ATMOS 397G Biogeochemical Cycles and Global Change Lecture 14: Methane and CO

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ATMOS 397GBiogeochemical Cycles and Global
ChangeLecture 14 Methane and CO

• Don Wuebbles
• Department of Atmospheric Sciences
• University of Illinois, Urbana, IL
• March 6, 2003

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Tropospheric OH July -- MOZART II
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Tropospheric OH (January) -- MOZART II
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OH concentrations

• OH depends non-linearly on atmospheric
  concentrations of many tropospheric gases
• most important gases CH4, CO, NOx, tropospheric
  O3, NMHCs
• other factors tropospheric water vapor, uv
  radiation flux which depends on stratospheric
  ozone

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Simplified CH4/OH/CO Chemistry

• CH4
• oxidizes to CO OH CH4
• CO OH CO OH
• NMHCs
• UV O3 H2O NOx

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From Daniel Jacob
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Decay of an Impulse Emission
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Methane Lifetime vs. Response Time

• Atmospheric lifetime of CH4m
• ? Burden / flux 9 years
• 9.6 years in IPCC (2001)
• Response time is the e-folding time after a
  perturbation
• Response time 1.4 x ? 13 years

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Trends in Tropospheric OH
Based on Krol et al. (2002)
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Estimated Changes in CH4 Source and Sink
Assumes fixed lifetime of CH4 8.9 years
From NOAA CMDL, E. Dlugokencky
Mean emission rate of period of observations is
550 Tg CH4 yr-1.
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Earlier Estimates for Trends in Anthropogenic CH4
Emissions
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Methane Growth Rate
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Methane Changes in Growth Rate

• 1991-1992
• After the eruption of Mt. Pinatubo, a large
  positive anomaly in growth rate was observed at
  tropical latitudes. It has been attributed to
  short-term decreases in solar UV in the tropics
  immediately following the eruption that decreased
  OH formation rates in the troposphere
  (Dlugokencky et al., 1996).
• A large decrease in growth was observed,
  particularly in high northern latitudes, in 1992.
  This feature has been attributed in part to
  decreased northern wetland emission rates
  resulting from anomalously low surface
  temperatures (Hogan and Harriss, 1994) and in
  part to stratospheric ozone depletion that
  increased tropospheric OH (Bekki et al., 1994
  Fuglestvedt et al., 1994).
• Records of changes in the 13C/12C ratios in
  atmospheric CH4 during this period suggest the
  existence of an anomaly in the sources or sinks
  involving more than one causal factor (Lowe et
  al., 1997 Mak et al., 2000).

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Methane Changes in Growth Rate

• 1998
• High northern and southern tropical latitudes
  have been linked to interannual variations in
  temperature and soil moisture content in wetland
  regions, thereby affecting CH4 emissions, and
  emphasizing the strong link between wetland CH4
  emissions and climate.
• Observations suggest that global emissions were
  greater than average by 24 Tg CH4
• A process based model, which included
  soil-temperature and precipitation anomalies, was
  used to calculate CH4 emission anomalies from
  wetlands of 24.6 Tg CH4, split nearly equally
  between high-northern latitudes and the southern
  tropics Dlugokencky et al., 2001.

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Modeling sources of CH4

• Prediction of methane emissions from scenarios
  of basic variables including
• temperature changes
• population growth
• evolution of land use patterns
• future energy demand and sources
• technological improvements

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2001 IPCC SRES Projected CH4 Emissions
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Derived Concentrations for Methane
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Observed
2-D Model 1992
Pre-ind.
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CH4 only
All GHGs
Ratio
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Reductions in CH4, CO and NOx
Reduce CO and NOx
Reduce CO but not NOx
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Change in Total Ozone () for 2 x CH4
? O3 3.4 University of Illinois 2-D Model
Latitude
Month
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Change in Local Ozone () for 2 x CH4
University of Illinois 2-D Model