Ocean fertilization: what we have learned from models

Transcription

1 Ocean fertilization: what we have learned from models Jorge L. Sarmiento Princeton University I. A tutorial on the biological pump and its role in controlling the air-sea balance of CO 2 II. A tutorial on the role of iron in controlling the biological pump III. Model studies of carbon mitigation by iron fertilization Sarmiento et al. (2010, Biogeosciences, 7: ) Marine Ecology Progress Series (2008, Theme section: Implications of large scale iron fertilization of the oceans)

2 I. The biological pump

3 The biological pump removes nutrients and CO 2 from the surface and adds them to the deep ocean Stochiometry of organic matter is typically C:N:P = 106:16:1 Photosynthesis (upper ~100 m) converts dissolved nutrients & CO 2 into organic matter. Remineralization converts organic matter back into dissolved nutrients and CO 2 Sarmiento & Gruber (2006)

4 The biological pump efficiently depletes surface nutrients almost everywhere Sarmiento & Gruber (2006)

5 Consequence: dissolved inorganic carbon (DIC) is higher at depth than at the surface. The biological pump accounts for ~2/3 rds of the increase. The remainder is due to the solubility pump (CO 2 is more soluble in cold deep waters than in warm surface waters)

6 Influence of biological pump on atmospheric CO 2 Models show that If the biological pump were shut off, CO 2 would escape the deep ocean and atmospheric CO 2 would rise by ~200 ppm. If the biological pump efficiency were increased, surface nutrients would be removed to the deep ocean along with CO 2, thus CO 2 in the deep ocean would increase, and atmospheric CO 2 would drop by ~100 ppm

7 II. The role of iron in controlling the biological pump

8 The biological pump efficiently depletes surface nutrients everywhere except in three major regions where the iron supply is insufficient Note: iron is used in electron transport proteins involved in photosynthesis & respiration and in the enzymes nitrate & nitrite reductase and nitrogenase (required for N 2 fixation) Sarmiento & Gruber (2006)

9 Evidence for iron limitation from short term (~1 month) small scale in situ iron fertilization experiments Experiment Reference Nitrate Drawdown (mmol m 3 ) North Pacific SEEDS, July 2001 Tsuda et al. [2003] >15 SERIES, July 2002 Boyd et al. [2004] >5 Equatorial Pacific IRONEX I, Oct., 1993 Martin et al. [1994] None IRONEX II, May, 1995 Coale et al. [1996] ~5 Southern Ocean SOIREE, Feb., 1999 Boyd et al. [2000] ~3 EisenEx, Nov., 2000 Gervais et al. [2002] <2 SOFeX, Jan-Feb., 2002 Coale et al. [2004] ~2

10 Chlorophyll response to iron fertilization at the SERIES site

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12 Query: Can we use the response of the ocean to iron fertilization for carbon mitigation?

13 III. Carbon mitigation by iron fertilization

14 III. A. Putting iron fertilization in context: Reference scenario = 130 ppm/century

15 III. B. Large scale iron fertilization

16 Effect of large scale iron fertilization on CO 2 CO 2 response to relief of iron stress for 100 years

17 Effect of large scale iron fertilization on ocean biogeochemistry Southern Ocean iron fertilization depletes surface nutrients, thereby blocking the main return pathway for nutrients lost to the deep sea by the biological pump. Models predict the effects on biological production outside the Southern Ocean would be catastrophic. Tropical iron fertilization leads to enhanced N 2 O production (a powerful greenhouse gas) and to loss of nitrate by denitrification in the low oxygen zones, which may reduce overall biological productivity

18 Large scale vertical overturning circulation in the Southern Ocean High surface nutrients The main return pathway for nutrients lost to the deep ocean by the biological pump is upwelling in the Southern Ocean and northward transport in Subantarctic Mode Water and Antarctic Intermediate Water (Sarmiento et al., 2004)

19 Zonally integrated biological productivity in the world ocean (Pg C/degree/yr) Nutrient depletion south of 30 S normal Southern Ocean surface nutrient depletion blocks the Southern Ocean nutrient resupply pathway & reduces low latitude biological productivity by ~75% (Marinov et al.., 2006)

20 The Southern Ocean plays a dominant role in deep water formation and, thus, preformed nutrient concentration The deep ocean meridional overturning circulation overlain on the radiocarbon distribution. Sarmiento & Gruber (2006)

