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The Contemporary Carbon Cycle Overview Marine Carbon cycle Marine sink for atmospheric CO2 The atmospheric imprint Atmospheric inversions The Terrestrial carbon sink Precision oxygen measurements. Reading. Overviews and reviews The IPCC Third assessment report.

brandon-luby
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  1. The Contemporary Carbon CycleOverviewMarine Carbon cycleMarine sink for atmospheric CO2The atmospheric imprintAtmospheric inversionsThe Terrestrial carbon sinkPrecision oxygen measurements

  2. Reading Overviews and reviews The IPCC Third assessment report. http://www.grida.no/climate/ipcc_tar/ Chapter 3, wg1 report “The scientific basis” Chapter 2-4, wg3 report “mitigation” Semi-popular overview of sinks for anthropogenic carbon: Sarmiento, J.L. and N. Gruber. Sinks for anthropogenic carbon, Physics Today, 55(8), 30-36, 2002. http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/sarmiento_pt_02.pdf Ocean - atmosphere fluxes: Watson, A. J. and Orr, J. C. (in press). Carbon dioxide fluxes in the global ocean . Chapter 5 in “Ocean Biogeochemistry : a JGOFS synthesis” eds Fasham, M. Field, J. Platt, T. & B. Zeitzschel. Available at: http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/Watson_and_orr_in_fasham(ed)_2004.pdf  Terrestrial net fluxes: Schimel DS, House JI, Hibbard KA, et al. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems NATURE 414: 169-172 2001 Available at http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/schimel_2001.pdf Atmospheric CO2 and O2 measurements Battle, M. et al. Global carbon sinks and their variability inferred from atmospheric O-2 and delta C-13. Science 287, 2467-2470 (2000). http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/battle_2000.pdf

  3. More Reading CO2 measurements in the atmosphere, and what you can do with them: Keeling, C.D., T.P. Whorf, M. Wahlen, and J. Vanderplicht, Interannual Extremes In the Rate Of Rise Of Atmospheric Carbon- Dioxide Since 1980, Nature, 375, 666-670, 1995. Keeling, C.D., J.F.S. Chin, and T.P. Whorf, Increased Activity Of Northern Vegetation Inferred From Atmospheric CO2 Measurements, Nature, 382, 146-149, 1996. Available at: http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/keeling_cd_1995.pdf http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/keeling_cd_1996.pdf Classic paper on Inverse atmospheric calculation, and the “missing sink”. Tans, P.P., I.Y. Fung, and T. Takahashi, Observational Constraints On the Global Atmospheric Co2 Budget, Science, 247 (4949), 1431-1438, 1990. http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/tans_et_al_1990.pdf Ocean uptake of CO2. Sarmiento, J.L., and E.T. Sundquist, Revised Budget For the Oceanic Uptake Of Anthropogenic Carbon-Dioxide, Nature, 356, 589-593, 1992. Watson, A.J., P.D. Nightingale, and D.J. Cooper, Modeling Atmosphere Ocean CO2 Transfer, Philosophical Transactions Of the Royal Society Of London Series B- Biological Sciences, 348, 125-132, 1995.

  4. Atmospheric CO2: Past, present and near future

  5. The global carbon cycle (Source, Sarmiento and Gruber, 2002)

  6. The global carbon cycle • Most of the “labile” carbon on Earth is in the deep sea. • The gross atmosphere-ocean and atmosphere-vegetation fluxes are of the same order. • The net atmosphere-ocean and atmosphere-vegetation fluxes are much smaller than the gross fluxes. • The flux through the marine biota (net productivity) is of the same order as that through the land vegetation. • The mass of the marine biota is 1000 times less than that of the land vegetation.

  7. The (almost) unperturbed marine carbon cycle: Global mean air-sea flux, calculated from pCO2 measurements

  8. Revision: Seawater Carbonate chemistry • Inorganic carbon exists as several forms in sea water: • Hydrated dissolved CO2 gas. • This rapidly reacts with H2O to form undissociated carbonic acid: CO2(g) + H2O  H2CO3 • Which can dissociate by loss of H+ to form bicarbonate ion: H2CO3  H+ + HCO3- • which can dissociate by further loss of H+ to form carbonate ion: HCO3-H+ + CO32- Typically, 90% of the carbon exists as bicarbonate, 9% as carbonate, 1% as dissolved CO2 and undissociated H2CO3 (usually lumped together).

