Greenhouse gases (GHGs) are atmospheric gases that increase the surface temperature of planets like Earth by absorbing the radiation emitted from them, resulting in the greenhouse effect. The Earth’s surface receives sunlight, which it then radiates as heat, primarily absorbed by these gases. Without them, Earth’s average temperature would be about −18 °C (0 °F), instead of the current 15 °C (59 °F).
The five most abundant GHGs in the atmosphere, in decreasing order of concentration, are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Concerns also arise from chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons, sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). Water vapor accounts for roughly half of the greenhouse effect and serves as a feedback mechanism.
Since the Industrial Revolution, carbon dioxide levels have risen by over 50%, and methane levels have increased by 150%. Carbon dioxide emissions contribute to about three-quarters of global warming, primarily from burning fossil fuels, while methane comes from agriculture and fossil fuel production. The carbon cycle takes thousands of years to absorb CO2 fully, whereas methane typically lasts around 12 years.
Natural carbon flows between the atmosphere, ecosystems, oceans, and sediments have been balanced over the past million years, but current CO2 levels are higher than they have been for three million years. If emission rates continue, global warming may surpass 2.0 °C (3.6 °F) between 2040 and 2070, a level deemed “dangerous” by the Intergovernmental Panel on Climate Change (IPCC).
