A greenhouse gas is a gaseous constituent of the atmosphere that absorbs and emits radiation at wavelengths associated with thermal infrared emissions. These gases produce the greenhouse effect, which helps maintain habitable temperatures on Earth. They include water vapor, carbon dioxide, methane, nitrous oxide, ozone, and several manufactured gases. Their natural presence is distinct from the increases caused by human activities, which strengthen the greenhouse effect and drive climate change. (ipcc.ch)
Physical mechanism
Earth receives energy from sunlight and returns energy to space as thermal radiation. Greenhouse gases absorb portions of this outgoing infrared radiation and emit radiation both upward and downward. They therefore influence the rate at which energy escapes to space and the temperatures of the surface and atmosphere. The effect is not an unlimited accumulation of heat: temperatures adjust until outgoing radiation again balances absorbed solar energy. (science.nasa.gov)
Increasing greenhouse-gas concentrations can initially reduce outgoing radiation, creating an imbalance in Earth’s energy budget. The surface and lower atmosphere then warm, increasing thermal emissions. Different gases absorb different parts of the infrared spectrum, and their absorption bands overlap. Consequently, a gas’s climatic influence depends on its concentration, absorption properties, and interactions with other atmospheric constituents, rather than simply its abundance. (science.nasa.gov)
Principal greenhouse gases
Water vapor is an abundant greenhouse gas whose atmospheric concentration responds strongly to temperature and the water cycle. Evaporation supplies it, while condensation and precipitation remove it. In the lower atmosphere, water vapor primarily acts as a feedback: warming permits greater atmospheric moisture, which strengthens greenhouse warming. It is therefore not treated as interchangeable with long-lived gases that initiate sustained changes in the energy balance. (science.nasa.gov)
Carbon dioxide (CO₂) exchanges naturally among the atmosphere, organisms, soils, and oceans through the carbon cycle. Human sources include fossil-fuel combustion, cement production, and land-use change. Deforestation can release stored carbon and reduce subsequent uptake. Plants remove CO₂ through photosynthesis, and the ocean absorbs part of human emissions. These sinks do not remove all emitted CO₂, allowing atmospheric concentrations to increase. (science.nasa.gov)
Methane (CH₄) comes from natural sources, especially wetlands, and human activities including livestock production, rice cultivation, waste disposal, and fossil-fuel extraction and distribution. Its atmospheric abundance is much lower than that of CO₂, but emissions have a stronger warming influence per unit mass over commonly used comparison periods. Methane also affects atmospheric chemistry, including the formation of ozone. (epa.gov)
Nitrous oxide (N₂O) has natural and human sources. Agricultural soil management, including fertilizer use, is an important human source; industrial processes, combustion, and wastewater treatment also contribute. Ozone (O₃) is another greenhouse gas. Its distribution and effects depend on altitude: stratospheric ozone and ozone in the lower atmosphere have different formation processes and environmental roles. (epa.gov)
Fluorinated gases include hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. Their sources include refrigeration, air conditioning, electronics manufacturing, and electrical equipment. Many have high warming effects per unit mass, although their atmospheric lifetimes and radiative properties vary greatly. Some fluorinated compounds also deplete stratospheric ozone, but ozone depletion and greenhouse warming are distinct processes. (epa.gov)
Atmospheric persistence and sources
Atmospheric concentration reflects the balance between emissions and removal processes, not emissions alone. Removal can involve chemical reactions, uptake by vegetation, or exchange with oceans. The Intergovernmental Panel on Climate Change assessed methane’s perturbation lifetime—the timescale over which an added atmospheric burden declines—as approximately 11.8 years, and that of nitrous oxide as approximately 109 years in its 2021 assessment. These quantities describe atmospheric responses rather than fixed lifespans for individual molecules. (ipcc.ch)
CO₂ has no single atmospheric lifetime because removal operates through several processes on different timescales. Some additional CO₂ is absorbed relatively quickly, while a fraction influences atmospheric concentrations for thousands of years. Certain fluorinated gases likewise persist for centuries or millennia. These differences mean that short-lived and long-lived gases produce different warming trajectories even when their emissions are expressed in a common unit. (ipcc.ch)
Comparing warming effects
Global warming potential (GWP) compares the time-integrated radiative forcing from a pulse emission of a gas with that from an equal mass of CO₂. CO₂ therefore has a GWP of 1 by definition. A time horizon must be specified, commonly 20 or 100 years. Methane has a larger GWP over 20 years than over 100 years because its atmospheric perturbation declines comparatively rapidly. (epa.gov)
Emissions expressed as carbon dioxide equivalent are calculated by multiplying each gas’s emitted mass by its selected GWP. This enables inventories to aggregate different gases, but does not imply identical effects on temperature at every future date. Comparisons require consistent time horizons and assessment values; GWP estimates can change as scientific understanding improves. (epa.gov)
Measurement and evidence
Atmospheric monitoring networks collect air samples and operate continuous instruments to identify long-term trends, seasonal cycles, and geographic differences. Measurements of isotopic composition help investigate sources and exchanges. Air preserved in ice cores provides evidence of earlier greenhouse-gas concentrations, extending the record beyond modern instrumental observations. These records complement emissions inventories, which estimate releases from particular activities rather than directly measuring atmospheric abundance. (gml.noaa.gov)