Green plants use carbon dioxide for photosynthesis

Green plants use carbon dioxide for photosynthesis and return oxygen to the atmosphere, even then carbon dioxide is considered to be responsible for green house effect. Explain why?

The amount of ${{CO}_{2}}$produced due to human activity such as burning of fossil fuels like coal, natural gas, petroleum, etc., and production of lime from limestone is much more than that consumed during photosynthesis. The consumption in photosynthesis has further decreased due to deforestation.