Efficacy of Methane Forcing: the Role of Fast Adjustments on Four-Month Timescale
Abstract
Methane is the 2nd most important anthropogenic greenhouse gas and it has been increasing rapidly at a rate of 6 ppb/year in the recent decade. Few modeling studies have investigated the effectiveness of methane (CH4) forcing in causing climate change when compared to CO2. In this study, using idealized step-forcing simulations, we investigate the efficacy of CH4 forcing relative to CO2 forcing where efficacy is the ratio of the equilibrium global mean temperature change per unit forcing by that agent relative to the equilibrium temperature change per unit CO2 forcing from the same initial climate state. For a 10 fold increase in CH4 concentration that produces the same long term global mean warming ( 1.7 K) as a one-third increase in CO2 concentration, we find that the efficacy of CH4 forcing is 82% in the NCAR CAM5 climate model (Fig.1a). We find that the feedback mechanisms associated with fast adjustment in the first 2-4 month period, which includes rapid changes in clouds, upper tropospheric thermal structure and land surface adjustments leads to a steeper slope in the regression between the TOA net radiative fluxes and surface temperature in the CH4 forcing case (Fig. 1a and 1b). This implies a stronger net negative feedback and hence correspondingly the need for a larger CH4 forcing than CO2 forcing for the same equilibrium surface warming. We further find that the hydrological sensitivity is weaker for CH4 forcing (2.05 %/K for CO2 versus 1.87 %/K for CH4 forcing). The differing fast response mainly contributes to this difference: the fast precipitation response to CH4 forcing is -1.25 % while it is only -0.75 % for the equivalent CO2 forcing. Our study indicates that CH4 is less effective in causing climate change when compared to CO2. However, further investigation is needed and we recommend a multi-model intercomparison framework to assess the robustness of our results.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A33B0205M
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSESDE: 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSESDE: 1626 Global climate models;
- GLOBAL CHANGEDE: 1627 Coupled models of the climate system;
- GLOBAL CHANGE