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

M Etminan,E J Highwood,K P Shine,G Myhre

doi:10.1002/2016gl071930

Abstract

AbstractNew 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.

Highlights

The radiative forcing (RF) due to changes in concentrations of the relatively well mixed greenhouse gases (WMGHGs) is the largest component of total RF due to human activity over the past century [Myhre et al, 2013a].The headline RF values for CO2, CH4, and N2O presented in recent Intergovernmental Panel on Climate Change (IPCC) assessments [e.g., Myhre et al, 2013a] are calculated using simplified expressions presented in the IPCC Third Assessment Report [Ramaswamy et al, 2001, section 6.3.5]
The RF results for all 48 cases are presented in Table S1 and include the shortwave RF, the instantaneous clear‐sky and all‐sky longwave RF, the adjusted longwave RF, and the net forcing
For high CO2 cases (2000ppm—Table S1, rows 37–48) the new calculations are typically 10% higher than the old fits, indicating that the CO2 forcing increases more rapidly than expected from a purely logarithmic dependence. Such high CO2 amounts were outside the range considered by MHSS98

Summary

Introduction

The headline RF values for CO2, CH4, and N2O presented in recent Intergovernmental Panel on Climate Change (IPCC) assessments [e.g., Myhre et al, 2013a] are calculated using simplified expressions presented in the IPCC Third Assessment Report [Ramaswamy et al, 2001, section 6.3.5]. These were largely based on the work of Myhre et al [1998, MHSS98]. The MHSS98 expressions have been widely used elsewhere, for example, for calculating WMGHG RF in simple climate models and in NOAA’s Annual Greenhouse Gas Index Other similar simplified expressions have been proposed [e.g., Shi, 1992; Byrne and Goldblatt, 2014], and Hansen et al [2000] presented updates to the Hansen et al [1988] fits

Methods

Results

Conclusion