Global Mean Radiative Forcings C Cycle - 6 Lectures IPCC (2007) AR4, Figure TS.5. PCC 588, Pt. II: The Global Carbon Cycle Thurs 1/29 Long-term carbon cycle (105-108 yr) Broecker 2005 pp. 79-130 Tues 2/3 Short-term C Cycle (100-102 yr). Atmosphere-ocean CO2 exchange I Emerson & Hedges 2007 Ch. 11 Thurs 2/5 Atmosphere-ocean CO2 exchange II Tues 2/10 Atmosphere-ocean CO2 exchange III. GlacialInterglacial CO2 (Mid-term C cycle, 103-105 yr) Thurs 2/12 Paper Discussion (or lecture if needed) Tues 2/17 Mid-term exam Thurs 2/19 Anthropogenic perturbation of C cycle Broecker 2005 pp. 130-156 Homework #3 out (due 3/3) Tues 2/24 Terrestrial C cycle (LJ) Thurs 2/26 Paper Discussion 1 Reservoirs, fluxes & the LongTerm Carbon Cycle Carbon Reservoirs on Earth's Surface To get gigatons of C (109 metric tons = 1015 gC) multiply by atomic mass of 12 Units = 1015 moles C Large size of ocean w.r.t. atmosphere reservoir + rapid exchange between = focus on ocean-atmosphere CO2 interactions 2 Primary Carbon Fluxes Today Feb 24 (LJ) Feb 19 Feb 3, 5, 10 Petagram = 1015 g = Gt, gigaton The Long-Term Carbon Cycle: Tectonics & Weathering On the Million to Billion Year Time Scale: Weathering of rocks consumes CO2 [106 yr] Seafloor spreading releases mantle CO2 [107 yr] o Its rate varies through time for reasons largely unknown Continental drift can result in increased or decreased weathering rates [108 yr] o depending on rainfall & temperature regime Mountain building increases weathering rates [108 yr] o by producing fresh, easily eroded rock, focusing precipitation, providing steep slopes for rapid runoff 3 Carbon Reservoirs & Fluxes The Long-Term View Most carbon in Earth's crust occurs in carbonate rocks (~1000x more than in ocean + atmosphere) & as organic material (kerogen) in rocks (~250x more than in ocean + atmosphere) Ocean + atmosphere C reservoir is small w.r.t. rock reservoir & the transfer rates between those reservoirs Transfer of C between rocks & ocean + atmosphere (>106 yr) can strongly perturb the CO2 greenhouse effect Weathering Carbonate Metamorphism & Volcanism Weathering Sedimentation & Burial 106 or million yr (units are 1000x larger than in previous figures!) Amended 1/30/09 The Long-Term Biogeochemical Carbon Cycle 4 Chemical Weathering = chemical attack of rocks by dilute acid C O2 + H2O <---> H2CO3 ---------------------------------------------- The Geochemical (or non-biological part of the) LT Carbon Cycle 1. Carbonate Weathering: C a C O3 + H2CO3 --> Ca2 + + 2 HCO3- Carbonate Rocks (e.g., limestone) 2. Silicate Weathering: C a S i O3 + 2 H2CO3 --> Ca2 + + 2HCO3- + SiO2 + H2O Silicate Rocks (most of the mantle & crust. E.g., granite) ------------------------------------ 2x CO2 consumption for silicates Carbonates weather faster than silicates http://en.wikipedia.org/wiki/Image:Yosemite_20_bg_090404.jpg http://en.wikipedia.org/wiki/Image:Burren_karst.jpg Carbonate rocks weather faster than silicate rocks! Granite (silicate) Rivers transport dissolved ions to ocean Limestone (carbonate) Adapted from Kump et al. (1999) 5 Products of weathering precipitaed as CaCO3 & SiO2 in ocean Diatom (SiO2) Radiolarian (SiO2) R, Protozoans L, Eukaryotic Phytoplankton Coccolithophorid (CaCO3) Foraminifer (CaCO3) Net reaction of Rock Weathering on Land & (Biogenic) Mineral Precipitation in the Ocean Carbonate Weathering: Carbonate Precipitation: ____________________________________________________________________________ CaCO3 + H2CO3 Ca2+ + 2HCO3Ca2+ + 2HCO3- CaCO3 + H2CO3 0 Note: Both reactions occur at Earth surface conditions Calcium-Silicate Weathering: CaSiO3 + 2H2CO3 Ca2+ + 2HCO3- + SiO2(aq) + H2O Note: Silicate minerals do not re-form at Earth surface conditions Carbonate Precipitation: Ca2+ + 2HCO3- CaCO3 + H2CO3 Opal (Biogenic