  9. Seawater Partial pressure of CO2 • The partial pressure of CO2 of the sea water (pCO2sw) determines whether there is flux from air to sea or sea to air: • Air-to-sea Flux is proportional to (pCO2air* - pCO2sw) • pCO2sw is proportional to dissolved CO2(g): [CO2(g)] =  x pCO2sw = where • is the solubility of CO2. The solubility decreases with increasing temperature. *pCO2air is determined by the atmospheric mixing ratio, i.e. if the mixing ratio is 370ppm and atmospheric pressure is 1 atm, pCO2air is 370 atm.

  10. What sets the net air-sea flux? The flux is set by patterns of sea-surface pCO2sw, forced by: • Ocean circulation; • Is surface water is cooling or heating? • Is water being mixed up from depth? • Ocean biology; • Is biological activity strong or weak? • Is calcium carbonate being precipitated? • The rising concentration of atmospheric CO2 • pCO2 of air is rising and this tends to favour a flux from atmosphere into the ocean.

  11. Circulation influence on air-sea flux • Warm currents, where water is cooling, are normally sink regions (NW Atlantic, Pacific). • Source regions for subsurface water, where water is cooled sufficiently to sink are strong sinks (N. N. Atlantic, temperate Southern ocean).. • Tropical upwelling zones, where subsurface water comes to the surface and is strongly heated, are strong sources (equatorial Pacific).

  12. The overturning thermohaline circulation Water cools and sinks Water warms and upwells? • The Northern North Atlantic is a region of strong cooling, associated with the North Atlantic drift. • Cooling water takes up CO2 and may subsequently sink. • The water upwells in other parts of the world ocean, particularly the equatorial Pacific. • Upwelling regions are usually sources of CO2 to the atmosphere – deep water has high CO2 and the water is being warmed. • This circulation controls how rapidly old ocean water is brought to the surface, and therefore how quickly the ocean equilibrates to changes in atmospheric CO2 concentration.

  13. Biological influence on air-sea flux. • Blooms of plankton fix carbon dioxide from the water and lower CO2, hence pCO2. • Particularly marked in the North Atlantic which has the most intense bloom of any major ocean region. • In the equatorial Pacific, plankton blooms are suppressed by lack of iron – part of the explanation for high pCO2there. • In the equatorial Atlantic, upwelling is less intense and there is more iron from atmospheric dust.

  14. Ocean carbon “pumps” • Deep water has higher (10-20%) total carbon content and nutrient concentrations than surface water. There are several processes contributing to this: • The "Solubility pump" tends to keep the deep sea higher in total inorganic carbon (CO2) compared to the warm surface ocean. • The “Biological pump(s)" – the flux of biological detritus from the surface to deep, enriches deep water concentrations. There are two distinct phases of the carbon in this material: • The "soft tissue" pump enriches the deep sea in inorganic carbon and nutrients by transport of organic carbon compounds. • The calcium carbonate pump enriches the deep sea in inorganic carbon and calcium.

  15. Carbonate Soft tissue Ocean biological pumps • Falling dead organisms, faecal pellets and detritus are "remineralised" at depth. Remineralization occurs • By bacterial activity. • By inorganic dissolution of carbonate below the lysocline. • The different phases have different depth profiles for remineralisation.

