ERS 765

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ERS 765 The Global Carbon Cycle
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MAUNA LOA
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MAUNA LOA
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Long wave radiation - absorbed by GHG's and
reflects back to earth
Sun
Short wave radiation -penetrates atmosphere
Earth
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Carbon Pools on Earth

• Earth contains about 1023 g C
• Most in sedimentary rocks,
• Organic compounds (1.56 x 1022)
• Carbonates (6.5 x 1022)
• Table 2.2, p. 28
• Active pools near earths surface 40 x 1018 g C
• Extractable fossil fuels 4 x 1018
• Fig 11.1, p. 359

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The Global Carbon Cycle
The Global Carbon Cycle
Fossil Fuels 6
Atmosphere 750 Annual Increase 3.2
Fire 5
Photosynthesis 120
Respiration 60
92
90
Detritus 60
Decomposition 60
Terrestrial Biota 560
Oceans 38,000
Soil and Litter 1500
Net veg destruction 0.9
Burial 0.1
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Carbon Pools on Earth

• Ocean is huge pool (38 x 1018 g C)
• Has large capacity to buffer CO2 by
  dissolution/degassing
• Henrys Law H2CO3 (Kh)pCO2
• Where
• H2CO3 H2CO3 dissolved CO2
• Kh Henrys Law constant
• pCO2 partial pressure of CO2 in atmosphere

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Carbon Fluxes on Earth

• Largest fluxes are into and out of
• Land vegetation (120 x 1015 g C)
• Oceans (90-92 x 1015 g C)
• Each CO2 molecule has the potential to be
  captured every 12.5 years by primary production
  on land alone
• Overall mean residence time(MRT) for CO2 in the
  atmosphere is 5 years

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The Global Carbon Cycle
The Global Carbon Cycle
Fossil Fuels 6
Atmosphere 750 Annual Increase 3.2
Fire 5
Photosynthesis 120
Respiration 60
92
90
Detritus 60
Decomposition 60
Terrestrial Biota 560
Oceans 38,000
Soil and Litter 1500
Net veg destruction 0.9
Burial 0.1
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Carbon Fluxes on Earth

• Because this MRT is less than mixing time,
  seasonal oscillations occur as a result of uptake
  and output differences by season.
• More variation in the northern hemisphere than in
  the southern because there is more seasonal land
  vegetation in the north
• Fig 3.6. p. 56

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Carbon Fluxes on Earth

• Seasonal fluctuation on Muana Loa equals about 13
  x 1015 g C
• Much less than annual primary productivity of
  plants
• This is due to the asynchrony of primary
  productivity and respiration and to ocean uptake

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Carbon Fluxes on Earth

• Release of CO2 by fossil fuels is one of the best
  known numbers
• Net release of CO2 by fossil fuels is less than
  annual increase in the atmosphere
• Therefore, a search is on for the missing sink
• The problem is, no one and yet everyone can find
  it - it is very small relative to other fluxes
  and especially other pools.

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The Global Carbon Cycle
The Global Carbon Cycle
Fossil Fuels 6
Atmosphere 750 Annual Increase 3.2
Fire 5
Photosynthesis 120
Respiration 60
92
90
Detritus 60
Decomposition 60
Terrestrial Biota 560
Oceans 38,000
Soil and Litter 1500
Net veg destruction 0.9
Burial 0.1
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Carbon Fluxes on Earth

• Role of oceans
• Uptake of 2 x 1015 g C yr-1
• Mostly due to dissolution of CO2 in water
  (Henrys Law)
• Note Henrys Law is affected by
• Salinity (decreases with salinity pour salt in
  beer sometime)
• Temperature (decreases with temperature)
• Little primary production increase

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The Global Carbon Cycle
The Global Carbon Cycle
Fossil Fuels 6
Atmosphere 750 Annual Increase 3.2
Fire 5
Photosynthesis 120
Respiration 60
92
90
Detritus 60
Decomposition 60
Terrestrial Biota 560
Oceans 38,000
Soil and Litter 1500
Net veg destruction 0.9
Burial 0.1
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Carbon Fluxes on Earth