Silica) Precipitation: SiO2(aq) SiO2(s) Ocean-atmosphere CO2 exchange: CO2 + H2O H2CO3 CaSiO3 + CO2 CaCO3 + SiO2 Ca2+ liberated from silicate weathering leaves ocean as CaCO3 2 mol H2CO3 req'd to weather CaSiO3 but only 1 mol H2CO3 liberated during CaCO3 precipitation Added 1/30/09 ____________________________________________________________________________ 6 Net Reaction of Rock Weathering + Carbonate and Silica Precipitation in Ocean CaSiO3 + CO2 --> CaCO3 + SiO2 CO2 consumed (~ 0.03 Gt C/yr) Net reaction of geochemical carbon cycle (Urey Reaction) Would deplete atmospheric CO2 in 20 kyr (R w.r.t. weathering) Plate tectonics returns CO2 via Volcanism and Metamorphism ------------------------------------Carbonate Metamorphism C a C O3 + SiO2 --> CaSiO3 + CO2 CO2 produced from subducted marine sediments On geologic time scales, rock weathering balanced by carbonate metamorphism Any imbalance can cause changes in atmospheric CO2 (1) (2) (3) (4) CarbonateSilicate Geochemical Cycle 1. CO2 released from volcanism dissolves in H2O, forming carbonic acid 2. H2CO3 dissolves rocks 3. Weathering products transported to ocean by rivers 4. CaCO3 precipitation in shallow & deep water 5. Cycle closed when CaCO3 metamorphosed in subduction zone or during orogeny. (4) (5) Stanley (1999) 7 Carbonate-Silicate Geochemical Cycle Kump et al. (1999) Geologic record indicates climate has rarely reached or maintained extreme Greenhouse or Icehouse conditions.... Negative feedbacks between climate and Geochemical Carbon Cycle must exist Thus far, only identified for CarbonateSilicate Geochemical Cycle: Temp., rainfall enhance weathering rates (Walker et al, 1981) How are CO2 levels kept in balance on >106yr time scales? Feedbacks (I.e., no obvious climate dependence of tectonics or organic carbon geochemical cycle.) Adapted from Kump et al. (1999) 8 Atmospheric CO2 During the Last 545 Ma 385 ppm 2008 http://commons.wikimedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png Short-Term (100-102 yr) Carbon Cycle Our Focus Photosynthesis, Respiration, Air-Sea Gas Exchange, Ocean Circulation 2/24 (LJ) 2/3,5,10 2/19 Black arrows natural fluxes IPCC 2007, Fig. 7.3 Red arrows anthropogenic fluxes 9 Table XI-1. Carbon reservoirs (excluding terrestrial rocks other than coal) and fluxes. The data are from the compilations of Pilson (1998), IPCC (2001), and Sabine (2004). Reservoirs (Pg): Atmosphere: CO2 (288 ppm in 1850) (369 ppm in 2000) Oceans: Biota DOC Org C in sediments (1 meter) DIC 612 784 1-2 700 1,000 38,000 600 1,500 44 90 3440 90 100 120 45 60 8-15 0.2 Global Carbon Fluxes & Reservoirs Details Terrestrial: Biota Soil Humus (1 meter) Fossil Fuels (identified reserves), gas oil coal, oil sand & shale Fluxes (Pg yr-1): Atmosphere-Ocean exchange Gross Primary Production Ocean Land Net Primary Production Ocean Land Net C export from the surface ocean Sedimentation of Org. C. in the ocean Anthropogenic Changes (Pg or Pg yr-1): Cumulative Changes (Pg): (1800-1994) Fossil Fuels Burnt & Cement Prod. Atmospheric Increase Storage in the Ocean Inferred Terrestrial Change Excludes crustal rocks (& mantle!) other than coal, oil & gas 244 165 118 -39 Partitioning of Anthropogenic Fluxes (1990s) (Pg yr-1) Fossil Fuel and Cement Production 6.3 0.4 Atmosphere Accumulation 3.2 0.1 Uptake by Terrestrial Biosphere -1.4 0.7 Ocean Uptake -1.7 0.5 _____________________________________________________________________________ Pg, petagram = 1015 g = Gt, gigaton Emerson & Hedges (2007) Table XI-1 Because ocean reservoir is large w.r.t. atmosphere, & rapid exchange occurs between them, the processes affecting these reservoirs & fluxes are central in controlling atmospheric CO2 (& GG forcing) on 101-102 time scales 8 1015 moles C 1015 moles C/yr 8 To get gigatons of C (109 metric tons = 1015 gC) multiply by atomic mass of 12 10 Modern Air-Sea Fluxes of CO2 Sinks Sources What Determines these Fluxes? IPCC 2007 Fig. 7.8 Processes Controlling the Exchange Of CO2 Between the Atmosphere & Ocean Chemical Physical Quikscat surface wind power density Carbonate buffer Biological SeaWifs ocean color Sabine et al. (2004) Science Vol. 305: 367-371 http://www.nasa.gov/images/content/ 257995main_quikscat-wind-browse.jpg http://oceancolor.gsfc.nasa.gov/SeaWiFS/ TEACHERS/sanctuary_7.html 11 Material in the following lectures was drawn from several sources Broecker (2005) The Role of the Ocean in Climate Yesterday, Today and Tomorrow, Eldigio Press, NY. Broecker & Peng (1982) Tracers in the Sea Eldigio Press, NY. Emerson & Hedges (2007) Chemical Oceanography and the Carbon Cycle. Cambridge University Press. Zeebe & Wolf-Gladrow (2001) CO2 in Seawater: Equilibrium, Kinetics, Isotopes. Elsevier Press. Sarmiento & Gruber (2006) Ocean Biogeochemical Dynamics. Princeton University Press. Ed Boyle (2008) Lecture Notes for 12.842: Climate Physics & Chemistry, MIT. Summary of Processes Influencing Air-Sea Exchange of CO2 Physical & Chemical Processes Biological Processes IPCC 2007 Fig. 7.10 12 Outline of Processes Influencing Air-Sea Exchange of CO2 1. Physical Processes (kinetics) o Air-sea gas exchange = f (wind speed, bubble injection, surfactants) o Ocean circulation 2. Chemical Processes o CO2 solubility = f (temperature, salinity) ["The Solubility Pump"] o Carbonate chemical equilibrium 3. Biological Processes ["The Biological Pump"] o Photosynthesis & respiration o Calcium carbonate production Physical & Chemical Processes Controlling CO2 Uptake by the Ocean Chemical equilibrium determines total possible transfer Carbonate equilibrium, summarized by Revelle Factor; not attained in most of the surface ocean Gas exchange dynamics across the air-sea interface determine the rate of approach to chemical equilibrium. Gas exchange = f (wind speed, bubble injection, surfactants) o Estimated from 222Rn deficit, 14C uptake, tracer release experiments (SF6, 3He, Bacillus globigii) CO2 that dissolves into surface mixed layer carried into ocean interior by ocean circulation 13 Outline of Processes Influencing Air-Sea Exchange of CO2 1. Physical Processes o Air-sea gas exchange = f (wind speed, bubble injection, surfactants) o Ocean circulation 2. Chemical Processes o o CO2 solubility = f (temperature, salinity) ["The Solubility Pump"] Carbonate chemical equilibrium 3. Biological Processes ["The Biological Pump"] o o Photosynthesis & respiration Calcium carbonate production Air-Sea Gas Exchange Under which conditions do you expect higher rates of air-sea gas exchange? http://z.about.com/d/cruises/1/0/u/k/3/Emerald_Princess_Asea.JPG http://ninjaradio.files.wordpress.com/2008/08/calm_sea_memory_470x353.jpg 14 Atmosphere-Ocean Gas Exchange Basics Gas exchange is driven by a disequilibrium in the partial pressure of gases between the ocean & atmosphere (e.g., from biological processes, temperature, ocean mixing) Although the direction & magnitude of net gas exchange is thermodynamically driven, it is limited by physical transport (diffusion & microadvection) through boundary layers at the surface of the ocean & bottom of the atmosphere. Physical motions in the boundary layers are restricted by surface tension (water) & friction (atmosphere) that can be enhanced by natural surfactants. Some gas exchange also caused by bubbles from breaking waves, esp. in high winds. Can help facilitate equilibrium (e.g., trapped gas can equilibrate with water & return equilibrated gas to surface), but can also create disequilibrium when a submerged bubble completely dissolves the atmospheric gases quantitatively into the water in