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This Week

This Week. READING: Chapter 6 of text Announcements Problem Set 1 due Fri Oct 12. Problem Set 2 due Tuesday Oct 16. Why N 2 , O 2 , etc? (Mars and Venus aren’t) Atmospheric Composition and Biogeochemical Cycles. The atmosphere as part of the Earth System

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This Week

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  1. This Week READING: Chapter 6 of text Announcements Problem Set 1 due Fri Oct 12. Problem Set 2 due Tuesday Oct 16. Why N2, O2, etc? (Mars and Venus aren’t) Atmospheric Composition and Biogeochemical Cycles • The atmosphere as part of the Earth System • Global Biogeochemical Cycles (Box-Model Heaven) • N2 • O2 • CO2

  2. Planetary Atmospheres

  3. Today: Earth System and N Cycle • Oxidizing Atmosphere • Earth System Surface Reservoirs • N2 Cycling—does it do anything?

  4. The Atmosphere: An Oxidizing Medium Gas phase radical chemistry Oxidation Oxidized gas/ aerosol Reduced gas Cloud Chemistry Deposition Uptake EARTH SURFACE Emission Reduction Geological or Biological

  5. Surface Reservoirs of the Earth System Atmosphere photo- synthesis decay air-sea exchange decay Biosphere decay assimilation assimilation Hydrosphere Soils runoff Lithosphere erosion What are the time scales of exchange between the various reservoirs of the Earth System?

  6. Oxidation States of Nitrogen N has 5 electrons in valence shell  9 oxidation states from –3 to +5 Increasing oxidation number (oxidation reactions) Decreasing oxidation number (reduction reactions)

  7. Nitrogen Cycle: Major Processes combustion lightning ATMOSPHERE N2 NO oxidation HNO3 biofixation denitri- fication deposition orgN decay NH3/NH4+ NO3- BIOSPHERE assimilation nitrification burial weathering LITHOSPHERE

  8. Box Model of the Nitrogen Cycle Tropospheric Fixed N (non-N2O) 5 40 Atmospheric N2 3x109 Combustion, biomass burning, lightning denitri-fication denitri-fication Agricult. biofixation rain biofixation rain 90 80 80 150 30 40 40 150 (NH3) 2530 Land biota 1x104 Soil 1x105 Ocean biota 1x103 40 2300 1650 1640 Deep ocean 1x106 Inventories in Tg N, 1Tg = 1x1012 g Flows in Tg N yr-1 10 burial 10 From Jaffe, 1992; Jacob text--modified Lithosphere 2x109 weathering

  9. Questions • If denitrification shuts off, while fixation continues, how long will it take for atmospheric N2 to be depleted? • How many times does an N atom cycle between atmospheric N2 and oceanic N before being transferred to the lithosphere? • Combustion and fertilizer use increase the rate of transfer of N2 from the atmosphere to the soil. Assume that these human activities have been in place and constant for the past 100 years, and prior to that they were negligible. By how much have humans increased the nitrogen contents of the total land reservoir (soil + land biota) and contributed to a global fertilization of the biosphere?

  10. N2O • Very important byproduct of nitrification/denitrification • source of reactive nitrogen in stratosphere • greenhouse gas IPCC [2001]

  11. Fast Oxygen Cycle: Atmosphere--Biosphere Source of O2: photosynthesis nCO2 + nH2O g (CH2O)n + nO2 Sink: respiration/decay (CH2O)n + nO2 g nCO2 + nH2O CO2 Photosynthesis - respiration O2 orgC O2 lifetime: ~ 5000 years orgC decay litter

  12. Fast O2 Cycle: Atmosphere-Biosphere Can photosynthesis/decay control O2 levels? I.e., if photosynthesis stopped, by how much would O2 decrease due to complete decay of all biomass? (figure from DJJ)

  13. Slow Oxygen Cycle: Atmosphere-Lithosphere O2 in atmosphere: 1.2x106 Pg O 0.4 Pg O/yr weathering CO2 O2 Photosynthesis decay runoff Fe2O3 H2SO4 CO2 O2 orgC FeS2 OCEAN CONTINENT orgC Uplift burial CO2 orgC: 1x107 Pg C FeS2: 5x106 Pg S microbes FeS2 orgC SEDIMENTS Compression subduction

  14. Question • Does atmospheric oxygen have a seasonal cycle? If so, when would it maximize? • Do you think humans are increasing or decreasing atmospheric O2, why?