  16. Ocean Carbon: The Biological (soft tissue) Pump • This mechanism acts continually to reduce the partial pressure of CO2 (pCO2) in the surface ocean, and increase it at depth. • Over most of the ocean, upwelling water is depleted of inorganic carbon and nutrients (nitrate and phosphate) by plankton. • In the process they remove about 10% of the inorganic CO2 in the water. Most of this goes to form organic matter via the reaction: CO2 + H2O  CH2O +O2. • Because the buffer factor ~10, this has a large effect on surface pCO2, decreasing it by 2-3 times. • The reverse reaction occurs by (mostly bacterial) respiration at depth, and increases CO2 concentration there. Depth

  17. Surface pCO2, nutrient and surface temperature in the North Atlantic 360 Mar Apr May Jun Jul Aug Sep Oct 340 18 SST SST (°C) 320 8 16 ) atm m ( 300 6 2 pCO 2 pCO 14 Nitrate ( 4 m 280 M ) 12 2 260 Nitrate

  18. The biological (calcium carbonate) pump. • This mechanism also transfers carbon from the surface ocean to the deep sea. • Some of the carbon taken up by the biota in surface waters goes to form calcium carbonate. • The CaCO3 sinks to the deep sea, where some of it may re-dissolve and some become sedimented. The redissolution can only occur below the lysocline, which is shallower in the Pacific than the Atlantic. • In contrast to the soft tissue pump, this mechanism tends to increase surface ocean pCO2 and therefore atmospheric CO2 . The net reaction is: Ca++ + 2HCO3- H2O +CO2 + CaCO3

  19. Coccolithophores -- calcite precipitating plankton Photo: courtesy D. Purdie: See the Ehux home page: www.soc.soton.ac.uk/SUDO/tt/eh/index.html

  20. The ocean sink for anthropogenic CO2 • The oceans are close to steady-state with respect to atmospheric CO2. • Prior to the industrial revolution, the oceans were a net source of order 0.5 Pg C yr-1 CO2 to the atmosphere. Today they are net sink of order 2 Pg C yr-1. • The main factor controlling ocean uptake is the slow overturning circulation, which limits the rate at which the ocean mixes vertically. • Three methods are being used to calculate the size of the ocean sink. • Integration of the pCO2 map (difficult and inaccurate). • Measurements of atmospheric oxygen and CO2 (see later). • Models of ocean circulation. These are of two types: • Relatively simple box-diffusion models “calibrated” so that they reproduce the uptake of tracers such as bomb-produced 14carbon. • Ocean GCMs which attempt to diagnose the uptake from the circulation. (However, the overturning circulation is difficult to model correctly. In practice these models are also tested against ocean tracers.)

  21. “Riverine” flux The pre-industrial steady state cycle: to balance the flux of carbon coming down rivers, there must have been a CO2 flux of the order of 0.5 Pg C yr-1 from ocean to atmosphere and from atmosphere to land. Volcanic activity and sedimentation fluxes provide smaller net inputs and outputs to the system.

  22. 300 Bomb radiocarbon x 1020 atoms 200 100 1950 1960 1970 1980 1990 Tropospheric bomb radiocarbon The atmospheric bomb tests of the 50s and 60s injected a “spike” of radiocarbon into the atmosphere which was subsequently tracked into the ocean. This signal provides a good proxy for anthropogenic CO2 over decadal time scales.

  23. 3-D model outputs for surface pCO2 • Capture the basic elements of the sources and sinks distribution. • Considerable discrepancies with one another and with the data (Southern Ocean, North Atlantic).

  24. How well is the global ocean sink known? Estimates of the global ocean sink 1990-1999 Reference Sink (Pg C yr-1) IPCC (2001) 1.7+/- 0.5 Estimate (Keeling oxygen technique) OCMIP-2 Model 2.5+/- 0.4 Intercomparison (ten ocean carbon models). Not very well!

  25. Will ocean uptake change in the future? • Yes: the models forecast that the sink will increase in the short term as increasing atmospheric CO2 forces more into the oceans. • But, the buffering capacity of the ocean becomes less as CO2 increases, tending to decrease uptake. • Also, if ocean overturning slows down, this would tend to decrease the uptake. • Changes in ocean biology may also have an impact….

  26. Source: Manabe and Stouffer, Nature 364, 1993

  27. 1994-1995 2002-2005 North Atlantic pCO2 • Data 1994-1995 • Near-continuous data 2002-present • Sharp decrease in ΔpCO2 relative to mid 90s

  28. Possible Marine biological effects on Carbon uptake, next 100 years. Iron fertilisation -- deliberate or inadvertent NO3 fertilisation pH change mediates against calcite- precipitating organisms Reduction in THC offset by increased efficiency of nutrient utilisation Other unforeseen ecosystem changes Process Effect on CO2 uptake ?