non-thermodynamic ratios. Gas exchange is occurring in both directions at all times (even when gas partial pressures are equal between water & air) Adapted from Ed Boyle 12.842 Lecture 2008 *** Stopped Here - 1/29/09 *** 15 Atmosphere-Ocean Gas Exchange For gas exchange without bubbles, net flux is proportional to the disequilibrium between the dissolved gas at equilibrium with the atmosphere & the dissolved gas concentration in the ocean mixed layer The proportionality constant depends on the gas, wind speed (& other factors such as surface slicks) where: Cm= dissolved gas conc. in ocean mixed layer (mol/m3) Co = dissolved gas conc. at equilibrium w/ atmosphere = gas conc. in air / H H = Henry's law const. (ratio of conc. in air to equil. conc. in H2O,T) k = proportionality const. relating 1-way gas flux to its conc. in H2O Flux units = moles/m2/yr; k units = [moles/m2/yr] / [mol/m3] = m/yr Microphysics of gas exchange not well understood. Conceptual models commonly used to estimate gas exchange rates. Flux = k * (Cm-Co) Adapted from Ed Boyle 12.842 Lecture 2008 Stagnant Film Model Flux = D*(Cm-Co) / zfilm Co zfilm Cm Where: Cm= dissolved gas conc. in ocean mixed layer Co= dissolved gas conc. at surface (equil. w/ atmos.) zfilm= thickness of stagnant film (20-200 m, varies with wind) D = diffusion coeff. of gas (~ 10-5 cm2/sec, varies with gas) Note: k = D / zfilm Thin film of "stagnant" water separates well-mixed air from well-mixed water Gases transferred between air & water by molecular diffusion through film Assumes gas conc. at equilibrium w/ air at top of film & = surf. ocean @ bottom Film thickness decreases as agitation (i.e., wind speed) increases (avg~30 m) Adapted from Ed Boyle 12.842 Lecture 2008, Broecker & Peng (1984) 16 Gas Exchange ( k = D / zfilm ) Coefficient Increases With Wind Speed Jahne & Hau ecker (1998) Air-water gas exchange, Ann. Rev. Fluid. Mech. Vol. 30: 443-468. Piston velocity k ( = D / zfilm ) is called the "piston velocity" because it has units of length per time & behaves like two pistons driving dissolved gases into & out of ocean mixed layer o May be more logical & intuitive to interpret k as an "exchange coefficient" instead of literally as a ratio of diffusion to film thickness The piston velocity for CO2 in the ocean is about 2000 m/yr! diffusion from water into air stagnant film diffusion from air into water Adapted from Ed Boyle 12.842 Lecture 2008 17 Ballpark Estimate of Exchange Rate of CO2 Between Ocean & Atmosphere 2000 m yr-1 * 10-5 moles kg-1 * 1000 kg m-3 = 20 moles m-2 yr-1 Piston velocity conc. of conversion gaseous Factor for dissolved CO2 H 2O Exchange rate of CO2 across air/sea interface How are Gas Exchange Rates (Coefficients) Determined? Radon-222 deficit Atmosphere-ocean 14C difference Tracer release experiments (SF6, 3He) Eddy covariance 18 Deficit Method For Determining Gas Exchange Rates zfilm = D / k = [DRn-222/Rn-222*h] * [1/((ARa-226/ARn-222)-1)] 222Rn Adapted from Ed Boyle 12.842 Lecture 2008 Air-Sea CO2 Exchange from "Bomb 14C" Above-ground nuclear bomb tests in 1950s-1960s doubled the inventory of 14C in the atmosphere Attenuated bomb 14C signal in ocean places strong constraint on CO2 gas exchange rate Bomb spike Krakauer et al. (2006) Tellus Vol. 58B: 390-417 19 Dual Gaseous Tracer Release Technique Used to separate advective & gas transfer components of mass loss in a deliberate tracer experiment Two gaseous tracers with widely different (known) rates of escape Nonvolatile tracer used to monitor dispersion of patch while change in nonvolatile to volatile tracer ratio over time near center of patch indicates loss due to gas transfer Combination of 3He & SF6 works well in ocean b/c escape rates differ by 3x, nontoxic, stable, & non-reactive Gas transfer velocity expressed as k = h/(t2- t1) In (Rwt1/Rwt2) where Rwt is the ratio of volatile (SF6) to nonvolatile (3He) tracer at time t. Wanninkhof et al. (1993) J. Geophys. Res., 98(C11), 20237-20248. Gas Exchange Increases with Wind Speed 3He/SF 6 Wind Speed Higher wind speeds lower 3He/SF6 ratio b/c they cause increased loss of less volatile gas (3He) Nightingale et al. (2000) Glob. Biogeochem. Cycl. Vol. 14(1): 373-387. 