Presentation is loading. Please wait.

Presentation is loading. Please wait.

The Contemporary Carbon Cycle Overview Marine Carbon cycle Marine sink for atmospheric CO2 The atmospheric imprint Atmospheric inversions The Terrestrial.

Similar presentations


Presentation on theme: "The Contemporary Carbon Cycle Overview Marine Carbon cycle Marine sink for atmospheric CO2 The atmospheric imprint Atmospheric inversions The Terrestrial."— Presentation transcript:

1 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

2 Reading Overviews and reviews The IPCC Third assessment report. 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, 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: Terrestrial net fluxes: Schimel DS, House JI, Hibbard KA, et al. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems NATURE 414: Available at Atmospheric CO 2 and O 2 measurements Battle, M. et al. Global carbon sinks and their variability inferred from atmospheric O-2 and delta C-13. Science 287, (2000).

3 More Reading CO 2 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, , Keeling, C.D., J.F.S. Chin, and T.P. Whorf, Increased Activity Of Northern Vegetation Inferred From Atmospheric CO2 Measurements, Nature, 382, , Available at: 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), , Ocean uptake of CO 2. Sarmiento, J.L., and E.T. Sundquist, Revised Budget For the Oceanic Uptake Of Anthropogenic Carbon-Dioxide, Nature, 356, , 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, , 1995.

4 Atmospheric CO 2 : 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 pCO 2 measurements

8 Revision: Seawater Carbonate chemistry Inorganic carbon exists as several forms in sea water: –Hydrated dissolved CO 2 gas. –This rapidly reacts with H 2 O to form undissociated carbonic acid: CO 2(g) + H 2 O H 2 CO 3 –Which can dissociate by loss of H + to form bicarbonate ion: H 2 CO 3 H + + HCO 3 - –which can dissociate by further loss of H + to form carbonate ion: HCO 3 - H + + CO 3 2- Typically, 90% of the carbon exists as bicarbonate, 9% as carbonate, 1% as dissolved CO 2 and undissociated H 2 CO 3 (usually lumped together).

9 Seawater Partial pressure of CO 2 The partial pressure of CO 2 of the sea water (pCO 2sw ) determines whether there is flux from air to sea or sea to air: –Air-to-sea Flux is proportional to (pCO 2air * - pCO 2sw ) pCO 2sw is proportional to dissolved CO 2(g) : [CO 2(g) ] = x pCO 2sw = where is the solubility of CO 2. The solubility decreases with increasing temperature. *pCO 2air is determined by the atmospheric mixing ratio, i.e. if the mixing ratio is 370ppm and atmospheric pressure is 1 atm, pCO 2air is 370 atm.

10 What sets the net air-sea flux? The flux is set by patterns of sea-surface pCO 2sw, 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 CO 2 –pCO 2 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 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 CO 2 and may subsequently sink. The water upwells in other parts of the world ocean, particularly the equatorial Pacific. Upwelling regions are usually sources of CO 2 to the atmosphere – deep water has high CO 2 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 CO 2 concentration. The overturning thermohaline circulation

13 Biological influence on air-sea flux. Blooms of plankton fix carbon dioxide from the water and lower CO 2, hence pCO 2. 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 pCO 2 there. 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 ( CO 2 ) 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 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. Soft tissue Carbonate

16 Ocean Carbon: The Biological (soft tissue) Pump This mechanism acts continually to reduce the partial pressure of CO 2 (pCO 2 ) 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 CO 2 in the water. Most of this goes to form organic matter via the reaction: CO 2 + H 2 O CH 2 O +O 2. Because the buffer factor ~10, this has a large effect on surface pCO 2, decreasing it by 2-3 times. The reverse reaction occurs by (mostly bacterial) respiration at depth, and increases CO 2 concentration there. Depth

17 Surface pCO 2, nutrient and surface temperature in the North Atlantic pCO 2 ( atm ) Nitrate ( M ) SST (°C) MarAprMayJunJulAugSepOct Nitrate SST pCO 2

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 CaCO 3 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 pCO 2 and therefore atmospheric CO 2. The net reaction is: Ca HCO 3 - H 2 O +CO 2 + CaCO 3

19 Photo: courtesy D. Purdie: See the Ehux home page: Coccolithophores -- calcite precipitating plankton

20 The ocean sink for anthropogenic CO 2 The oceans are close to steady-state with respect to atmospheric CO 2. Prior to the industrial revolution, the oceans were a net source of order 0.5 Pg C yr -1 CO 2 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 CO 2 (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 14 carbon. 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 CO 2 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 Bomb radiocarbon x atoms 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 CO 2 over decadal time scales.

