1. This Week The atmosphere as part of the Earth System Global Biogeochemical Cycles (Box-Model Heaven) N 2 O 2 CO 2 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

2. Planetary Atmospheres Planet Earth VenusMars Radius (km) 6400 61003400 T avg (K)250700200 P s (atm)1916x10 -3 N2N2 0.78.030.027 O2O2 0.210.0070.0015 CO 2 4x10 -6 0.960.95

3. Today: Earth System and N Cycle Oxidizing Atmosphere Earth System Surface Reservoirs N 2 Cycling—does it do anything?

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

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

6. Oxidation States of Nitrogen -30+1+2+3+4+5 NH 3 --Ammonia NH 4 + --Ammonium R 1 N(R 2 )R 3 --Organic N N2N2 N 2 O --Nitrous oxide NO --Nitric oxide HONO --Nitrous acid NO 2 - --Nitrite NO 2 --Nitrogen dioxide HNO 3 --Nitric acid NO 3 - --Nitrate Decreasing oxidation number (reduction reactions) Increasing oxidation number (oxidation reactions) N has 5 electrons in valence shell  9 oxidation states from –3 to +5

7. Nitrogen Cycle: Major Processes ATMOSPHERE N2N2 NO HNO 3 NH 3 /NH 4 + NO 3 - orgN BIOSPHERE LITHOSPHERE combustion lightning oxidation deposition assimilation decay nitrification denitri- fication biofixation burial weathering

8. Box Model of the Nitrogen Cycle Inventories in Tg N, 1Tg = 1x10 12 g Flows in Tg N yr -1 From Jaffe, 1992; Jacob text--modified Atmospheric N 2 3x10 9 Tropospheric Fixed N (non-N 2 O) 5 Land biota 1x10 4 Soil 1x10 5 Ocean biota 1x10 3 Deep ocean 1x10 6 Lithosphere 2x10 9 Combustion, biomass burning, lightning 40 2530 2300 80 150 Agricult.biofixation 150 denitri- fication 10 80 (NH 3 ) 90 40 30 biofixation denitri- fication rain 10 1640 1650 weathering burial

9. 1.If denitrification shuts off, while fixation continues, how long will it take for atmospheric N 2 to be depleted? 2.How many times does an N atom cycle between atmospheric N 2 and oceanic N before being transferred to the lithosphere? 3.Combustion and fertilizer use increase the rate of transfer of N 2 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? Questions

10. N2ON2O Very important byproduct of nitrification/denitrification source of reactive nitrogen in stratosphere greenhouse gas IPCC [2001]

11. Fast Oxygen Cycle: Atmosphere--Biosphere Source of O 2 : photosynthesis nCO 2 + nH 2 O  (CH 2 O) n + nO 2 Sink: respiration/decay (CH 2 O) n + nO 2  nCO 2 + nH 2 O O2O2 CO 2 orgC litter Photosynthesis - respiration decay O 2 lifetime: ~ 5000 years

12. Fast O 2 Cycle: Atmosphere-Biosphere (figure from DJJ) Can photosynthesis/decay control O 2 levels? I.e., if photosynthesis stopped, by how much would O 2 decrease due to complete decay of all biomass?

13. Slow Oxygen Cycle: Atmosphere-Lithosphere O2O2 CO 2 Compression subduction Uplift CONTINENT OCEAN FeS 2 orgC weathering Fe 2 O 3 H 2 SO 4 runoff O2O2 CO 2 Photosynthesis decay orgC burial SEDIMENTS microbes FeS 2 orgC CO 2 orgC: 1x10 7 Pg C FeS 2 : 5x10 6 Pg S O 2 in atmosphere: 1.2x10 6 Pg O 0.4 Pg O/yr

14. 1.Does atmospheric oxygen have a seasonal cycle? If so, when would it maximize? 2.Do you think humans are increasing or decreasing atmospheric O 2, why? Question

15. Recent Growth in Atmospheric CO 2 Arrows indicate El Nino events Notice: atmospheric increase is ~50% of fossil fuel emissions large inter-annual variability Where is rest of CO 2 going? IPCC 2001

16. Uptake of CO 2 by Oceans CO 2 (g) CO 2. H 2 O HCO 3 - + H + HCO 3 - CO 3 2- + H + K H = 3x10 -2 M atm -1 K 1 = 9x10 -7 M K 2 = 7x10 -10 M pK 1 Ocean pH pK 2 Net uptake: CO 2 (g) + CO 3 2- 2HCO 3 -- CO 2. H 2 O HCO 3 - CO 3 2- OCEAN ATMOSPHERE

17. F calc = 0.03  97% of CO 2 resides in the oceans This is definitely wrong! It greatly underestimates the fraction of CO 2 that resides in atmosphere (F true ~ 70%)…Why? What’s wrong with this estimate? Want to know fraction of atmospheric and oceanic CO 2 that is in atmosphere at equilibrium V ocean = 1.4x10 18 m 3 pH ocean = 8.2P CO2 = 375 x 10 -6 atm Equilibrium Partitioning of CO 2

18. CO 2 Uptake Limited by Ocean Mixing Inventories in 10 15 m 3 water Flows in 10 15 m 3 yr -1 Uptake by oceanic mixed layer only (V OC = 3.6x10 16 m 3 ) would give f = 94% of added CO 2 remains in atmosphere…now estimate is too small…?!

19. CO 2 Uptake also Limited By Ocean Alkalinity Equilibrium calculation pCO 2, ppm 100 200 300 400 500 8.6 8.4 8.2 2 3 4 1.4 1.6 1.8 1.9 2.0 2.1 Ocean pH [CO 3 2- ], 10 -4 M [HCO 3 - ], 10 -3 M [CO 2. H 2 O]+[HCO 3 - ] +[CO 3 2- ], 10 -3 M  uptake of CO 2 is limited by the existing supply of CO 3 2- To increase supply of CO 3 2-, CaCO 3 in sediments/deep ocean must dissolve: CaCO 3  Ca 2+ + CO 3 2- …which takes place over a time scale of thousands of years

20. 1.Marine biota take in CO 2 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 CO 2 between the atmosphere and ocean? 2. Does the growth of corals/shells (Ca 2+ + CO 3 2-  CaCO 3 ) cause atmospheric CO 2 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 CO 2 emissions? Questions

21. Evidence For Land Uptake of CO 2 Trends in O 2, 1990-2000

22. Atmosphere--Terrestrial Biosphere C Cycle 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 790 From DJJ 2000

23. Global Preindustrial Carbon Cycle 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 CO 2. How long-term is this perturbation to the carbon cycle? (from DJJ)

24. A Long View of Fossil Fuel Perturbation It takes a long time for fossil fuel CO 2 to completely leave the atmosphere.

25. Future Atmospheric CO2 2000 230021002200 Using estimates about future population growth, energy needs, etc. project future CO 2 emissions. Using a climate model with a carbon cycle, predict CO 2 based on projected emissions and sinks. CO 2 double pre-industrial value by ~ 2150

26. Stabilization Scenarios 2000 230021002200 To make CO 2 growth rate 0, sources must balance sinks These calculations show what our emissions can be for different CO 2 levels. Note that sinks are predicted to get smaller. To stop CO 2 increase now, we’d have to cut our emissions by 50%

27. Projected Trends in CO 2 Sinks IPCC [2001]

28. Questions 1.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? 2.To keep CO 2 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 CO 2 to start increasing again, we’d have to cut our emissions even lower. Why might this be? 3.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?