Greenhouse Gases Compared
Five gas families account for essentially all anthropogenic climate forcing. Carbon dioxide is the largest contributor and the longest-lived. Methane has 80× the warming power of CO₂ over 20 years but breaks down in roughly a decade. Nitrous oxide and fluorinated gases are smaller in volume but extremely potent per molecule and very long-lived.
Key insights
Time horizon changes the methane story
Methane traps heat much more efficiently than CO₂ but disappears from the atmosphere relatively quickly. Over 100 years it has about 28× CO₂'s warming effect (GWP-100); over 20 years it has roughly 80× (GWP-20). The choice of horizon is not just methodological — it changes which mitigation actions look most urgent.
F-gases are small but troublesome
Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆) account for ~2% of greenhouse-gas emissions today but have GWPs in the thousands to tens of thousands and atmospheric lifetimes up to 50,000 years. The Kigali Amendment to the Montreal Protocol (2016) commits countries to phase down HFCs — analogous to the original CFC ban that closed the ozone hole.
Aerosols partially offset greenhouse warming
Sulphur and nitrogen aerosols emitted alongside fossil-fuel CO₂ reflect sunlight and cool the planet by roughly 0.5°C — partially masking the underlying greenhouse warming. As air-quality regulations cut aerosol emissions (good for health) the masked warming surfaces. This is not a reason to relax aerosol regulation; it is a reason to expect a small acceleration of measured warming as the cleanup proceeds.
Radiative forcing by greenhouse gas (since 1750)
Watts per square metre, current concentrations vs pre-industrial
Key Finding: CO₂ accounts for ~65% of total well-mixed greenhouse-gas forcing; methane ~17%.
Global warming potential (GWP-100) — selected gases
CO₂-equivalent warming over 100 years
Key Finding: On a 100-year basis, methane is 28× more warming per kg than CO₂; SF₆ is 23,500×.
Methodology & caveats
Radiative forcing explained
Radiative forcing measures the change in net energy received at the top of the atmosphere relative to a baseline (usually 1750). It's measured in watts per square metre. A positive forcing warms the climate; a negative forcing cools it. The TCRE (transient climate response to cumulative emissions) provides the conversion from cumulative emissions to eventual temperature change.
GWP and its discontents
Global Warming Potential is a ratio: the integrated radiative forcing of a unit mass of one gas over a chosen horizon, divided by that of CO₂ over the same horizon. The choice of horizon is arbitrary and changes the answer substantially for short-lived gases like methane. GWP* and GTP (Global Temperature Potential) are alternative metrics designed to handle short-lived gases more honestly.
Atmospheric lifetime
CO₂ does not have a single decay constant — about 50% is absorbed by oceans and land within ~30 years, another 30% over centuries, and ~20% remains for tens of thousands of years. Methane has a relatively clean ~12-year decay via reaction with hydroxyl radicals. Nitrous oxide lasts ~120 years. Long-lived fluorinated gases (PFCs, SF₆) persist for thousands to tens of thousands of years.