23 3-D model outputs for surface pCO 2 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 ReferenceSink (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 CO 2 forces more into the oceans. But, the buffering capacity of the ocean becomes less as CO 2 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 North Atlantic pCO Data 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 NO 3 fertilisation pH change mediates against calcite- precipitating organisms Reduction in THC offset by increased efficiency of nutrient utilisation Other unforeseen ecosystem changes ProcessEffect on CO 2 uptake ?

29 Marine carbon cycle summary: The ocean CO 2 sink is affected both by circulation and biology. Changes in either would affect how much CO 2 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 Small variations correlate with climate change (?) – i.e little ice age ~ 1600.

32 Atmospheric CO 2 variations since 1000 AD

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

34 The budget for anthropogenic CO 2 (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 Ocean uptake 1.9 Pg C yr -1 Fossil fuel release 5.4 Pg C yr -1 Accumulation in atmosphere 3.3 Pg C yr -1 Land uptake? (1.9 by difference) Deforestation 1.7 Pg C yr -1 ? 1980s budget of anthropogenic carbon dioxide.

36 The Mauna Loa atmospheric record. Accurate measurements of CO 2 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. Late 1990s measurement network C. Dave Keeling

37 The Mauna Loa atmospheric record. Overall increase in atmospheric CO 2 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 CO 2, 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 CO 2 are highly variable. Though the land sink dominates variability, ocean is also important

39 Overall increase in atmospheric CO 2 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. The Mauna Loa atmospheric record…contd. Accurate measurements of CO 2 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:

40 Keeling, C.D., et al., Nature, 382, , 1996

41 Distribution of CO 2 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 CO 2 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 Combined constraints from observed interhemispheric gradient with ocean surface pCO 2 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. Observational constraints on the global atmospheric CO2 budget, Sciecne 247, 1431 (1990).

44 Tans et al Fig 5: Observed mean annual CO 2 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 CO 2 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 QuantitySymbolValue PgC a -1 Fossil fuel fluxFF5.4 Accumulation in the atmosphere AA3.4 Interhemispheric flux* IF1.9 Northern hemisphere ocean sink NO Southern hemisphere + equatorial ocean sink SO Northern hemisphere land sink NL Equatorial + Southern land sink EL * We take the interhemispheric gradient to be g = 2 x v/v The residence time wrt interhemispheric exchange is τ = 1 yr The Mass of the atmosphere M = 1.6 x mol The interhemispheric flux is then = 1.9 PgC

47 . (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= (4) Total ocean sink by model and observation: NO+SO = 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 ) natural sinks were nearly double this. Today they are in the range 3 – 4 Pg C yr -1 Box model calculations for the period

48 Possible causes of the NH mid-latitiude sink Land use Change Anthropogenic fertilization, chiefly nitrogen deposition CO 2 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): JUL

51 Sources of anthropogenic nitrogen Agricultural fertilizer Animal husbandry: –Runoff from farms –Ammonia emissions NO y emissions from transport, other fossil fuels

52 Current deposition of atmospheric NO y (mmol N m -2 yr -1 )

53 Cross-section of trunk of Picea abies from the fertilised and irrigated (IL) treatment at the Flakaliden study site -- Boreal forest, Northern Sweden. Effect of fertilization on tree growth

54 Effect of beta-factor C / C 0 P/P 0 CO 2 Fertilization effect. CO 2 is a limiting factor on growth of plants. Higher CO 2 may therefore stimulate net growth. CO 2 fertilization is usually quantified by the "beta factor"; where is usually in the range P,P 0 are the carbon assimilation rates at CO 2 concentrations C,C

55 Free-air CO 2 Enrichment (FACE) experiments Designed to enrich the CO 2 in air over a circle of vegetation, with minimal other disturbance. A ring of towers able to release CO 2, sensors to detect wind speed and direction and measure CO2 concentration. Continuous rapid monitoring of the CO 2 concentrations. Control system to decide which towers to release from and adjust release rates to keep concentration constant.

