Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
The increased amount of CO2 in the air is mainly responsible for global warming. A large amount of carbon dioxide gets released into the atmosphere. Excess of CO2 in the air is removed by green plants and this maintains an appropriate level of CO2 in the atmosphere. The actual cause of unbalance CO2 level is a large amount of carbon dioxide produced by the combustion of fossil fuels.