20 Different techniques for determining wind speed dependence of gas exchange rates vary by a factor of 2 or more... ...& uncertainty for all techniques higher at high wind speeds Eddy covariance: Flux = v(t)C(t) dt Jahne & Hau ecker (1998) Ann. Rev. Fluid. Mech. Vol. 30: 443-468. McGillis et al. (2001) Mar. Chem. Vol. 75(4): 267-280. Surface Slicks (Surfactants) Retard Gas Exchange Surfactants increase surface pressure of water Increasing surface pressure results in decreased gas exchange rate Surface Pressure McKenna & McGillis (2004) Int. J.Heat Mass Transfer Vol. 47(3): 539-553. 21 = K, piston velocity Measuring the Surfactant Effect in Seawater Most surfactants are organic Gas exchange rates of oxygen were inversely proportional to DOC concentration Implies that the organic content affects the interface properties that control gas transfer Emerson & Hedges Fig. 10.13, Redrawn from Frew (1997). http://www.venocoinc.com/images/seepsheen.jpg Effect of Bubbles on Air-Sea Gas Exchange Some gas exchange occurs from bubbles formed by breaking waves, especially in high winds Can help facilitate equilibrium (e.g., trapped gas can equilibrate with water & return equilibrated gas to surface) Can also create disequilibrium when a submerged bubble completely dissolves the atmospheric gases quantitatively into the water in non-thermodynamic ratios. Emerson & Hedges (2007) Chem. Oceanogr., Fig. 10.10 22 Effect of Bubbles on AirSea Gas Exchange Bubble-induced gas transfer velocity as a function of wind speed for He, O2 & CO2 (from a model) Bubbles may make an important contribution to net gas exchange at wind speeds >10 m/sec. The global-mean supersaturation of CO2 induced by bubbles is not larger than 0.3% & is likely around 0.08%. Total gas transfer velocity Bubble contribution A major uncertainty results from a lack of information on production rates & distributions of large bubbles. Keeling, R.F. (1993) J. Mar. Res. Vol. 51: 237-271. Measuring the Bubble Injection Effect Supersaturation of inert gases with different solubilities allow quantification of bubble-induced injection of air to net air-sea gas exchange For more on this topic, see excellent chapter 10 in Emerson & Hedges (2007) Chem. Oceanogr.. This is Fig. 10.12. 23 Outline of Processes Influencing Air-Sea Exchange of CO2 1. Physical Processes o Air-sea gas exchange = f (wind speed, bubble injection, surfactants) o Ocean circulation 2. Chemical Processes o o CO2 solubility = f (temperature, salinity) ["The Solubility Pump"] Carbonate chemical equilibrium 3. Biological Processes ["The Biological Pump"] o o Photosynthesis & respiration Calcium carbonate production The Role of Ocean Circulation in Air-Sea Exchange of CO2 CO2 in the atmosphere equilibrates with the ocean mixed layer on a timescale of ~1 yr. o We will do this calculation after discussing the chemistry of ocean uptake of CO2. But when atmospheric CO2 rises (e.g., from fossil fuels) the ocean's uptake of that CO2 is limited by the rate of penetration of surface waters into the ocean interior. o That is why the mean age of fossil-fuel CO2 is ~28 years. Ocean circulation & the rate at which surface waters enter the deep sea are therefore central in determining air-sea CO2 exchange (on 101-102 yr time scales). 