56 Uncertainties about CO 2 Fertilization Easily measurable in many plants in greenhouse situations, but it is difficult to extrapolate this to the natural world. Questions include: How big is the effect in natural ecosystems? How is it modified by other limiting nutrient availabilities? Does it result in continuous storage of carbon in plants and soils, or is a new equilibrium state rapidly reached?

57 Sink saturation? Assume that the sink is mostly due to CO 2 fertilization. Rising CO 2 has an immediate effect on photosynthesis –Leading to net ecosystem uptake of CO 2. Rising CO 2 has a delayed effect on global temperatures. Rising temperatures will enhance respiration in the future –Leading to net ecosystem release of CO 2 Therefore presently observed uptake of CO 2 may be a transitory phenomenon only, and the sink will saturate. The sink may be even more transitory if it is due in whole or in part to land use change, or nitrogen fertilization.

58 Courtesy John Grace, U. Edinburgh

59 Sink saturation? FACE experiments suggest uptake of CO 2 due to CO 2 fertilization is itself transitory. But: soil warming experiments suggest that the temperature effect on soil respiration may also be transient.

60 Carbon cycle:change of carbon in vegetation and soils according to the Hadley Centre coupled carbon- climate model.

61 Precision atmospheric oxygen measurements Since 1990, direct measurements of oxygen concentrations, at ppm accuracy, have been made at certain sites throughout the world, by R. F. Keeling and others. Locations of precision O 2 measurements

62 ….Precision atmospheric oxygen measurements The concentrations are affected by: fossil fuel burning net land vegetation net uptake Seasonal uptake/release of oxygen from both the land and the ocean biota -- unlike the case of CO 2 which is little affected by ocean seasonal cycle, because of the long air/sea exchange time for CO 2.

63 Oxygen and CO 2 Comparison O 2 decrease year- on year of the same order as the CO 2 increase. Seasonal cycles of O 2 in antiphase with those of CO 2. The Southern Hemisphere O 2 seasonal signal is much larger than is the case for CO 2.

64 Deductions from Oxygen 1 ) Net land and ocean sinks of carbon: The molar ratio of oxygen utilisation relative to carbon dioxide release during the following three processes are all known; a) fossil fuel burning, R ff = (DO 2 /DCO 2 )~-1.3 b) photosynthesis/ respiration, R pr =(DO 2 /DCO 2 )~-1.1 c) ocean uptake of CO 2 (DO 2 /DCO 2 )=0 They can be plotted on a vector diagram of mean annual O 2 change versus CO 2 change. From a knowledge of how much fossil fuel has been burned, the size of the net ocean and land sinks can be determined.

65 Complications with the O 2 signal.. Ocean release: The oceans are not in general neutral w.r.t atmospheric oxygen. –As the ocean warms, O2 becomes less soluble and some dissolved oxygen outgasses from ocean to atmosphere. –Seasonally mixed ocean regions such as the N. Atlantic tend to be oversaturated with O2 in summer and undersaturated in winter, when deep mixing brings up old water. If mixing patterns change this can mean a net source or sink. –From these causes, there is evidence that the oceans have outgassed O2 in recent decades. Uncertainty in C:O ratios: –Different fossil fuels (e.g. coal, gas) have markedly different ratios of carbon released to oxygen used. –Different vegetation types may have different C:O ratios: ratios may vary between soil and vegetation, and may change seasonally even for the same vegetation type.

66

67 Conclusions The carbon cycle was closely in steady state prior to the industrial revolution. It is now substantially perturbed. Natural CO 2 sinks both on land and in the ocean are very important for slowing the rate of global warming due to greenhouse gases. These sinks change substantially from year to year and decade to decade. Changes are synchronous with climate oscillations such as El Niño/ La Niña cycles. The cause of the land sink for anthropogenic CO 2 is poorly understood. It may turn into a source because of global warming, in the next 100 years. The ocean CO 2 sink is affected both by circulation and biology. Changes in either would affect how much CO 2 is taken up by the ocean. Climate change may cause both.

