Adrián Tichno

Looking specifically at methane (CH₄) in the red-highlighted band near the infrared spectrum (10µm range), we observe its absorption band in a relatively narrow wavelength region. However, when comparing this with the absorption by water vapor (H₂O) in the cyan-highlighted area, it becomes clear that water vapor absorbs broadly and significantly across much of the infrared spectrum, including wavelengths that overlap methane's absorption range. This means that water vapor is already absorbing the majority of the radiation in these overlapping bands. Water vapor is vastly more abundant in the atmosphere than methane, with concentrations measured in parts per thousand (ppt) compared to methane's parts per billion (ppb). The dominance of water vapor absorption effectively "saturates" these overlapping wavelengths, leaving methane with only a minimal additional impact on the total radiative transfer. This is why slight increases in methane concentrations due to anthropogenic or natural emissions have a negligible effect on Earth's energy balance compared to the influence of water vapor. In summary, while methane is a greenhouse gas on a per-molecule basis, its contribution is limited by the overwhelming and already saturated absorption capabilities of water vapor in the same regions of the spectrum.