APS March 2015 – Talk D33.00002
Observational Determination of
Surface Radiative Forcing by CO2 and CH4
Daniel Feldman
Lawrence Berkeley Lab
William Collins, and Margaret Torn
Lawrence Berkeley Lab
With assistance from
Jonathan Gero
University of Wisconsin, Madison
Eli Mlawer
Atmospheric and Environmental Research
Timothy Shippert
Pacific Northwest National Laboratory
GHG Concentrations and Forcing
IPCC AR5
• While the concentrations are observed,
these forcings are strictly modeled.
• Can we observe climate forcing?
CO2 Infrared Absorption
• CO2 symmetric molecular structure leads to a series of infrared
absorption features that absorb the energy the Earth emits.
• Databases tabulate these features but are periodically updated as
measurements and theory improve.
Rothman, 2013
3
Goody and Yung, 1989
Radiative Forcing and Climate Models
HITRAN Line
Parameters
Climate Model
Integrations
Radiative Transfer
Code
Radiative
Forcing
Line Mixing,
Continuum Absorption
4
Objectives of This Study
• Determine whether the basis of climate change
projections, namely GHG forcing, can be
observed outside the laboratory.
• Provide direct observational evidence how
changes in GHGs are altering the
surface energy balance.
• Determine whether climate models are
reproducing surface forcing from WMGHGs
(other work).
0
1
2
3
4
5
6
3 3.2 3.4 3.6 3.8 4 4.2 More
Fre
qu
en
cy
Forcing ( W / m 2 )
2 xCO2 Forcing in I PCC Models
Collins et al, 2005
Rasch et al, 2014
ARM Sites
6
The Atmospheric Radiation Measurement (ARM) program run by the
US Department of Energy has sponsored decades of intensive
measurements at sites in Alaska and Oklahoma
The Data and Tools We Use
In situ data on WMGHGs Active Remotely-Sensed Cloud Locations
Radiosonde Atmospheric Profiles
AERI
Microwave Water Retrieval
The Role of Simulations
90% of energy within 3σ Noise-Equivalent Radiance
The Spectral Fingerprint of CO2
Trend is zero based on linear least-squares fit and a χ2 test at the 0.01 level.
1st Detection of CO2 Surface Forcing
(a) (b)
(c) (d)
• First empirical confirmation of surface radiative forcing:
(from 23 ppmv increase in atmospheric CO2 from 2000-2010).
SGP
NSA
Task 1 CO2 Time Series Task 4 CO2 Forcing Time Series and Power Spectra
Feldman et al, 2015
0.2 W/m2/decade
0.2 W/m2/decade
Growing Surface CH4 Forcing?
Methane growth rate by latitude
E G Nisbet et al. Science 2014;343:493-495
Recent global methane concentrations
• We can use an analogous procedure to derive CH4 surface longwave forcing.
CH4 Time Series at ARM Sites
• North Slope of Alaska site shows clear CH4 hiatus in 2000s ending ~ 2005.
• Southern Great Plains shows much higher CH4 concentrations than NSA.
1st Detection of CH4 Surface Forcing
Observational Determination of:
• Time series of radiative forcing by CH4: Signals of “hiatus” and recent rise.
• Quantification of impact of CH4 on the Earth’s surface energy budget
• Determination of the radiative signal from seasonal cycle.
• We have quantified trends in
surface radiation from ΔCO2 and
from ΔCH4.
• Trends in forcing:
• CO2: 0.2 + 0.05 Wm-2 / decade
• CH4: 0.2 + 0.1 Wm-2 / decade
• We have independently determined
the increasing greenhouse effect
due to rising CO2 and CH4.
• We find the basis for climate forcing
by GHGs to be robust and
consistent with observations.
• For more details, see:
Nature (2015) doi:10.1038/nature14240
Backup Slides
Global Sources of Methane
Anthropogenic sources (70%) • Fossil fuel 100 Tg CH4 yr-1 • Ruminant animals 80 Tg CH4 yr-1
• Rice agriculture 60 Tg CH4 yr-1 • Landfills 60 Tg CH4 yr-1 • Biomass burning 50 Tg CH4 yr-1
Natural sources (30%) • Wetlands 100 Tg CH4 yr-1 • Termites 20 Tg CH4 yr-1
• Geological 10 Tg CH4 yr-1
• Hydrates 5 Tg CH4 yr-1 • Oceans 4 Tg CH4 yr-1
• Wildfires 2 Tg CH4 yr-1
Denman et al. (2007) (IPCC, Ch. 7)
Thanks to I. Simpson, UC Irvine
Global Sources of Ethane
Anthropogenic sources • Fossil fuel 8-9 Tg yr-1 • Biomass burning 2-3 Tg yr-1
• Biofuel 2-3 Tg yr-1
Minor sources • Oceans 0-0.5 Tg C yr-1 • Vegetation Negligible
Xiao et al. (2008); Pozzer et al. (2010)
• Ethane has a fairly simple source budget
• Ethane’s major sources are shared with methane
Thanks to I. Simpson, UC Irvine
Using Ethane to Understand Methane
Ethane + methane sources: • Fossil fuel • Biomass burning
Methane-only sources: • Wetlands • Rice paddies • Ruminants • Landfills
Time Time
Co
nce
ntr
atio
n
Co
nce
ntr
atio
n Methane Methane
Ethane
Ethane
Thanks to I. Simpson, UC Irvine
Evidence for Joint (M)Ethane Source
• High correlation with ethane in Oklahoma.
• Low correlation with ethane on Alaska coastline.
Absence of Combustion Signatures
• Little correlation with CO, a product of combustion.
• Negligible correlation with AOD, including smoke AOD.
Signals of Other Agents in CH4 Band
• Methane forcing signal is convolved with effects of N2O, H2O, and T (of course).
• Uncertainties in non-CH4 agents introduce large uncertainties in FLBLRTM (t,t0).