greenhouse gas (GHG)

A greenhouse gas (GHG) is any atmospheric gas that absorbs and re-emits infrared radiation in wavelengths emitted by the Earth's surface, trapping heat in the lower atmosphere and warming the planet's surface above what would otherwise prevail. The defining property is selective absorption in the 4-25 micrometre infrared range, where the Earth's surface emits most strongly. Naturally occurring water vapour is the largest single contributor by mass, but human-emitted CO2, methane (CH4), nitrous oxide (N2O), and fluorinated gases (HFCs, PFCs, SF6, NF3) are the targets of climate policy because their atmospheric concentrations have risen sharply since the industrial revolution.

Each GHG is characterised by:

• Global Warming Potential (GWP): how much heat the gas traps over a defined time horizon, normalised to CO2 (set to 1).
• Atmospheric lifetime: how long it persists before being removed by chemical reactions or biological/geological sinks.

Key 100-year GWP values from the IPCC AR6 (the basis of current climate accounting):

• CO2: 1 (reference), lifetime varies from decades to millennia.
• Methane (CH4): 27-30, atmospheric lifetime 12 years.
• Nitrous oxide (N2O): 273, lifetime 109 years.
• HFC-134a (refrigerant): 1,530, lifetime 14 years.
• Sulfur hexafluoride (SF6, electrical switchgear): 25,200, lifetime 3,200 years.

Carbon markets — both compliance (such as India's CCTS launched 2024) and voluntary (VCS/Verra, Gold Standard) — assign monetary value to GHG emission reductions, currently 600-2,500 INR per tonne CO2e for typical Indian voluntary projects. CBG plants, plastic recycling, e-waste, battery recycling, and tyre pyrolysis all generate substantial GHG reductions versus their fossil-fuel/virgin-material counterfactuals and can register projects with carbon registries to monetise the avoided emissions.

The trade-off in GHG mitigation strategy is short-term versus long-term impact. Reducing methane (12-year lifetime, high GWP) delivers fast climate benefit and is the focus of the Global Methane Pledge that India endorsed in part. Reducing CO2 (long lifetime, GWP=1) is essential for long-term stabilisation but slower to translate into temperature outcomes. Industrial GHG management programmes typically address all gases together using the CO2-equivalent framework, with internal carbon pricing increasingly used by Indian corporates to inform capex decisions on energy efficiency, fuel switching, and process improvements.