Carbon cycling and greenhouse gases

Photosynthesis takes carbon dioxide from air and incorporates its carbon into plant material. Respiration returns carbon dioxide as organisms release energy from organic compounds. Combustion of biomass and fossil fuels also returns carbon dioxide. These flows help regulate atmospheric carbon dioxide, but they do not guarantee a constant concentration if inputs exceed removals.

Carbon dioxide and methane are greenhouse gases. They absorb outgoing infrared radiation and contribute to warming of the atmosphere and surface. More greenhouse gas strengthens this effect. Fossil-fuel combustion and deforestation add CO₂ or reduce its removal. Methane sources include livestock, landfill decomposition, wetlands and fossil-fuel leakage.

Potential consequences of increased greenhouse-gas levels include more extreme weather events and melting polar ice. Melting land ice contributes to rising sea level; floating sea ice and land ice should not be treated as identical sea-level mechanisms. Warming also changes rainfall patterns and stresses habitats.

For bioethanol, plant growth can take up carbon dioxide later released by burning the fuel. The comparison with fossil fuels depends on how the crop is grown, processed and transported and whether land-use changes release additional carbon. A carbon-flow explanation is stronger than the claim that renewable fuels produce no carbon dioxide.