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Atmospheric Methane. Paul Wennberg. Radiative Properties – as a greenhouse gas Atmospheric Chemistry – contributes to control of [OH] Budgets – what processes determine the concentration of CH 4 ?. Climate Change 2001: Working Group I: The Scientific Basis

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atmospheric methane

Atmospheric Methane

Paul Wennberg

  • Radiative Properties – as a greenhouse gas
  • Atmospheric Chemistry – contributes to control of [OH]
  • Budgets – what processes determine the concentration of CH4?
slide2

Climate Change 2001:Working Group I: The Scientific Basis

Figure 2: Long records of past changes in atmospheric composition provide the context for the influence of anthropogenic emissions.(a) shows changes in the atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over the past 1000 years. The ice core and firn data for several sites in Antarctica and Greenland (shown by different symbols) are supplemented with the data from direct atmospheric samples over the past few decades (shown by the line for CO2 and incorporated in the curve representing the global average of CH4). The estimated positive radiative forcing of the climate system from these gases is indicated on the right-hand scale. Since these gases have atmospheric lifetimes of a decade or more, they are well mixed, and their concentrations reflect emissions from sources throughout the globe. All three records show effects of the large and increasing growth in anthropogenic emissions during the Industrial Era. (b) illustrates the influence of industrial emissions on atmospheric sulphate concentrations, which produce negative radiative forcing. Shown is the time history of the concentrations of sulphate, not in the atmosphere but in ice cores in Greenland (shown by lines; from which the episodic effects of volcanic eruptions have been removed). Such data indicate the local deposition of sulphate aerosols at the site, reflecting sulphur dioxide (SO2) emissions at mid-latitudes in the Northern Hemisphere. This record, albeit more regional than that of the globally-mixed greenhouse gases, demonstrates the large growth in anthropogenic SO2 emissions during the Industrial Era. The pluses denote the relevant regional estimated SO2 emissions (right-hand scale).[Based upon (a) Chapter 3, Figure 3.2b (CO2); Chapter 4, Figure 4.1a and b (CH4) and Chapter 4, Figure 4.2 (N2O) and (b) Chapter 5, Figure 5.4a]

slide5
The growth in atmospheric methane also contributes to moistening of the stratosphere. Air entering the stratosphere is desiccated to 3-4 ppmv by the very cold temperatures present at the tropical tropopause (190-200 K). As air remains in the stratosphere the methane is oxidized to CO2 and H2O and the moisture increases to nearly 8 ppmv in the 'oldest' air (i.e. where [CH4]  0). Because H2O in the lower stratosphere is such a good GHG (it is cold and has high extinction), this source of H2O must be considered in future climate predictions.
slide6

As illustrated in the IPCC CH4 figure, the growth rate of atmospheric methane has been highly variable. During the 1980s, the trend was fairly constant at 10 ppbv /yr. During the 1990s the rate has oscillated with some years with essentially no trend (e.g. 1992-1993, 1999-2000).

There is no consensus view on what causes the variability in the methane growth rate. Some research has pointed to variability in the sources driven by changes in the atmospheric hydrological cycle; others have pointed to possible variability in [OH] driven by changes in stratospheric ozone. Given the lack of understanding of recent trends, prediction of future methane concentration remains highly uncertain.

slide9

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide10

Photochemistry – The primary sink of atmospheric methane

Methane is destroyed primarily in the troposphere (90% of the loss) when it is oxidized by the hydroxyl radical (OH) – (ESE/Ch/Ge 171):

OH + CH4 CH3 + H2O

OH is the premier oxidant in Earth's atmosphere. It is formed in the daytime via gas phase photochemistry; its major source is:

O3 + hO (1D) + O2

O (1D) + H2O  OH + OH

h represents a photon of wavelength < 315 nm. O (1D) is the first electronically excited oxygen atom. In the troposphere, OH has an average mixing ratio of ~ 1 x 106 molecule cm-3. At 273 K, the rate coefficient for the reaction of OH with methane is about 3.5 x 10-15 cm3 molecule-1 s-1. Thus:

CH4 = [CH4]/(kOH+CH4 [OH][CH4]) = 1/( kOH+CH4 [OH]) = 1/(2.8 x 108 s-1) = 9 years.

