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

• Published in: Geophysical research letters Vol. 43; no. 24; pp. 12,614 - 12,623
• Main Authors: Etminan, M., Myhre, G., Highwood, E. J., Shine, K. P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-12-2016
• Subjects: Absorption; Agreements; Air pollution; Assessments; Carbon dioxide; Climate; Climate change; Environmental assessment; Formulas (mathematics); Gases; Global warming; Greenhouse effect; Greenhouse gases; Human influences; Intergovernmental Panel on Climate Change; International agreements; Loads (forces); Mathematical analysis; Methane; Methane emissions; Methods; Nitrous oxide; Nitrous oxides; Radiation; Radiative forcing; Solar radiation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2016GL071930

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 CO2 and N2O; for N2O forcing, the CO2 overlap can be as important as the CH4 overlap. The 1750–2011 CO2 RF is within 1% of IPCC's value but is about 10% higher when CO2 amounts reach 2000 ppm, a value projected to be possible under the extended RCP8.5 scenario.
Plain Language Summary
“Radiative forcing” is an important method to assess the importance of different climate change mechanisms, and is used, for example, by the Intergovernmental Panel on Climate Change (IPCC). Increased concentrations of greenhouse gases, such as carbon dioxide, methane and nitrous oxide, are the major component of the human activity that led the IPCC, in its 2013 Assessment, to conclude that “it is extremely likely that human influence is the dominant cause of warming since the mid‐20th century.” In this letter, we report new and detailed calculations that aimed to update the simpler methods of computing the radiative forcing that have been used in IPCC assessments, and elsewhere. The major result is that radiative forcing due to methane is around 20‐25% higher than that found using the previous simpler methods. The main reason for this is the inclusion of the absorption of solar radiation by methane, a mechanism that had not been included in earlier calculations. We examine the mechanisms by which this solar absorption causes this radiative forcing.The work has significance for assessments of the climate impacts of methane emissions due to human activity, and for the way methane is included in international climate agreements.
Key Points
Calculated CH4 radiative forcing is about 25% higher than earlier estimates
New simplified expressions for forcing are presented for CO2, N2O, and CH4
Forcing for high CO2 concentrations is 9% higher than previous expressions