24 Ocean Circulation & Air-Sea CO2 Exchange Ocean circulation overview Transient tracers *** Ended Here - 2/3/09 *** 25 Ocean Surface Currents Driven by winds & Earth's rotation Hence the term "wind-driven circulation" http://mynasadata.larc.nasa.gov/images/L9_OceanCurrentsUSNOO.gif Processes Driving the Surface Circulation Ekman Spiral & Drift Coriolis force Friction Drives water to the right (left) of the wind stress in N (S) hemisphere Equatorial Upwelling Ekman divergence Coastal Upwelling Offshore Ekman drift Adapted frpm http://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm 26 Surface Circulation of the North Atlantic from Drifters Decadal average surface velocities in 1 grid boxes Fratantoni (2001) J. Geophys. Res. Vol. 106(C10): 22067-22093. Global Ocean Surface Velocities from an Ocean GCM MITgcm - MIT Climate Modeling Initiative - NASA 27 Deep Ocean Circulation Driven by wind, tidal & buoyancy forcing* Volume transports: Intermediate Layer (985 - 2200 m) Deep Ocean (2200 m seafloor) *For a great review see Wunsch & Ferrari (2004) Vertical mixing, energy, and the general circulation of the oceans. Annual Reviews of Fluid Mechanics, 36, 281-314. Lu & Stammer (2004) J. Phys. Oceanogr. Vol. 34(3): 605-622 Conversion of Surface Water to Deep Water Ocean Mixing Unlike the atmosphere, the ocean is both heated & cooled from the same level (i.e., the surface of the ocean) & ought to be very stably stratified. Conversely, the atmosphere is heated from below and cooled from above, making it convectively unstable and conducive to mixing Why then doesn't the ocean fill up completely with cold water, except for a thin skin of warm surface water at low latitudes? The answer lies in the turbulent processes that mix ocean waters & that derive their energy from the wind & from tides *A good review: Wunsch & Ferrari (2004) Vertical mixing, energy, and the general circulation of the oceans. Annual Reviews of Fluid Mechanics, 36, 281-314. 28 Eddies Small-scale (~100-102 km), time-varying components of the circulation Spatio-temporal integrations produce the large-scale "steady" flows Contain 90-99% of the kinetic energy of the flow (Wunsch, 2004, 2007, in press) Ubiquity & importance recognized since `70s; widely observed since `90s Without eddies (1 grid size) With eddies (1/6 grid size) http://www.oar.noaa.gov/climate/images/modeling_oceancirc.jpg Simple Model of Thermohaline (Meridional Overturning) Circulation From John Marshall, MIT 29 Processes Driving Combined Ocean Circulations (into page) Ekman pumping Convection Ekman divergence o Causing isopycnal surfaces to shoal at equator Anderson et al. (2002) Deep-Sea Res. II Vol. 49: 1909-1938. http://www.iitap.iastate.edu/gcp/sealevel/images/thermocline.gif Western Atlantic Potential Temperature* Section Combined circulations produce the observed temperature field Rather different than might be expected for a stagnant fluid heated (and cooled) at the surface! * The temperature a fluid would have if adiabatically (no heat loss or gain) brought to a standard reference pressure, usually 1000 millibars (used b/c fluid is heated when pressurized) 30 Transient Tracers Tritium (3H) & 14C were added to the atmosphere from nuclear bomb tests in the 1950's-60's, & chlorofluorocarbons (CFCs) began to be added in ~1950. Unlike CO2 these tracers began entering the ocean only within last ~50 yr. They can therefore be used to estimate how much of the ocean has been in contact with the surface during that time. Adapted from Ed Boyle 12.842 lecture notes, MIT, 2008 CFC 11 in the North Atlantic Ocean 31 CFC-11 in the Pacific Ocean 100 m 500 m 1000 m WOCE Atlas Vol. 2, Pacific Ocean, 2007 in the Pacific Ocean 100 m 14C 500 m 1000 m Units are 14C, or relative to 1950 wood WOCE Atlas Vol. 2, Pacific Ocean, 2007 32 CCl4 zonal section in the South Atlantic (11.7 S) Roether & Putzka (1996) 33