What Are Greenhouse Gases?

Greenhouse gases are atmospheric gases that absorb and re-emit infrared radiation, trapping heat that would otherwise escape to space and producing the greenhouse effect that keeps the surface of the Earth roughly 33 degrees Celsius warmer than it would be without an atmosphere. The property depends on molecular structure. Diatomic molecules such as nitrogen and oxygen, which make up most of the air, have no dipole moment that changes during vibration and are effectively transparent to infrared, while molecules with three or more atoms have vibrational modes that couple to thermal radiation. Water vapor, carbon dioxide, methane, nitrous oxide, ozone, and synthetic fluorinated compounds all fall into that category.

Human activity has raised the concentration of several of these gases well above pre-industrial levels, which is why the term now appears more often in emissions accounting than in atmospheric physics. The US Environmental Protection Agency's overview of greenhouse gases sets out the principal species, their emission sources, and their relative heat-trapping strength.

The Principal Gases and Their Sources

Carbon dioxide is the largest anthropogenic contributor, released by fossil fuel combustion, cement manufacture, and land use change, and it is removed only slowly by ocean uptake and rock weathering, so a fraction of each emission persists for centuries. Methane comes from oil and gas systems, coal mining, ruminant livestock, rice cultivation, and landfills; it has an atmospheric lifetime near twelve years but a far higher instantaneous absorption per molecule. Nitrous oxide arises chiefly from nitrogen fertilizer use, manure management, and some industrial chemistry, and it also depletes stratospheric ozone. Fluorinated gases, including hydrofluorocarbons used as refrigerants, perfluorocarbons from aluminum smelting and semiconductor etching, sulfur hexafluoride used as an insulating medium in high-voltage switchgear, and nitrogen trifluoride from display manufacturing, are emitted in small quantities but persist for thousands of years in some cases. Water vapor is the strongest absorber overall, yet it acts as a feedback rather than a forcing because its concentration is set by temperature.

Radiative Forcing and Global Warming Potential

Comparing gases requires a common measure. Radiative forcing, expressed in watts per square meter, quantifies the change in the energy balance at the top of the atmosphere caused by a change in concentration. Global warming potential converts a mass of gas into the mass of carbon dioxide that would produce the same integrated forcing over a chosen horizon, conventionally 100 years, which is the basis for the carbon dioxide equivalent unit used in inventories built to the Greenhouse Gas Protocol. NOAA's Annual Greenhouse Gas Index applies these quantities to observed concentrations, tracking the combined forcing of carbon dioxide, methane, nitrous oxide, and nineteen minor halogenated gases against a 1990 baseline. The choice of time horizon matters: a short-lived gas such as methane looks far more significant on a 20-year horizon than on a 100-year one.

Measurement and Monitoring

Concentration data comes from three complementary systems. Surface flask sampling and continuous in-situ analyzers, coordinated through NOAA's Global Greenhouse Gas Reference Network, provide the long baseline records that begin with the Mauna Loa carbon dioxide series in 1958. Aircraft and tall tower profiles extend those measurements vertically. Satellite spectrometers, including Japan's GOSAT series and NASA's Orbiting Carbon Observatory instruments, retrieve column-averaged dry air mole fractions globally, and newer methane-focused missions locate individual super-emitting sources. Inverse modeling combines these observations with atmospheric transport models to estimate surface fluxes independently of self-reported inventories.

Applications

Greenhouse gas science and measurement have applications in a range of fields, including:

• National emissions inventories and international climate reporting
• Corporate carbon accounting and decarbonization planning
• Remote sensing and satellite instrument design
• Atmospheric and climate modeling
• Agricultural and land management practice
• Refrigerant and industrial process substitution