  29. Marine carbon cycle summary: • The ocean CO2 sink is affected both by circulation and biology. Changes in either would affect how much CO2 is taken up by the ocean. Climate change may cause both. • Because different methods agree roughly on the size of the global ocean sink, it has generally been assumed that we know it reasonably well. • However, there is an increasing discrepancy between the most accurate methods. Our present understanding allows us to specify the sink only to ~35%. • We cannot at present specify how it changes from year to year or decade-to-decade. • Acccurate knowledge of the ocean sink would enable us (via atmospheric inverse modelling) to be much more specific about the terrestrial sinks – useful for verification of Kyoto-type agreements.

  30. The atmospheric imprint of anthropogenic carbon

  31. Pre-industrial steady state. • Fluxes into and out of the atmosphere were approximately at steady state before 1750. • Small variations correlate with climate change (?) – i.e little ice age ~ 1600.

  32. Atmospheric CO2 variations since 1000 AD

  33. Fossil Fuel Emissions • Well quantified from econometric data (Marland, Andres)

  34. The budget for anthropogenic CO2 (1980s: numbers in Pg C yr-1.) • Well-known numbers (<10% uncertainty): • 1) Rate of fossil fuel release 5.4 • 2) Rate of build-up in the atmosphere: 3.3 • Poorly known number ( 0.8 Pg uncertainty?) • 3) uptake by ocean 1.9 • Very poorly known number ( 1.3 Pg C yr-1). • 4) Rate of (mostly tropical) deforestation: 1.7 • Extremely poorly known number calculated • to balance budget (ie 1 +4 – 2 - 3). • 5) Uptake by extra-tropical vegetation 1.9

  35. Accumulation in atmosphere 3.3 Pg C yr-1 1980s budget of anthropogenic carbon dioxide. Land uptake? (1.9 by difference) Deforestation 1.7 Pg C yr-1? Fossil fuel release 5.4 Pg C yr-1 Ocean uptake 1.9 Pg C yr-1

  36. The Mauna Loa atmospheric record. Accurate measurements of CO2 mixing ratio in dried air have been made by C. Dave Keeling since 1958 at Mauna Loa observatory, Hawaii. From the 70s on, there have been an regular measurements at an increasing number of stations around the globe. C. Dave Keeling Late 1990s measurement network

  37. The Mauna Loa atmospheric record. • Overall increasein atmospheric CO2 of~4% per year. • Inter-annual and inter-decadal changes in the rate of rise not due to changes in fossil fuel emissions -- indicate changes in the “natural” sinks. • An increasing amplitude of the northern hemisphere seasonal cycle correlating with increased global temperatures. • Increasing length of the growing season.

  38. Variation in the growth rate of atmospheric CO2, 1957-1999 • Rate of growth is highly variable – not due to change in fossil fuel source. • Variation correlates with Southern oscillation – El Ninos. • Indicates the “Natural” sinks for atmospheric CO2 are highly variable. • Though the land sink dominates variability, ocean is also important

  39. The Mauna Loa atmospheric record…contd. Accurate measurements of CO2 mixing ratio in dried air have been made by C. D. Keeling since 1958 at the Mauna Loa Laboratory in Hawaii, and more recently at many other stations around the world. The Mauna Loa record shows: • Overall increasein atmospheric CO2 of~4% per year. • Inter-annual and inter-decadal changes in the rate of rise not due to changes in fossil fuel emissions -- indicate changes in the “natural” sinks. • An increasing amplitude of the northern hemisphere seasonal cycle correlating with increased global temperatures. • Increasing length of the growing season.

  40. Keeling, C.D., et al., Nature, 382, 146-149, 1996

  41. Distribution of CO2 in the atmosphere • Seasonality is most pronounced at high latitudes Northern Hemisphere. Southern Hemisphere seasonality is small. • The seasonality is mostly due to the land biota – almost all in the N. Hemisphere. • The marine biological signal is buffered by carbonate chemistry and its seasonality is smoothed out – not apparent in the atmospheric signal.

