Which best describes how the atmosphere's CO2 content was regulated before the Industrial Revolution? Which best describes how the atmosphere's CO2 content was regulated before the Industrial Revolution? Inputs of CO2 to the atmosphere resulted from cellular respiration and widespread use of fossil fuels. These inputs were much greater than the uptake of CO2 by producers. CO2 was taken up by organisms for use in cellular respiration; a similar amount of CO2 was released by plants and other producers. CO2 was released by organisms as a by-product of cellular respiration; photosynthesis contributed additional CO2 to the atmosphere. CO2 was released by organisms as a by-product of cellular respiration; a similar amount of CO2 was absorbed by plants and other producers.