Connecting atmospheric composition with climate variability and change Seminar in Atmospheric Science, EESC G9910 9/19/12 Observed methane trends in recent

Connecting atmospheric composition with climate variability and change

Seminar in Atmospheric Science, EESC G9910

9/19/12 Observed methane trends in recent decades: Emission trends or climate variability?

1. Aydin et al., Nature, 2011 (fossil fuel)Study period: 20th century; ethane:methane in firn air

2. Kai et al., Nature, 2011 (NH microbial sources)study period: 1984-2005; isotopic source signature

3. Hodson et al., GRL, 2011 (ENSO and wetlands)study period: 1950-2005; process modeling

GMD monitoring network

http://www.esrl.noaa.gov/gmd/dv/site/map1.php

The Methane Mystery: Leveling Off then Reboundinghttp://www.esrl.noaa.gov/gmd/aggi/

Help characterizing sources from isotopes + co-emitted species Inverse constraints on sinks (confidence?)[Montzka et al., 2011]

Dlugokencky et al., GRL, 2009

The uptick: observational evidence suggests natural sources in 2007 and 2008:

2007 Arctic depleted in 13C (wetlands) Warm Arctic Temp

2008 tropics (zero growth rate in Arctic) La Nina, tropical precip

The Methane Mystery: Leveling Off then Rebounding

Heimann, Science, 2011, “news and views”

Possible sources of variability/trends in recent decades

SOURCES: 1. Wetlands: At present 2/3 tropics, 1/3 boreal;

estimated at 170-210 Tg CH4 (ENSO-driven; Hodson et al.)-- T and water table (seasonal, interannual)2. Biomass burning3. clathrate/permafrost degassing4. fossil fuel (also landfills/waste management) Aydin et al.5. rice agriculture Kai et al. (+ wetlands – they can distinguish “microbial”)6. ruminants

SINKS:Atmospheric Oxidation (primarily lower tropical troposphere)

-- feeds back on any source change-- amplified by changes in biogenic VOC (but chemistry uncertain!)-- photolysis rates (e.g., due to overhead O3 columns; affects OH source)-- water vapor (affects OH source)-- shift in magnitude / location of NOx emissions (OH source)

Aydin et al., Nature, 2011

METHODS:1) Firn air measurements (flasks) at 3 sites:Summit, South Pole, WAIS-D, analyzed with GC-MS

2) Derive annual mean high latitude tropospheric abundances of ethane (1-D firn-air model + synthesis inversion)

3) Explore role of biomass burning + fossil fuel in contributing to observed ethane time histories (2-box model, informed by 3-D model)

Use Ethane as a proxy for fossil fuel methane 2nd most abundant constituent in natural

gas Released mainly during

production+distribution (same as CH4) Major loss by OH, ~2 month lifetime

M Aydin et al. Nature 476, 198-201 (2011) doi:10.1038/nature10352

Ethane mixing ratios in firn air at three sites, and the Atmospheric histories derived from these measurements.

Possible atmospheric histories(different PI ethane)

contemporary

Not used in inversion

modeled

Shaded regions not constrainedDue to uncertainties in PI levelsS Pole can constrain ramp-upStarting 1910 5x by 1980

All 3 site show 1980 peak, thendecline (~10%) despite increase in FF use


Ethane source emissions and the resulting atmospheric histories. Derived with 2-box model 3D model used to relatehow air reaching firn responds to changing hemispheric mean ethane levels

1. FF dominates observed time history2. Decline of CH4 growth rate parallels

ethane decline3. Now steady recent “uptick” not

due to FF CH4


Ethane and methane emissions from fossil fuels, biofuels and biomass burning.

1. FF ethane differs from bottom-up CH42. BB agrees with independent estimates

Are the CH4 inventories wrong?

Could methane-to-ethane Emission ratios have changed?

