Answer to Question 1
ANSWER: On land, carbon is held as biomass, in live plant material, and in dead material on the ground and in the soil. During photosynthesis, plants take up CO2 from the atmosphere and release oxygen while growing, the amounts varying by season. The addition of CO2 due to human activities is significantly larger than the plants and oceans can take up; thus, these activities add carbon into the atmosphere.
Answer to Question 2
ANSWER: They permit radiant energy from the sun to heat Earth's surface, but prevent that heat from escaping back into space. The trapped heat causes increased temperatures on Earth.