, plants utilize CO2 and release O2. Due to excessive combustion, the concentration of CO2 increases beyond certain limit and some CO2 remains unutilized which absorbs heat irradiated by earth. Some of the heat is dissipated back in the atmosphere and the remaining is redirected back towards earth surface. This increases the temperature and is known as green house effect which results in global warming.