21 The biogeochemical divide: CO 2 effects confined primarily to blue outcrop (AABW), productivity effects to red (AAIW & SAMW) SAMW AAIW CDW AABW NADW Marinov et al. (2006)

22 Influence of nutrient depletion in different regions of the Southern Ocean Reduction in atmospheric CO 2 Red outcrop Blue outcrop Marinov et al. (2006) Production north of 35 S (Pg C yr 1 )

23 Conclusions - 1 Large scale iron fertilization is potentially disastrous. So why has interest in iron fertilization as a mitigation option continue to draw attention? Some reasons: The role of iron in ocean biogeochemistry continues to be one of the most important scientific problems in our field today. For commercial interests, the opportunity to make money is potentially quite large. Carbon markets already exist and iron fertilization is cheap (?). The more recent emphasis has been on relatively small scale patch fertilization.

24 III. C. Patch scale iron fertilization

25 Patch iron fertilization scenarios in GFDL/Princeton model (based on Dutkiewicz et al., 2006) Flux of 0.02 mmol m -2 y -1 bio-available iron (also 5x, 10x, 100x, and 1000x multiples of this) Four patch locations ~Patch size (10 3 km 2 ) PAPA (N. Pac) 50 N, 145 W 72 EqPac 3.5 S, 104 W 106 S. Ocean 60 S, 170 W 116 Ross Sea 76 S, 176 E 104 Four fertilization scenarios: 1x 1 mo, 1 time 10x 1 mo/yr, 10 yrs 100x 1 mo/yr, 100 yrs 1200x Continuous for 100 years Total of 4 x 4 = 16 scenarios Includes atmosphere with pco 2 set initially at 278 ppm

26 CO 2 uptake from atmosphere after 10 years of continuous fertilization (gc m -2 ) Princeton/GFDL model

27 Local dissolved iron response PAPA S. Oc. Eq Pac Ross

28 Local nitrate response PAPA S. Oc. Eq Pac Ross

29 Local DIC response PAPA S. Oc. Eq Pac Ross

30 Effect of patch fertilization on CO 2 for fertilization in the eastern equatorial Pacific Year 1 Years 2-9 Gnanadesikan et al. (2003), macronutrient fertilization, flux to bottom.

31 Cumulative CO 2 uptake (Mtons of C) YEAR 10 PAPA Eq Pac S. Ocean Ross Sea 1x x x YEAR 100 PAPA Eq Pac S. Ocean Ross Sea 1x x x x Note: these are all simulations with a fixed atmosphere, i.e., they do not include the return flux to the atmosphere

32 Estimated cost of iron fertilization 1x ship time costs ~$2M, so from our simulations, the cost for Ross Sea 1x, 10x, and 100x is <$6.00/ton of C (~$2/ton of CO 2 ) Boyd (2008)

33 Effect of patch fertilization on nitrate over 100 yrs (Figure shows global horizontal mean of nitrate perturbation. Note: diazotroph C:N:P = 366:50:1) PAPA S. Oc. Eq Pac Ross

34 Nitrate budget in 100 year continuous fertilization simulations Denitrification causes 4 Tmol NO 3 loss in Eq Pac!

35 Conclusions-2 Quantification and verification of CO 2 uptake from the atmosphere for patch fertilization Direct verification is not possible because the relevant processes are global in scale and too small to measure Indirect verification by models requires understanding both the physical and biological efficiency and there are many uncertainties (cf. Gnanadesikan et al., 2003; & Sarmiento et al., 2010) Consequences (studied in models) Increased N 2 O production and degassing, which can counteract some or all of the reduction in radiative forcing by fertilization (Jin & Gruber, 2003) Decreased oxygen, which leads to increased hypoxia and net loss of nitrate by denitrification (notably when fertilization occurs in the eastern Equatorial Pacific) Loss of macronutrients from the upper ocean reduce biological productivity in other regions when fertilization ends (not shown) Fundamental alteration of nutrient ratios and the limiting nutrients: factors that structure surface ecosystems

36 Overall conclusions The only way to get reference scenario scale removal is by very large scale fertilization. The impacts of this on global ocean ecosystems would be devastating. Patch fertilization would not contribute significantly to carbon mitigation and there are other reasons not to view it favorably including the difficulty of verification and the likelihood of significant negative impacts. The focus should be on the science of the iron cycle in the ocean and the role of iron in it. Other methods of carbon mitigation have much greater potential.