Less venting while production increased?15-30 Tg CH4 yr-1 decrease 1980 to 2000

Shift in distribution / Cl sink estimated to be small

Kai et al., Nature, 2011

METHODS:1) measurements from UCI, NIWA, and SIL networks

2) Examine various hypotheses for explaining decline in CH4 growth rate (2-box model including CH4 and its isotopes)

3) Empirical, process-based biogeochemical model to estimate changes in rice agriculture

Use CH4 abundance plus 13C/12C of CH4 to distinguish microbial vs. fossil sources, also distinguish sinks by looking at D/H

information in inter-hemispheric difference (IHD)

Conclusion: Isotopic constraints exclude reductions in fossil fuel as primary cause of slowdown. Rather, large role for Asian rice agriculture (+fertilizer, -water use




FM Kai et al. Nature 476, 194-197 (2011) doi:10.1038/nature10259

Long-term trends in atmospheric CH4, 13C-CH4, and D-CH4.


Long-term trends in atmospheric CH4, 13C-CH4, and D-CH4.



Possible driving factors of trend towards NH enriched C isotopes of CH4

1. Decrease in isotopically depleted source (microbial: agriculture, landfills, wetlands)

2. Increase in enriched source (FF or BB)

3. Increase in removal by OH (but dD relatively constant suggests no change in sink)

Considering CH4 alone, leveling off canbe explained by both FF and agricultural emissionsbut isotopic time histories differ for FF / agriculture

dig deeper into the isotopic constraints


Variations in CH4 fluxes and the impacts of source composition on

isotopic trends.

Assume all change due to FF,IHD of d13C-CH4 widens, notConsistent with obs

Agricultural source changes can(or wetlands / better landfill management) They posit wetland source hasn’t

changed in consistent way

Conclusions from scenario analysis:

31 Tg CH4 yr-1 decrease (~6% total budget)

FM Kai et al. Nature 476, 194-197 (2011)

doi:10.1038/nature10259

Evidence for intensification of rice agriculture in Asia.

Increase in chemical fertilizer use

Increase in industrial water use;New mid-season drainage of rice paddies

15.5 +/- 1.9 Tg CH4 yr-1 1984 to 2005

Follow-up (2012 Nature: Levin et al.)

Different isotope datasetsDo not support change in IHD (so flat microbial source)

Response of Kai et al: Need to bring together all datasets; value of isotopic measurements.

Hodson et al. GRL, 2011

Method: Use simple dynamic vegetation wetland model and compare with ENSO index

Conclusions: Repeated El Nino events in 1980s and 1990s contributed toreducing CH4 emissions and atmospheric abundance leveling off

E (x,t) = F(x) b Rh(x,t) S(x,t)

x= each 0.5° grid cellt = monthlyE = wetland emission flux (Tg CH4 grid cell-1 month-1)F=ecosystem dependent scaling factorb = 0.03 mol CH4/mol C respiredRh = heterotrophic respiration (mol C respired) from LPJ DGVM (T, CO2)S = areal extent of wetland (satellite 1993-2000); fitted to runoff in LPJ

Also account for differences in emitting capacity btw boreal + tropics (empirical)

Multivariate Enso index

http://www.research.noaa.gov/climate/observing1.html

“An index of six observed variables (such as pressure, air and sea-surface temperatures, winds, cloudiness) over the tropical Pacific is used to monitor the coupled ocean-atmosphere phenomenon known as the El Ni ño-Southern Oscillation (ENSO). Areas with large positive values of the index (large red spikes) depict the "El Niño" warm phase of the ENSO phenomenon. [From the NOAA Climate Diagnostics Center”

Hodson et al., GRL, 2011: FIGURE 1

N. Temperate (27%) and Tropics (44%)Dominate variability

Tropics responds toVariability in inundated area;Boreal to Rh (T)

R2 = 0.56

Hodson et al., GRL, 2011: TABLE 1

During events, wetland response > prior estimates for fires;Possibility of offsetting influences during El Nino (+fires; -wetlands) Contributed to slow down (citing other work for anthropogenic sources)

Hodson et al., GRL, 2011: Table 2

Potential amplification if boreal wetland emissions increase in the future

Some overall discussion points

Why so many competing hypotheses?

How strong a constraint is there on the OH sink and trends therein?

Confidence in proxies we have for CH4 source attribution? How well do we know isotopic source/sink signatures?

Representativeness of “reference” measurement stations

Large interannual “wiggles” in data: real? Artifacts of combining measurements for different places / periods?

Connections of microbial emissions to other pollutants/GHGs (acid deposition; N2O production)

Documents

Connecting atmospheric composition with climate variability and change Seminar in Atmospheric Science, EESC G9910 9/19/12 Observed methane trends in recent