Understanding Methane Concentrations

Nicola Warwick1,

Euan Nisbet2, John Pyle1

1- Centre for Atmospheric ScienceUniversity of Cambridge

2 – Royal Holloway, University of London

Quest CH4 Workshop, June 2004

Outline

• CH4 observations

• Understanding the global distribution and seasonal cycles

• Understanding variability and trends

• Outlook

Factors controlling CH4

• Emissions (natural and anthropogenic)• Sinks (OH, soil)• Meteorology (winds, temperatures, rainfall)

July model surface CH4 (ppbv)

July Surface OH in model (106molecules/cm3)

Methane Measurements

• In-situ and flask data:Global network of 40-50 stations mainly including: US-NOAA-CMDLAustralian CSIRONew Zealand NIWAMeth-MonitEUr – European Network

• Satellite observations

• Ice-cores

Observed CH4 1992-2002

• Observed CH4 mixing ratios (Marine Boundary Layer) from 1992 to 2001, NOAA-CMDL

Southern Hemisphere

Northern Hemisphere

CH4 Measuring Stations

Mace Head

AlertNy-Ålesund

Mace HeadParisHeidelberg /SchauinslandKrakow / KasprowySt. PetersburgOb RiverLondon

Main methane sourceregions

PallasWetlandand gasAtlantic background airreaches west Europeanmargins

Meth-MonitEUr: Methane monitoring in the European region.

How Mean Carbon-Gas Concentrations have varied with Time

0

200

400

600

800

1000

1200

Dec-95 Dec-96 Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02 Dec-03

CO/ppb - 90 CH4/ppb - 1800 (CO2/ppm - 360) x 10

Ny-Alesund

Ny-Alesund

Mace Head

AlertNy-Ålesund

Mace HeadParisHeidelberg /SchauinslandKrakow / KasprowySt. PetersburgOb RiverLondon

Main methane sourceregions

PallasWetlandand gasAtlantic background airreaches west Europeanmargins

Meth-MonitEUr: Methane monitoring in the European region.

How Mean Carbon-Gas Concentrations have varied with Time

0

200

400

600

800

1000

1200

Dec-95 Dec-96 Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02 Dec-03

CO/ppb - 90 CH4/ppb - 1800 (CO2/ppm - 360) x 10

Royal Holloway College, W London (provided by E. Nisbet, D. Lowry)

Testing our understanding of CH4

• Global modelling (forward and inverse)

• Regional modelling

• Back trajectory analyses / diurnal experiments

• Isotopes

December surface CH4 by MATCH model (provided by Kim Holmen, NILU)

13C-CH4 Isotopic Fractionation of CH4

Sources

Light Heavy

Methane sources have a wide range of distinct 13CH4 signatures further constraint on emission scenarios.

Adapted from Chanton et al. [2000]

-47%

KIE

Global CH4 and 13CH4 Distributions

Wetlands

Fossil Fuel

Rice Surface 13CH4 (‰) - January monthly

mean

Surface CH4 (ppbv) - January monthly mean

How well do measurements describe the CH4 burden?

Blue line: modelled zonal mean CH4 and 13C-CH4

Red Circles: modelled CH4 and 13C-C-CH4 at measuring station locations

Black squares: observed CH4 and 13C-CH4 [Dlugokencky et al., 1998, Miller et al., 2002]

Houwelling et al. (1999): relative contribution of NH sources decrease from 77% to 67%.

Breakdown of seasonal cycles by source

Southern Hemisphere

(Modelled CH4 at Ascension Island,

Atlantic, 7°S)

Northern Hemisphere

(Modelled CH4 at Alert, Canada, 82°N)

Alternative Emission Scenarios

Source / Tg Scenario 1 (base)

Scenario 2 (wtnd)

Scenario 3 (ff)

Scenario 4

(wtld&ff)

Wetlands 115 61 115 85

Fossil Fuel 89 89 35 65

Biomass Burning

54 108 108 108

Other 254 254 254 254

Global Total (Tg/yr)

512 512 512 512

Alternative Emission Scenarios: Results

Niwot Ridge, Colorado

(106W, 40N)

Cape Grim, Tasmania

(145E, 41S)

South Pole

(25W, 90S)

Measurements courtesy of NOAA/CMDL/CCGG

Understanding Interannual Variability and Trends

Feedback Timescales

Year-to-Year Millennial and Longer…

Petit, J.R., et al., 2001, Vostok Ice Core Data for 420,000 Years, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2001-076. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

Dlugokencky et al., 1998, 2003

NOAA Observed Interpolar Gradient

Dlugokencky et al., GRL, 2003

CH4 Variability due to Meteorology

• Black line = observed growth rate

• Dashed line = modelled growth rate

Correlation Coefficients:

Key Biscayne, r=0.61 Ascension, r=0.59

Modelled and Observed CH4 Variations

Simulated CH4 interannual variability arising from changes

in the circulation

Observed CH4 growth rate (NOAA CMDL Carbon Cycle

Greenhouse Gases)

• Changes in modelled interpolar gradient resulting from meteorology: ~up to 0.5%

Influence of Meteorology on CH4

• Tropics: – OH (will vary with humidity) – Tropical forest fires / wetlands– El Niño

Influence of el Nino on fire emissions – van der Werf et al., 2004.

Measured CH4 at Ascension Island (D. Lowry, E. Nisbet)

Influence of Meteorology on CH4

• Tropics: – OH (will vary with humidity) – Tropical forest fires / wetlands– El Nino– Winds (alter inter-hemispheric mixing)

• Northern Latitudes:– Northern wetlands

1700

1800

1900

2000

2100

2200

2300

2400

2000 2001 2002 2003Year

ÑÍ4 concentration, ppb

Yamal

Teriberka

St. Cape Kamenny St. New Port

CH4 measurements at Yamal Peninsula compared to Teriberka background level

Problems and Outlook

• Model simulations still have trouble reproducing CH4 observations– Need more continental data (e.g. South America, Africa

Asia) to test models

• Satellites: – look for abrupt changes (e.g tropical fires)– infer surface fluxes (need high precision)

• Isotopes:– High-precision ground-based isotope measurements can

distinguish sources (help quantify wetland source)

The End

The global CH4 burden may be higher than previously thought.....

• Use modelled radon concentrations to distinguish between ‘clean’ and ‘dirty’ air

• True global mean = 1.3% greater than ‘clean’ global mean• By 2008-12, Kyoto Protocol envisages reduction in GHG emissions of

~5% w.r.t. 1990 values

Modelled annual mean surface CH4 / ppbv

Observed and modelled CH4 seasonal cycles

The Atmospheric CH4 Record: 1000 to 2000

Change in CH4 abundance for the last 1000 years.

IPCC TAR Fig 4-1.

Carbon Gas Measurements: 1996-2003

Carbon gas measurements at Royal Holloway College, W London (provided by E. Nisbet, D. Lowry)

How Mean Carbon-Gas Concentrations have varied with Time

0

200

400

600

800

1000

1200

Dec-95 Dec-96 Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02 Dec-03

CO/ppb - 90 CH4/ppb - 1800 (CO2/ppm - 360) x 10

CH4 Variability due to Sinks

• Prinn et al. 2001: 1.4 ± 2.1% yr-1 upward trend 1979-1989

2.3% yr-1 downward trend 1990-2000

• Krol et al. 1998, 2001: 1978-1993, 0.46 ± 0.6% yr-1

• Krol and Lelieveld 2003: possible problems with CH3CCl3 data.

• Dentener et al. 2003: 1979-1993, 0.24 ± 0.06% yr-1

large interannual variability of OH (1.5%)