Time Evolving Ghg Radiative Forcing Calculator

Radiative forcing is the change in net downward energy flux at the tropopause, in watts per square metre, produced by a perturbation such as a rise in greenhouse gas concentration, evaluated after the stratosphere has adjusted thermally but before the surface responds. Each gas gets a different functional form because each occupies a different part of the infrared spectrum and is saturated to a different degree. Carbon dioxide takes a logarithmic form, 5.35 x ln(C/C0), because its 15 micrometre band is already opaque at line centre, so extra molecules only widen the band wings; that is why every doubling adds a near-constant 3.7 W/m2 regardless of starting concentration. Methane and nitrous oxide sit on weaker, less saturated bands and follow square-root forms, with an overlap term subtracted because their absorption features near 7.7 micrometres shadow one another. Against a 1750 baseline of roughly 280 ppm CO2, 700 ppb CH4 and 270 ppb N2O, present concentrations give a well-mixed greenhouse gas forcing near 3.3 W/m2, of which CO2 supplies about two thirds. Equilibrium warming is estimated here as ECS x F / 3.7, where the IPCC Sixth Assessment gives a likely equilibrium climate sensitivity range of 2.5 to 4.0 degrees C with a best estimate of 3.0. Three caveats matter: these are simplified fits accurate to a few percent, not line-by-line radiative transfer; halocarbons, ozone and aerosols are excluded, and aerosols alone offset roughly 1 W/m2; and equilibrium warming is not present-day warming, since ocean heat uptake delays the surface response by decades to centuries.

The physical foundation was laid by John Tyndall, who demonstrated in laboratory work reported in 1859 that carbon dioxide and water vapour absorb infrared radiation while nitrogen and oxygen do not. Svante Arrhenius published the first quantitative estimate in 1896, calculating by hand that halving or doubling atmospheric carbonic acid would change surface temperature by several degrees, and arrived at a sensitivity broadly in the range still discussed today. Guy Stewart Callendar revived the argument in 1938, and Gilbert Plass in the 1950s used newly available spectroscopic data and early computers to show that the CO2 band was not saturated in the way critics had assumed. The modern framework emerged with Syukuro Manabe and Richard Wetherald, whose 1967 radiative-convective model produced a doubling response near 2 degrees C, and with the 1979 US National Academy of Sciences report chaired by Jule Charney, which set the long-standing 1.5 to 4.5 degrees C sensitivity range. Radiative forcing was adopted as the common currency for comparing climate agents in the early IPCC assessments. The specific expressions used here come from Gunnar Myhre and colleagues, whose 1998 Geophysical Research Letters paper fitted simplified formulas to three independent line-by-line models and fixed the 5.35 coefficient and the CO2 doubling value of 3.71 W/m2. Etminan and colleagues revised these in 2016, raising methane forcing by about 25 percent by accounting for shortwave absorption previously neglected.