Download WHY DO WE CARE ABOUT METHANE?

METHANE
TOPICS FOR TODAY
1.
Why do we care about methane?
2.
What are the sources and concentrations of methane in
the atmosphere?
3.
Uncertainties in the current methane budget
4.
Methane in a changing climate
WHY DO WE CARE ABOUT METHANE?
(1) GHG #2
(22 times more effective than CO2)
(2) Contributes to background O3
(GHG #3 and air quality)
(3) Controls oxidizing capacity of
troposphere
(via OH)
(4) Source of stratospheric
water vapour
(enhances HOx catalyzed O3
depletion)
(5) Methane in stratosphere can
reduce O3 depletion
(termination of ClOx cycling)
IPCC [2007]
HISTORICAL TRENDS IN METHANE
The last 20 years
The last 1000 years
Currently, atmospheric concentration of methane is 1774 ppm
(unprecedented in last 650 kyr)
IPCC [2007]
LOOKING BACK EVEN FURTHER…
TOPICS FOR TODAY
1.
Why do we care about methane?
2.
What are the sources and concentrations of methane
in the atmosphere?
3.
Uncertainties in the current methane budget
4.
Methane in a changing climate
GLOBAL METHANE EMISSIONS
~300 Tg CH4 yr-1 Anthropogenic [EDGAR 3.2 Fast-Track 2000; Olivier et al., 2005]
~200 Tg CH4 yr-1 Biogenic sources [Wang et al., 2004]
>25% uncertainty in total emissions
Clathrates?
Melting permafrost?
Plants?
BIOMASS BURNING
+ BIOFUEL
ANIMALS
30
WETLANDS
90
180
GLOBAL METHANE
SOURCES
(Tg CH4 yr-1)
Slide c/o Arlene Fiore (GFDL)
TERMITES RICE 40
20
LANDFILLS +
WASTEWATER
50
GAS + OIL
60
COAL
30
A.M. Fiore
SINKS OF ATMOSPHERIC METHANE
I. Transport to the Stratosphere (40 TgCH4/yr)
Only a few percent, rapidly destroyed  BUT the most
important source of water vapour in the dry stratosphere
II. Tropospheric oxidation (511 TgCH4/yr)
O2
CH4 + OH  CH3O2CO + other products
OH estimated to
have decreased by
10-30% from PI to
PD  trend in CH4
therefore related to
sources not sinks
[Wuebbles and Hayhoe, 2002]
III. Biological oxidation in soil (30 TgCH4/yr)
Tropospheric Lifetime ~ 9 years
IPCC [2007]
SPACE-BASED METHANE COLUMN OBSERVATIONS
by solar backscatter at 2360-2385 nm
GLOBAL DISTRIBUTION OF METHANE
NOAA/GMD surface air measurements
MLO site
TOPICS FOR TODAY
1.
Why do we care about methane?
2.
What are the sources and concentrations of methane in
the atmosphere?
3.
Uncertainties in the current methane budget
4.
Methane in a changing climate
ESTIMATES OF CURRENT METHANE EMISSIONS
Total CH4 source ~600 Tg yr-1, ~60% anthropogenic [IPCC AR-4]
>25% uncertainty in present-day CH4 sources
Plants?
anthropogenic
c/o Michael Raupach, CSIRO, Australia; studies cited in IPCC TAR, AR-4
Methane sink (OH) also ~30% uncertain [Stevenson et al., 2006]
Slide c/o Arlene Fiore (GFDL)
WHY HAVE METHANE CONCENTRATIONS LEVELED OFF?
How well do we understand interannual variability?
CH4 (ppb)
Major driver for 97-98 anomaly?