Because the reaction of OH with CH4 is an important sink of tropospheric OH, the lifetime of methane is not independent of the concentration of methane. As the concentration of methane increases, the lifetime of methane also increases - a positive feedback. [Prather et al.,Lifetimes and Eigenstates in atmospheric chemistry, GRL, 21, 801, 1994]

slide11

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide12

Modeled Methane Emissions - Biome4

Present Wetlands

(140 Tg/yr)

LGM Wetlands

(107 Tg/yr)

Kaplan

(GRL, 2002)

factors controlling methane emissions from wetlands
Factors Controlling Methane Emissions from Wetlands?

Temperature, Water balance, Productivity

Bubier and Moore, Trends in Ecology and Evolution (1994)

slide14

Vostok (Antarctica) Ice Core-Orbital Cycles

Inter-

glacial

Inter-

glacial

Inter-

glacial

Inter-

glacial

Warm

*

Glacial

Glacial

Glacial

Cold

Petit et al. (Nature, 1999)

* D/H, relative to a standard - proxy for surface air temperature

slide17

El Niño

E.J. Dlugokencky et al, GRL, 30 (19), 2003.

slide18

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide19

Rice Paddies are Man-made Wetlands

http://www.gsfc.nasa.gov/topstory/2002/1204paddies.html

engineering solutions
Engineering Solutions

Abstract:Decreased methane emissions from paddy rice may have contributed to the decline in the rate of increase of global atmospheric methane (CH4) concentration over the last 20 years. In China, midseason paddy drainage, which reduces growing season CH4 fluxes, was first implemented in the early 1980s, and has gradually replaced continuous flooding in much of the paddy area. We constructed a regional prediction for China's rice paddy methane emissions using the DNDC biogeochemical model. Results of continuous flooding and midseason drainage simulations for all paddy fields in China were combined with regional scenarios for the timing of the transition from continuous flooding to predominantly mid-season drainage to generate estimates of total methane flux for 1980-2000. CH4 emissions from China's paddy fields were reduced over that period by ~ 5 Tg CH4 yr(-1).

  • Reduced methane emissions from large-scale changes in water management of China's rice paddies during 1980-2000, Li CS, Qiu JJ, Frolking S, Xiao XM, Salas W, Moore B, Boles S, Huang Y, Sass RGEOPHYSICAL RESEARCH LETTERS 29 (20): art. no. 1972 OCT 15 2002
rice agriculture
Rice Agriculture
  • The case for human causes of increased atmospheric CH4Ruddiman WF, Thomson JSQUATERNARY SCIENCE REVIEWS, 20 (18): 1769-1777 DEC 2001

The anthropogenic greenhouse era began thousands of years ago. W.F. Ruddiman. (2003). CLIMATIC CHANGE 61(3): 261-293.

slide22

The anthropogenic greenhouse era began thousands of years ago. W.F. Ruddiman. (2003). CLIMATIC CHANGE 61(3): 261-293.

slide23

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide24

Biomass Burning Source

Fig. 3. Atmospheric CO (A), CO2 (B), and CH4 (C) growth rate anomalies from the NOAA/CMDL network (solid lines). Fire contributions to the observed growth rates from the forward model are represented with a dotted line, and inversion model estimates are represented with a dashed line (with 1 error bars). The error estimates were obtained by combining the errors (in quadrature) from the emission factors reported in Table 1 with the uncertainties on the scalars obtained from the least squares inversion.

Continental-Scale Partitioning of Fire Emissions During the 1997 to 2001 El Niño/La Niña Period

Guido R. van der Werf,James T. Randerson,G. James Collatz,Louis Giglio,Prasad S. Kasibhatla,Avelino F. Arellano, Jr.,Seth C. Olsen,Eric S. Kasischke, Science, Vol 303, Issue 5654, 73-76 , 2 January 2004

a signature of water or fire
A signature of water or fire?