• Role of oceans
• Turnover time of 11 years on surface
• Turnover time of 350 years in entire ocean
• If oceans well mixed, uptake could be 6 x 1015 g
  C yr-1
• Thus, oceans could take up all fossil fuels
• Therefore, rate of fossil fuel CO2 release gt
  rate of ocean uptake, and is definitive in net
  increase

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Carbon Fluxes on Earth

• Role of oceans
• If the release of CO2 from fossil fuels were
  curtailed, nearly all the CO2 that has
  accumulated in the atmosphere would eventually
  dissolve in the oceans and the global C cycle
  would return to steady-state (p. 361)
• Is this true?

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Carbon Fluxes on Earth

• Ocean uptake (2) and net accumulation in
  atmosphere (3.8) account for 89 of fossil fuel
  emissions
• But most terrestrial ecologists feel that net
  forest destruction causes a net loss of about
  0.9, so we are still looking for the missing sink
• Terrestrial ecologists and atmospheric modelers
  think they see a net sink in North America
• Are we chasing our tails over small numbers?

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Carbon Fluxes on Earth

• 13C and 14C ratios show unequivocal net
  release of CO2 from terrestrial system over the
  last century, possibly as large as fossil fuels
  (p. 362).
• In 1990, net deforestation in the tropics (1.6 x
  1015 g C yr-1) was partially offset by net uptake
  in Northern Hemisphere (0.7 x 1015 g C yr-1)
  (eastern US and Europe)
• But recall shaky- incorrect - assumptions about
  changes in soil C with harvesting
• Houghton et al (1983) estimated 40-50 soil C
  loss with harvesting
• We found none, on average.

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Effects of Forest Harvesting on soil C
(from Johnson and Curtis, 2001)
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Cultivation nearly always results in a loss of
soil C
From Johnson, 1992
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Carbon Fluxes on Earth

• Mechanisms by which land C sequestration could
  increase (the beta factor)
• Elevated CO2 and increased primary production
• Increased N deposition (but Nadelhoffer et al
  disagree)
• Better silviculture in forests (Europe Kauppi et
  al)

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Carbon Fluxes on Earth What are the
uncertainties?

• What about fire?.
• A flux that has not changed in recent times, no
  matter how large, is not likely to affect the
  concentration of atmospheric CO2 (Houghton et
  al., 1983). For example, the release of CO2 in
  forest fires is of no consequence to changes in
  atmospheric CO2 unless the frequency or area of
  forest fires has changed in recent times.
• What are the recent changes in global fire area
  and frequency?

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The Global Carbon Cycle
The Global Carbon Cycle
Fossil Fuels 6
Atmosphere 750 Annual Increase 3.2
Fire 5
Photosynthesis 120
Respiration 60
92
90
Detritus 60
Decomposition 60
Terrestrial Biota 560
Oceans 38,000
Soil and Litter 1500
Net veg destruction 0.9
Burial 0.1
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• Effects of Fire on soil C pools are not
  straightforward
• Little net change in most cases with fire
• Large net gain if N-fixers invade after fire

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Effects of Forest Fire on soil C (from Johnson
and Curtis, 2001)
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Effects of N-fixers on soil C (from Johnson
and Curtis, 2001)
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Carbon Fluxes on Earth What are the
uncertainties?

• A small change in a large pool or flux can make a
  big difference
• For example
• A 1 change in soils 15 x 1015 g C
• A 1 change in vegetation 5.6 x 1015 g C
• A 1 change in oceans 38 x 1015 g C

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Carbon Fluxes on Earth What are the feedbacks?