  15. Recent Growth in Atmospheric CO2 • Notice: • atmospheric increase is ~50% of fossil fuel emissions • large inter-annual variability Where is rest of CO2 going? IPCC 2001 Arrows indicate El Nino events

  16. Uptake of CO2 by Oceans CO2(g) CO2.H2O CO2.H2O HCO3- + H+ ATMOSPHERE KH = 3x10-2 M atm-1 OCEAN K1 = 9x10-7 M Ocean pH K2 = 7x10-10 M HCO3- CO32- + H+ pK2 pK1 CO2.H2O HCO3- CO32- Net uptake: CO2(g) + CO32- 2HCO3--

  17. Equilibrium Partitioning of CO2 Want to know fraction of atmospheric and oceanic CO2 that is in atmosphere at equilibrium Vocean = 1.4x1018 m3 PCO2 = 375 x 10-6 atm pHocean = 8.2 Fcalc = 0.03  97% of CO2 resides in the oceans This is definitely wrong! It greatly underestimates the fraction of CO2 that resides in atmosphere (Ftrue~ 70%)…Why? What’s wrong with this estimate?

  18. CO2 Uptake Limited by Ocean Mixing Uptake by oceanic mixed layer only (VOC= 3.6x1016 m3)would give f = 94% of added CO2 remains in atmosphere…now estimate is too small…?! Inventories in 1015 m3 water Flows in 1015 m3 yr-1

  19. CO2 Uptake also Limited By Ocean Alkalinity Equilibrium calculation 2.1 auptake of CO2 is limited by the existing supply of CO32- To increase supply of CO32-, CaCO3 in sediments/deep ocean must dissolve: CaCO3 Ca2+ + CO32- …which takes place over a time scale of thousands of years 2.0 [CO2.H2O]+[HCO3-] +[CO32-], 10-3M 1.9 1.8 [HCO3-], 10-3M 1.6 1.4 4 [CO32-], 10-4 M 3 2 8.6 Ocean pH 8.4 8.2 100 200 300 400 500 pCO2 , ppm

  20. Questions • Marine biota take in CO2 during photosynthesis to make OrgC. About 10% of this OrgC sinks to the ocean bottom (fecal matter, dead tissue, etc), and is buried into the sediments. How does this process affect the equilibrium partitioning of CO2 between the atmosphere and ocean? • 2. Does the growth of corals/shells (Ca2+ + CO32- CaCO3) cause atmospheric CO2 to increase or decrease? • 3. A consequence of global warming is melting of the polar ice caps. This melting decreases deep water formation. Why? Would this effect reduce or amplify warming caused by anthropogenic CO2 emissions?

  21. Evidence For Land Uptake of CO2 Trends in O2,1990-2000

  22. Atmosphere--Terrestrial Biosphere C Cycle 2000 790 From DJJ Inventories in PgC Flows in PgC yr-1 Time scales are short: ~ 12 yrs w.r.t uptake; ~ 160 yrs w.r.t soil emission

  23. Global Preindustrial Carbon Cycle (from DJJ) Inventories in PgC Flows in PgC yr-1 When we burn fossil fuels, we take C from the sediments and put it into the atmosphere as CO2. How long-term is this perturbation to the carbon cycle?

  24. A Long View of Fossil Fuel Perturbation It takes a long time for fossil fuel CO2 to completely leave the atmosphere.

  25. Future Atmospheric CO2 2100 2200 2300 2000 Using estimates about future population growth, energy needs, etc. project future CO2 emissions. Using a climate model with a carbon cycle, predict CO2 based on projected emissions and sinks. CO2 double pre-industrial value by ~ 2150

  26. Stabilization Scenarios 2100 2200 2300 2000 To make CO2 growth rate 0, sources must balance sinks These calculations show what our emissions can be for different CO2 levels. Note that sinks are predicted to get smaller. To stop CO2 increase now, we’d have to cut our emissions by 50%

  27. Projected Trends in CO2 Sinks IPCC [2001]

  28. Questions • The Kyoto Protocol (heard of it?) aimed to cut emissions to be 6% lower than the 1990 values. Emissions would be only slightly less than 7 GtC/yr. Why was this even considered potentially useful? • To keep CO2 constant at its current value 380 ppm, we’d have to cut emissions by 50% to 4 GtC/yr. This would match the current sink rate. After a few hundred years, if we didn’t want CO2 to start increasing again, we’d have to cut our emissions even lower. Why might this be? • Fossil fuel abundance is estimated at ~ 5000 GtC. If we burn this much eventually, will the terrestrial biosphere be of much significance as a sink/storage of this carbon?

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