68 Conclusions We know 3 or 4 possible reasons for the global vegetation sink, but presently we cannot be sure which of these are most important. We cannot be sure how long the sink will continue, and whether it will increase or decrease. Many lines of evidence point to a decrease.

69

70 Questions Should land sequestration of carbon be considered as a serious option for climate change mitigation, given –our poor understanding of current land sinks – their possibly transitory nature –their vulnerability to climate change The precautionary principle: if near-catastrophic outcomes of present practices cannot be ruled out, should we be putting maximum effort into emissions reductions?

71 Distribution of CO 2 in the atmosphere The space and time- averaged Northern hemisphere concentration is about 2ppm higher than that in the Southern hemisphere. This is because nearly all fossil fuel emissions occur in the North. Combined with atmospheric transport model, we can estimate the latidudinal distribution of natural sinks. Two views of global CO 2 versus time and latitude between 1986 and 1993

72 Box inverse model NH SH + equatorial region Accumulation 0.45AA Accumulation 055AA Fossil fuel (90%) 0.9FF Fossil fuel (10%) 0.1FF ---Unknown sinks ---- NO NL SO EL Interhemispheric flux IF 10ºN

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

74

75 Atmospheric CO 2 variations since 1000 AD

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

77 The Mauna Loa atmospheric record. Overall increase in atmospheric CO 2 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.

78 Riverine flux The pre-industrial steady state cycle: to balance the flux of carbon coming down rivers, there must have been a CO 2 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.

79 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 CO 2 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.

80 Tans et al Fig 5: Observed mean annual CO 2 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 CO 2 concentration.

81 ….Precision atmospheric oxygen measurements The concentrations are affected by: fossil fuel burning net land vegetation net uptake Seasonal uptake/release of oxygen from both the land and the ocean biota -- unlike the case of CO 2 which is little affected by ocean seasonal cycle, because of the long air/sea exchange time for CO 2.

82 Deductions from Oxygen 1) Net land and ocean sinks of carbon: The molar ratio of oxygen utilisation relative to carbon dioxide release during the following three processes are all known; a) fossil fuel burning, R ff = (DO 2 /DCO 2 )~-1.3 b) photosynthesis/ respiration, R pr =(DO 2 /DCO 2 )~-1.1 c) ocean uptake of CO 2 (DO 2 /DCO 2 )=0 They can be plotted on a vector diagram of mean annual O 2 change versus CO 2 change. From a knowledge of how much fossil fuel has been burned, the size of the net ocean and land sinks can be determined.

83 Deductions from O 2 continued…. 2) Gas exchange rate and primary productivity of the oceans To examine the contribution of the oceans to the oxygen signal, –1) The year-to-year decrease is removed leaving only a seasonal signal. –2) The CO 2 seasonal signal is multiplied by the ratio R pr of O 2 /CO 2 changes due to land vegetation, and subtracted from the O 2 signal; this leaves only the signal due to seasonal release/uptake by the oceans. –3) The magnitude of this signal at different stations is a function of the amount of oxygen released by the marine biota, and the rate at which it escapes into the atmosphere. Using additional information on the sea-surface concentrations of oxygen, estimates of both of these can be made.

84 Precision atmospheric oxygen measurements Since 1990, direct measurements of oxygen concentrations, at ppm accuracy, have been made at certain sites throughout the world, by R. F. Keeling and others. Locations of precision O 2 measurements

85 Oxygen and CO 2 Comparison O 2 decrease year-on year of the same order as the CO 2 increase. Seasonal cycles of O 2 in antiphase with those of CO 2. The Southern Hemisphere O 2 seasonal signal is much larger than is the case for CO 2.


Download ppt "The Contemporary Carbon Cycle Overview Marine Carbon cycle Marine sink for atmospheric CO2 The atmospheric imprint Atmospheric inversions The Terrestrial."

Similar presentations


Ads by Google