Human beings may now be altering this balance by adding unusual amounts of carbon dioxide to the atmosphere. This could make the global climate warmer than at any other time in human history. And this change would be irreversible. It is essential for photosynthesis, the process by which the sun's energy is converted into forms usable by plants and animals. It also helps regulate the critical heat balance of the planet, thus maintaining liquid water.
Carbon dioxide affects the heat balance by acting as a one-way screen.
It is transparent to incoming visible sunlight and allows the sun's heat to warm the ocean and the land. But carbon-dioxide molecules block some of the infrared heat radiated back into space. This reflected heat is absorbed into the lower atmosphere. This is the so-called greenhouse effect . . . The excerpt discusses the way in which the sun's energy is converted into food for plants and animals and the way in which carbon dioxide handle incoming and outgoing radiation.