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Observed differences in surface radiative forcing trends from methane

 
Poster PDF

Authors

Daniel Feldman — Lawrence Berkeley National Laboratory
William D. Collins — Lawrence Berkeley National Laboratory
Sebastien Christophe Biraud — Lawrence Berkeley National Laboratory
David D. Turner — NOAA- Global Systems Laboratory
Jonathan Gero — University of Wisconsin
Shaocheng Xie — Lawrence Livermore National Laboratory
Eli Jay Mlawer — Atmospheric and Environmental Research, Inc.
Timothy R. Shippert — Pacific Northwest National Laboratory
Margaret S. Torn — Lawrence Berkeley National Laboratory

Category

Radiation

Description

As the second most important anthropogenic greenhouse gas, methane has exhibited an as-yet-unexplained hiatus in rising atmospheric concentrations between 1995 and 2005 and spatially heterogeneous trends. Even though methane exhibits remarkable spectroscopic complexity and has been frequently updated in the HITRAN databases, its radiative forcing has only been calculated but not measured. Here we measure that clear-sky longwave surface radiative forcing using spectroscopic observations and ancillary data at Atmospheric Radiation Measurement (ARM) program sites. During the hiatus, the trend in forcing does not differ significantly from zero, but subsequently, the trend in forcing at the Southern Great Plains site is 0.3±0.1 W/m2/decade and is thus 50% higher than from CO2. At the North Slope of Alaska site, the trend is 0.1±0.05 W/m2/decade, indicating that CH4 forcing trends are more spatially heterogeneous than for CO2, and are larger near anthropogenic extraction than in pristine environments. We also find that the forcing from CH4 depends on humidity and trends thereof, which can lead to more spatial heterogeneity in forcing.