Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing

TLDR

The authors present new calculations and simplified expressions for the radiative forcing of CO₂, CH₄, and N₂O. The new expressions incorporate overlap between CO₂ and N₂O, a feature absent from previous IPCC formulations. Methane’s radiative forcing from 1750–2011 rises 25 % to 0.61 W m⁻² and its 100‑year GWP is 14 % higher than IPCC values, while CO₂ forcing is within 1 % of IPCC but 10 % higher at 2000 ppm under RCP8.5.

Abstract

Abstract New calculations of the radiative forcing (RF) are presented for the three main well‐mixed greenhouse gases, methane, nitrous oxide, and carbon dioxide. Methane's RF is particularly impacted because of the inclusion of the shortwave forcing; the 1750–2011 RF is about 25% higher (increasing from 0.48 W m −2 to 0.61 W m −2 ) compared to the value in the Intergovernmental Panel on Climate Change (IPCC) 2013 assessment; the 100 year global warming potential is 14% higher than the IPCC value. We present new simplified expressions to calculate RF. Unlike previous expressions used by IPCC, the new ones include the overlap between CO 2 and N 2 O; for N 2 O forcing, the CO 2 overlap can be as important as the CH 4 overlap. The 1750–2011 CO 2 RF is within 1% of IPCC's value but is about 10% higher when CO 2 amounts reach 2000 ppm, a value projected to be possible under the extended RCP8.5 scenario.