E = d[CH4]/dt + [CH4]/Tau

Tau = 8.9 years

25 Tg “anomalous source”

E.J. Dlugokencky et al, GRL, 30 (19), 2003.

slide26

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

engineering solutions28
Engineering Solutions

A Novel Way to Reduce Methane Emissions from Domestic Ruminants

Fievez, V., Dohme, F., Danneels, M., Raes, K. and Demeyer, D.  2003.  Fish oils as potent rumen methane inhibitors and associated effects on rumen fermentation in vitro and in vivo.  Animal Feed Science and Technology104: 41-58.

What was doneThe authors studied the effects of two types and seven levels of fish-oil feed additives on methane production associated with the rumen fermentation of sheep by means of both in vitro and in vivo experiments.

What was learnedA maximal 80% decline in ruminant methane production was observed with fish oil containing n-3-eicosapentaenoic acid.

slide29

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide31
Global methane emissions from landfill are estimated to be between 30 and 70 Tg each year. Most of this landfill methane currently comes from developed countries, where the levels of waste tend to be highest.
  • Landfills provide ideal conditions for methanogenesis, with lots of organic material and anaerobic conditions prevalent. The huge amounts of waste that are buried in landfill sites can mean that methane is produced for years after the site is closed, due to the waste slowly decaying under the ground.
  • Methane escapes from landfills either directly to the atmosphere or by diffusion through the cover soil.
  • Highly active communities of methanotrophs can develop in these overlying soils and these can greatly reduce the amounts of methane emitted.
engineering solutions32
Engineering Solutions
  • Cover and flare / burn for energy
  • Cover with soil / encourage methanotrophs
  • Liners
slide33

IPCC TAR Methane Budget Summary

Sinks

Anthropogenic

Sources

Natural

slide34
30-50 Tg methane emission per year are thought to be released by coal mining. The bulk of global methane emissions come from only a few relatively deep mines around the world. Coalbed methane now viewed as an economically viable resource.
slide35
Oil: As with coal, the geological formation of oil can result in large methane deposits associated with the oil. During oil drilling and extraction, the trapped methane is released to the atmosphere. Targeted collection of the methane associated with oil can vastly reduce emissions from this source, with the collected methane being flared off as carbon dioxide or even providing an additional fuel source.
  • Gas: The loss of methane during natural gas extraction is obviously something which has both a direct greenhouse gas and an economic cost. Nevertheless, significant amounts of methane are lost during both extraction and transfer of methane above ground. It is estimated, for example, that in the 1990's around 6 percent of the methane piped across Russia was lost due to leaks. More efficient collection techniques, improved targeting of buried methane deposits and better maintained transfer pipelines could all help reduce incidental methane emissions from this source.
  • Former soviet republics: Estimates by Olivier [2002] suggest that methane emissions from fsu decreased significantly between 1990 and 1995 (11 Tg). About 8 Tg of the decrease result from decreased fossil fuel production (mid-late1990s production = 2/3 or late 1980s). Natural gas production declined 17% between 1991 and 1997. Several reports estimate emissions of CH4 from natural gas industry decreased more than 20 Tg yr-1 ($4 billion)!
the future of methane
The future of methane

“Atmospheric methane levels off: Temporary pause or new steady-state?”

E = d[CH4]/dt + [CH4]/Tau

Tau = 8.9 years

25 Tg “anomalous source”

Interpolar difference (N-S gradient )

E.J. Dlugokencky et al, GRL, 30 (19), 2003.

satellite measurements of ch 4
Satellite Measurements of CH4

SCIAMACHY measurements of column-averaged CH4 (units of ppbv), August to November, 2003.

Frankenberg et al., Science Express, 17 March 2005

satellite measurements of ch 438
Satellite Measurements of CH4

Global chemistry-transport model calculations of column-averaged CH4, August to November, 2003, using current emission inventory, for time and location of the SCIA measurements.

Difference between SCIA measurements of CH4 column and model calculation. Largest differences are over tropical broadleaf evergreen forests in South America, central Africa, and Indonesia.

Frankenberg et al., Science Express, 17 March 2005