Without enough light, a plant cannot photosynthesize very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature. Increasing the light intensity increases the rate of photosynthesis, until some other factor - a limiting factor - becomes in short supply.
Carbon dioxide is returned to the atmosphere primarily by respiration of humans, (exhalation to be precise). The return of carbon dioxide to the atmosphere is also possible by the decay of plants and animals and combustion, which is the process of burning something.