  42. Calculation of sinks by inversion • Principle: Models of global atmospheric transport are used to deduce where the net source/sinks must be, in order to give rise to the observed (small) variations in atmospheric CO2 concentrations. • If the locations of the (anthropogenic) sources are known, the (natural) sinks can be specified. • Good for inter-hemispheric distributions. • Less good for latitudinal distributions. • Poor for longitudinal distributions.

  43. Tans, Fung and Takahashi Observational constraints on the global atmospheric CO2 budget, Sciecne 247, 1431 (1990). • Combined constraints from observed interhemispheric gradient with ocean surface pCO2 data. • N. Hemishere ocean data suggested N.H ocean uptake <= 0.6 Pg yr-1. • They deduced: • Global net ocean sink <= 1 Pg C yr-1 • Large N. Hemisphere mid-latitidue terrestrial sink (2-3 Pg C yr-1) • Subsequently it has been found that their ocean sink was too small, land sink too large, but the existence of a substantial NH land sink is now established.

  44. Tans et al Fig 5: Observed mean annual CO2 concentrations (circles and solid curve) as a function of sine of latitude (-1 is S. Pole). These are compared with calculations from a model (squares and dashed curve), and expressed as deviations from a mean CO2 concentration.

  45. Present distribution of Land sources and sinks Firm conclusions: • A substantial sink in the Northern Hemisphere mid-latitudes. • Unknown distribution among the continents • The tropical land areas are thought to be nearly neutral. • All sinks are variable from year to year and decade to decade. N. hemisphere Tropics S. hemisphere

  46. Box inverse model • * We take the interhemispheric gradient to be g = 2 x 10-6 v/v • The residence time wrt interhemispheric exchange is τ = 1 yr • The Mass of the atmosphere M = 1.6 x 1020 mol • The interhemispheric flux is then • = 1.9 PgC

  47. Box model calculations for the period 1980-1989 . (1) Total Mass balance: NO+SO+NL +EL = FF-AA = 2.0 (2) N. hemisphere mass balance: 0.9FF-NO-NL-IF = 0.45AA= 1.5 (3) N. hemisphere ocean sink by observation: NO= 0.60.15 (4) Total ocean sink by model and observation: NO+SO = 1.40.5 This is a system four equations in four unknowns: Calculation: From (3) and (4): SO=0.8 Sub values for FF, NO, IF in (2):NL=0.9FF-NO-IF-0.45AA = 0.9 Sub (4) into (1):NL + EL = 0.6: Hence EL = -0.3 These calculations imply a modest NH land sink in mid latitudes, and a small net source in the land tropics (could be lots of deforestation + lots of re-growth). Note the sensitivity of the calculations to errors. This arises because the sinks are calculated as comparatively small differences between large numbers. In the 1980s, the total “natural” sink (fossil fuel input - accumulation in atmosphere) was on average 2.0. In the early 1990s, (period 1991-1994) natural sinks were nearly double this. Today they are in the range 3 – 4 Pg C yr-1

  48. Possible causes of the NH mid-latitiude sink • Land use Change • Anthropogenic fertilization, chiefly nitrogen deposition • CO2 fertilization

  49. Land-Use change • “REVERSE PIONEER” REGROWTH OF FOREST • In the last century, large areas of forest near population centres in N. America were cleared for crops. • With the coming of the railways, the centres of crop production moved to the mid-western prairies. Farmland was abandoned and new-growth forest re-established. • The process is continuing today. • Similar, less dramatic trend in Europe and Russia. • FOREST CONSERVATION: • Suppression of fire • Suppression of insect infestation • INCREASED ORGANIC SEDIMENTATION IN RESERVOIRS?

  50. Land use change and the US carbon budget:estimates from “carbon accounting” Houghton RA, Hackler JL, Lawrence KT The US carbon budget: Contributions from land-use change SCIENCE 285 (5427): 574-578 JUL 23 1999

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