37 Atmospheric reservoir effect

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39 I. The biological pump

40 To put the ocean carbon pump in context, atmospheric CO 2 has increased by 111 ppm, from 280 ppm to 391 ppm since the industrial revolution began (Ice core data from Barnola, 1999; Mauna Loa from SIO, D. Keeling & T. Whorf, 2000)

41 II. The role of iron in controlling the biological pump

42 What happens to the air-sea CO 2 balance if you eliminate iron limitation? (A model study) The highest response is in the Southern Ocean with lower response in the tropics (though higher than in previous models), and lowest response in the North Pacific.

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44 C:Fe ratio etimates de Baar et al.(2008)

45 C:Fe ratio etimates de Baar et al.(2008)

46 C:Fe ratio etimates de Baar et al.(2008)

47 C:Fe ratio etimates de Baar et al.(2008)

48 III. Carbon mitigation by iron fertilization

49 III. A. The wedges concept

50 What do we mean by carbon mitigation? The wedges concept: 14 Billion of Tons of Carbon Emitted per Year 14 GtC/y Seven wedges 7 Historical emissions Flat path O 7 GtC/y Pacala & Socolow (2004)

51 What is a Wedge? A wedge is a strategy to reduce carbon emissions that grows in 50 years from zero to 1.0 GtC/yr. The strategy has already been commercialized at scale somewhere. Total = 25 Gigatons carbon 1 GtC/yr 50 years Cumulatively, a wedge redirects the flow of 25 Pg C (11.8 ppm) in its first 50 years. A solution to the CO 2 problem should provide at least one wedge

52 Fill the Stabilization Triangle with Seven Wedges Energy Efficiency Methane Management 14 GtC/y Decarbonized Electricity Forests & Soils Stabilization Triangle GtC/y Decarbonized Fuels Fuel Displacement by Low-Carbon Electricity Pacala & Socolow (2004)

53 Updating the wedge carbon budget: fossil fuel emissions have surged since 2002 Carbon emissions from fossil fuels and cement 8 Pg C/yr Growth rate Period (%/yr) (20 yrs) (20 yrs) (5 yrs) 4.0 ( ) Year (G. Marland, 2007 & pers. comm.; 2005 and 2006 estimates based on BP energy statistics; cf. Canadell et al., 2007)

54 Stabilization wedges update 2008 Pacala & Socolow (2004) Pacala & Socolow (2004) updated Worst case scenario (no CO 2 fertilization or other land sink) Emissions (Pg C/yr) 7 Avoided 180 Pg C (7 wedges) Allowable 350 Pg C Avoided 250 Pg C (10 wedges) Allowable 350 Pg C Avoided 375 Pg C (15 wedges) Allowable 225 Pg C

55 III. B. Large scale iron fertilization

56 A science detour: Air-sea balance of CO 2 is determined by the transformation of dissolved inorganic carbon (and nutrients) from the preformed to the remineralized pool High S Oc nutrients Low S Oc nutrients The figure shows: potential atmospheric pco 2 (in models with rapid gas exchange) versus the remineralized DIC pool (Pg C) for a wide range of model simulations Marinov et al. (in preparation)

57 Effect of large scale iron fertilization for 100 years Denman (2008)

58 III. C. Patch scale iron fertilization

59 Cumulative CO 2 uptake (Mtons of C) YEAR 10 PAPA Eq Pac S. Ocean Ross Sea 1x x x YEAR 100 PAPA Eq Pac S. Ocean Ross Sea 1x x x x x ship time costs ~$2M, so cost for Ross Sea 1x, 10x, and 100x is <$6.00/ton of C (~$2/ton of CO 2 ) Note: these are all simulations with a fixed atmosphere, i.e., they do not include the return flux to the atmosphere

60 Nitrate budget in 100 year continuous fertilization simulations Denitrification causes 4 Tmol NO 3 loss in Eq Pac!

61 Iron addition/aeolian flux ratio PAPA EqPac S.Oc. Ross 1 MIT LANL PRINCETON STANFORD

62 Local nitrate response over first 10 years of continuous fertilization PAPA S. Oc. Eq Pac Ross

63 Most efficient fertilization options out of 4 locations x 3 scenarios = 12 (Year 10) Rank Location Fertilization scenario CO 2 uptake/fe added Frequency Duration (mol C:mmol Fe) 1 Ross 1 mo/yr 10 yrs " 1 mo Once Eq Pac. 1 mo/yr 10 yrs S. Oc. 1 mo/yr 10 yrs " Continuous 10 yrs Note: all 10 & 100 year duration scenarios are identical at year 10, so there are only 3 rather than 5 different scenarios.