• Patterns with increased mean annual temperature
• Increases in primary productivity
• Even greater increases in decomposition
• Net result greater biomass but lower litter and
  soil C in warm climates
• Greater proportion of active and slow C in cold
  climates
• Patterns with precipitation not as clear, except
  in arid environments where C goes up with precip

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Carbon Fluxes on Earth What are the feedbacks?

• Climate warming
• Possible interim net release of N from soil
  (e.g., VanCleve et al, 1978)
• Soil CN 10-50
• Vegetation CN 50-300
• Net increase in C sequestration

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Carbon Fluxes on Earth What are the feedbacks?
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Methane (CH4)

• Seemingly a minor issue all sources 1012 to
  1014 g C yr-1 Table 11.2
• Atmospheric methane concentration 1.75 ppmv, vs
  365 ppmv for CO2
• However, each methane molecule has 25 x the
  greenhouse warming potential as a CO2 molecule
• Because the absorption by CO2 is reaching 100 in
  the wavebands in which it absorbs, methane may be
  a more important greenhouse gas in coming decades

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Methane (CH4)

• Atmospheric methane has increased approx 1 per
  year during the 1980s (Fig 11.6)
• Atmospheric methane increased much less during
  the early 1990s
• Causes?
• Reduced natural gas leakage in former Soviet
  Union
• Global cooling following the eruption of Mt.
  Pinatubo in June 1991
• Reduced stratospheric ozone allowed more uv to
  penetrate to troposphere where it created more
  hydroxyl radicals
• Atmospheric methane increases resumed the 1
  increase the late 1990s

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Methane (CH4)

• Causes of methane increase are not obvious - many
  sources
• Methanogenesis in wetlands is major natural
  source
• Changes in the global distribution of wetlands
  may contribute to increase in atmospheric methane
  - more rice fields
• Forest fires produce some methane (incomplete
  combustion)

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Methane (CH4)

• Farting as a global problem
• Grazers and termites create methane in their
  digestive tracts
• Termites may be a mobile wetland
• Grazer flatulence is a significant contribution
  to methane flux

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Methane (CH4)

• Human contributions to methane
• Fossil fuels (incomplete combustion)
• Landfills
• Release during mining
• Increase biomass burning (13C signal)

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Methane (CH4)

• Major sink for methane is the hydroxyl radical in
  the atmosphere
• Some diffusion and oxidation in surface soils
  (methanotropic bacteria)
• Some methanotropic activity is due to nitrifying
  bacteria
• Can use CH4 as a substitute for NH4 at times
• This is reduced with N fertilization and probably
  also with increased N deposition and land
  clearing (which stimulates N mineralization)

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Methane (CH4)

• Future potential increases
• Warming and drying of wetlands may reduce
  emissions
• But methane producing bacteria respond more to
  temperature than methane-oxidizing bacteria
• Catastrophic release of methane hydrate from
  marine sediments?

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Carbon monoxide (CO)

• In the atmosphere
• Low concentration (45 - 250 ppb)
• Limited greenhouse gas activity
• Main effect is probably slowing CH4 oxidation
• Role in controlling the levels of tropospheric
  ozone (CO OH radicals)
• Increasing 1 yr-1, but slowed up in the 1990s
• Perhaps due to the slowing of CH4, for which CO
  is an oxidation product
• Short lifetime 2 months

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Carbon monoxide (CO)

• CO budget dominated by anthropogenic sources
  (Table 11.3)
• Concentrations 3 x higher in northern than in
  southern hemisphere
• Most important sink is OH oxidation
• CO often lumped with CO2 budget

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Oxygen (O2)

• Significant buildup only after photosynthesis
• Present atmospheric levels represent a balance
  between production and reaction with crustal
  minerals (i.e., FeS2)
• Cycle is dominated by photosynthesis and
  respiration (Fig. 11.7)
• Annual variation is small 0.0020 of a
  background of 20.946
• Mean residence time in atmosphere is 4000 yr
• Does the carbon cycle on earth drive the oxygen
  cycle or vice-versa?

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