Greenhouse Gases: Mechanisms and Impacts

Greenhouse effect and heat-trapping gases

Greenhouse gases (GHGs) are atmospheric gases that trap heat by absorbing and re-emitting infrared radiation emitted by Earth’s surface. This absorption reduces the rate at which heat escapes to space, warming the planet. Without greenhouse gases, Earth’s average surface temperature would be far colder (about −18 °C) than it is today (about 15 °C). The main greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone, with additional human-relevant gases such as CFCs, HCFCs, HFCs, perfluorocarbons, SF6, and NF3. The strength of the greenhouse effect depends on how gases interact with radiation (they are “infrared active”), their atmospheric abundance, and their lifetime. Radiative forcing is a common way to quantify how changes in greenhouse gas concentrations alter Earth’s energy balance at the top of the atmosphere; increased greenhouse gases produce positive forcing (warming), while some other factors can produce negative forcing (cooling). Water vapor is the largest contributor to the greenhouse effect, but its global concentration is mainly controlled by temperature rather than direct human emissions, making it a key feedback mechanism. To compare different gases, scientists use global warming potential (GWP), which expresses how much heat a gas traps over a specified time relative to CO2. Because methane has a shorter atmospheric lifetime than CO2, its GWP is much larger over shorter time horizons (e.g., 20 years) but smaller over longer periods (e.g., 100 years). Human activities since the Industrial Revolution have increased CO2 and methane substantially, driving enhanced (human-caused) warming, while natural carbon and methane cycles partially offset emissions but do not fully prevent long-term accumulation of CO2.

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Earth temperature without greenhouse gases

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