Global Land Sink

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Global Land Sink

• Inverse models infer the location of sources
  and sinks on the Earths surface from information
  about atmospheric circulation and distribution of
  CO2.
• Inverse models suggest more than half the land
  sink is in the Northern Hemisphere.
• Further, more than half the NH sink is in the
  terrestrial biosphere.
• Studies of US show its land biosphere sink is due
  to land use changes, not fertilization.
• If this is the case in the rest of the world, the
  land sink is limited and temporary.

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Carbon Sink Feedbacks

• As climate warms, land sink may become a source
• disappearance of the Amazonian rainforest due to
  increased dryness of soils
• an increased rate of bacterial breakdown of
  organic carbon in soils due to increased soil
  temperatures
• Oceans may lose CO2 solubility
• higher water temperatures
• slowed vertical exchange
• changes in biological processes that redistribute
  carbon within the ocean.
• Combined land and ocean sinks predicted for 2100
  are 190 Gt-C rather than 750 Gt-C,

3
Positive Feedbacks

• CO2 could reach 980 ppmv by 2100, temperatures
  rise 5.5C (8C on land).

4
Methane CH4

• Current mixing ratio 1.8 ppmv
• Pre-industrial 0.8 ppmv, increasing 0.8/year
• Atmospheric lifetime 10 years
• IR absorption per molecule 21 x CO2
• Atmospheric burden 3.5 Gt-C CO2/200

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Methane Sources and Sinks

• Sources (425 Mt C yr-1)
• Bacterial production wetlands, rice paddies,
  digestion in bovines termites
• Natural gas leakage
• Landfills
• Biomass burning
• Tundra/clathrates
• Sinks (375 Mt C yr-1)
• Soil uptake
• Reaction with OH
• Atmospheric accumulation

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Nitrous Oxide (N2O)

• Lifetime 150 years
• IR absorption per molecule 200 x CO2
• Mixing Ratio 0.31 ppmv CO2/1000

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N2O Sources and Sinks

• Sources (11 Mt N yr-1)
• Denitrifying bacteria in soils
• Agricultural combustion
• Fossil fuels
• Biomass
• Oceans
• Sinks (13 Mt C yr-1)
• Photolysis in the stratosphere
• Atmospheric accumulation

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Halocarbons

• Lifetime 1.5-140 years
• IR absorption per molecule 105 x CO2
• Mixing Ratio 1 ppbv CO2/105

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Ozone (O3)

• Ozone has strong absorption features in the
  longwave IR.
• Most ozone is in the lower stratosphere, which is
  very cold, making it a good greenhouse gas.
• Ozone depletion decreases greenhouse warming.
• This decrease is counterbalanced by increasing
  halocarbons, which are depleting ozone.

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Water Vapor Feedback

• Atmospheric water vapor doubles every time
  temperature rises by 3C
• Greenhouse warming by H2O doubles the direct
  greenhouse effect of most gases (positive
  feedback).
• Increased water may increase clouds, cooling the
  earth (negative feedback).

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Paleoclimate

• Temperature (from isotope ratios) correlates with
  CO2 and CH4.
• Temperature changes precede CO2, CH4.
• As temperature ?
• CO2 sinks (ocean, land biosphere) ?
• CH4 sources (biosphere) ?
• CH4, CO2 amplify temperature changes.

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Recent Temperature Trends

• Global mean surface temperatures estimated since
  preindustrial times shown as anomalies relative
  to the 1961-1990 mean. Bars give annual average
  values of combined near-surface air temperature
  over continents and sea surface temperature over
  ocean. The solid curve gives a smoothing similar
  to a decadal running average. Adapted from IPCC
  (2001).

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Difficulties in determining T trends

• Year-to-year variation is comparable to trend
  over century
• Accuracy of historical measurements
• Geographical distribution
• Differences between sea surface temperature and
  air temperature
• Urban "heat island" effects
• Land/ocean bias

There is general agreement that day-night
temperature difference is decreasing.