-- Biomass burning
(CH4 emission, OH suppression via CO)
[e.g.Butler et al., 2005; Duncan et al., 2003;
Bousquet et al., 2006;Langenfelds et al., 2002]
CH4 growth rate (ppb yr-1)
-- Wetlands
[e.g. Dlugokencky et al. 2001;
Cunnold et al., 2002; Wang et al., 2004,
Mikaloff Fletcher 2004, Chen and Prinn, 2006]
Slide c/o Arlene Fiore (GFDL)
MODEL STUDIES INDICATE DIFFERENT DRIVERS FOR
OBSERVED DECADAL TRENDS
Study
Period
Approach
Major driver of trends / IAV*
Law and Nisbet, 1996
1980-1994
2D CTM
1991 on FSU emis decline
Bekki & Law, 1997
1980-1992
2D CTM
wetlands & OH
Dlugokencky et al, 2003 1984-2002
Obs. analysis
approach to steady state?
Karlsdottir and Isaksen,
2000
1980-1996
3D CTM (met fixed)
OH (+anthrop SE Asian
emis, -strat. o3)
Johnson et al, 2002
20 years
3D CTM (emis fixed)
OH (water vapor)
Warwick et al, 2002
1980-1998
3D CTM (OH fixed)
transport
Dentener et al, 2003ab
1979-1993
semi-inverse 3D CTM
OH (mainly water vapor)
Wang et al, 2004
1988-1997
3D CTM
anthrop emis & OH (-strat
O3)
Dalsoren and Isaksen,
2006
1990-2001
3D CTM (only emis. vary)
OH (+anthrop. CO, NOx,
NMVOC emissions)
Fiore et al, 2006
1990-2004
3D CTM (only met. varies)
OH (lightning NOx) + T
Bousquet et al, 2006
1984-2003
inverse 3D CTM
anthrop emis; post-1998
(OH from CH3Cl inversion) +anthrop - wetlands
Khalil et al, 2007
23 years
Obs. analysis
constant emis. and lifetime
Drevet et al., 2008
1990-2005
3D CTM
Anthrop. emis + OH
* Many of these studies also identify a large role for wetlands and BB on IAV
TRENDS IN THE METHANE SINK?
OH difficult to measure
directly, concentrations
inferred from methyl
chloroform
Significant interannual
variability, but no clear
trend
IPCC, 2007
METHANE EMISSION FROM PLANTS?
SCIAMACHY-TM3 Model
 Scale lab observations to estimate a
global source of 62–236 Tg yr-1.
Suggest that this can reconcile high
CH4 observed over tropical forests
Production mechanism unknown
 VERY CONTROVERSIAL
Frakenberg et al., Science 2005
Following this,
Houweling et al. [2006] set 125 Tg CH4 yr-1 as an upper limit (inverse modeling)
Kirschbaum et al. [2006] estimate source at 10-60 Tg CH4 yr-1
Kirschbaum et al. [2008] confirmed that source is not due to absorption/desorption
Important implications for tree-planting to mitigate climate change!
TOPICS FOR TODAY
1.
Why do we care about methane?
2.
What are the sources and concentrations of methane in
the atmosphere?
3.
Uncertainties in the current methane budget
4.
Methane in a changing climate
PROJECTIONS OF FUTURE METHANE EMISSIONS
(Tg CH4 yr-1)
Only accounting for growth in
anthropogenic emissions (not
feedbacks)
IPCC SRES, 2001
IMPACT OF CLIMATE CHANGE ON METHANE BUDGET
I. Most natural sources (wetlands, landfills, rice agriculture, biomass burning)
are affected by T and moisture
= positive feedback on CO2-induced warming
 Degree of emission enhancement affected by water
management and methods of cultivation (irrigated, rain-fed,
deepwater, etc.)
II. Feedbacks on the sink: OH = f(T, H2Ovap, NOx)
METHANE HYDRATES: “ICE THAT BURNS”
Increasing usage as
an energy source?
Destabilized with
warming events?
Could be a large
positive feedback.
MELTING OF THE PERMAFROST AND METHANE RELEASE
FROM PEATLANDS?
Methane emitted from thaw lakes and soils
(1) How much of permafrost carbon pool will be
converted to methane?
(2) What fraction of the pool will thaw under
anaerobic vs aerobic conditions?
Zhuang, EOS, 2009
Chapman and Thurlow [1996] extrapolated the
relationship between methane fluxes and
temperature at bogs in Scotland to predict a
CH4 emission increase of 17, 30 and 60% for
warmings of 1.5°C, 2.5°C, and 4.5°C