METHANEMETHANE

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)

(5) Methane in stratosphere can reduce O3 depletion (termination of ClOx cycling)

(3) Controls oxidizing capacity of troposphere (via OH)

(4) Source of stratospheric water vapour(enhances HOx catalyzed O3 depletion)

IPCC [2007]

HISTORICAL TRENDS IN METHANE

The last 1000 years

The last 20 years

IPCC [2007]

Currently, atmospheric concentration of methane is 1774 ppm (unprecedented in last 650 kyr)

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

ANIMALS90

LANDFILLS +WASTEWATER

50GAS + OIL60

COAL30RICE 40TERMITES

20

WETLANDS180

BIOMASS BURNING + BIOFUEL 30

GLOBAL METHANESOURCES

(Tg CH4 yr-1)

Clathrates?Melting permafrost?Plants?

A.M. FioreSlide c/o Arlene Fiore (GFDL)

SINKS OF ATMOSPHERIC METHANEI. 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)

CH4 + OH CH3O2CO + other products

III. Biological oxidation in soil (30 TgCH4/yr)

O2

Tropospheric Lifetime ~ 9 years

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]

IPCC [2007]

SPACE-BASED METHANE COLUMN OBSERVATIONSby 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

c/o Michael Raupach, CSIRO, Australia; studies cited in IPCC TAR, AR-4

ESTIMATES OF CURRENT METHANE EMISSIONS

Methane sink (OH) also ~30% uncertain [Stevenson et al., 2006]

Total CH4 source ~600 Tg yr-1, ~60% anthropogenic [IPCC AR-4]>25% uncertainty in present-day CH4 sources

anthropogenic

Plants?

Slide c/o Arlene Fiore (GFDL)

WHY HAVE METHANE CONCENTRATIONS LEVELED OFF?How well do we understand interannual variability?

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]

-- Wetlands [e.g. Dlugokencky et al. 2001; Cunnold et al., 2002; Wang et al., 2004, Mikaloff Fletcher 2004, Chen and Prinn, 2006]

CH4 (ppb)

CH4 growth rate (ppb yr-1)

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 (OH from CH3Cl inversion)

anthrop emis; post-1998 +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?

Significant interannual variability, but no clear

trend

OH difficult to measure directly, concentrations

inferred from methyl chloroform

IPCC, 2007

METHANE EMISSION FROM PLANTS?

Scale lab observations to estimate a global source of 62–236 Tg yr-1. Suggest that this can reconcile high CH4 observed over tropical forestsProduction mechanism unknown VERY CONTROVERSIAL

SCIAMACHY-TM3 Model

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)

IPCC SRES, 2001

Only accounting for growth in anthropogenic emissions (not

feedbacks)

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?

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

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