64 Most efficient fertilization options out of 4 locations x 5 scenarios = 20 (Year 50) Rank Location Fertilization scenario CO 2 uptake/fe added Frequency Duration (mol C:mmol Fe) 1 Ross 1 mo/yr 100 yrs " 1 mo/yr 10 yrs 82 3 " 1 mo Once 80 4 S. Oc. 1 mo/yr 100 yrs 63 5 " Continuous 100 yrs 59 38

65 Most efficient fertilization options out of 4 locations x 5 scenarios = 20 (Year 100) Rank Location Fertilization scenario CO 2 uptake/fe added Frequency Duration (mol C:mmol Fe) 1 Ross 1 mo/yr 100 yrs 85 2 " 1 mo/yr 10 yrs 60 3 " 1 mo Once 59 4 S. Oc. 1 mo/yr 100 yrs 42 5 " Continuous 100 yrs 40 29

66 Conclusions Queries Ross Sea has the highest efficiency but only for 1 mo/yr fertilization S. Oc. is next and both 1 mo/yr and continuous fertilization are good Eq Pac is good after 10 yrs of 1 mo/yr, but drops out of contention thereafter QUERY: Why is Ross Sea good only for 1 mo/yr where S. Oc. is also good for continuous fertilization? QUERY: Why is Eq Pac good only for the short term? QUERY: Why is PAPA so poor?

67 Seasonal behavior of nitrate response to fertilization S. Oc. Both the Southern Ocean and Ross Sea fertilization sites have no nutrient drawdown in the wintertime. However, the winter is much longer in the Ross Sea. Ross

68 Surface nitrate concentrations show that Papa and Eq Pac are near nitrate limitation during bloom time Annual mean nitrate in top 10 m of fertilization region after 10 years of continuous fertilization. Iron flux multiple Control Fertilization Papa Eqpac South Ross Nitrate at the time of the nutrient min. Iron flux multiple Control Fertilization Papa Eqpac South Ross Nitrate halfsaturation constant = 0.5 mmol m -3 Nitrate at the time of the nutrient max. Iron flux multiple Control Fertilization Papa Eqpac South Ross

69 Queries Answers QUERY: Why is Ross Sea good only for 1 mo/yr where S. Oc. is also good for continuous fertilization? ANSWER: Ross Sea has no light in the winter and so no response to iron addition. S. Oc. has light for longer in the year. QUERY: Why is Eq Pac good only for the short term? ANSWER: Eq Pac nutrients are lower, so there is less to work with QUERY: Why is PAPA so poor? ANSWER: Same as for Eq Pac.

70 So why do Dutkiewicz et al. (2006) see highest CO 2 uptake in tropical Pacific, and low uptake in Southern ocean? Dutkiewicz et al. (2006) atmospheric CO 2 uptake after (a) 10 years and (b) 100 years of iron input in MIT adjoint model (ton C/ton Fe) Additional iron input = 0.02 mmol m -2 yr -1 Cost function:

71 Response to iron addition Response to light increase Reason: The Dutkiewicz et al. model is light rather than iron limited in the high latitudes. Figures show response of net community production to 10 year (a) iron and (b) light increase in MIT adjoint model (gc m -2 ) Increased iron input = 0.02 mmol m -2 yr -1, light increase = 30 W m 2 Cost function:

72 KEY UNKNOWNS IN MODELS

73 Analysis of physical and biological efficiency The cumulative CO 2 uptake depends on the cumulative efficiency of the physical response (atmospheric drawdown efficiency) the cumulative C:Fe of the biological response CO ΔΦ 2 air sea C:Fe = e physical R iron utilization ΔFe added, where CO e physical =ΔΦ 2 air sea Org C & CaCO ΔΦ 3 export C:Fe R iron utilization Org C & CaCO =ΔΦ 3 export ΔFe added

74 Physical efficiency depends on depth of CO 2 removal Atmospheric uptake efficiency Low efficiencies result from: large area fertilization high light sensitivity studies Fraction of additional POC export across 100 m that occurs at base of top model layer (10 m) Jin & Gruber (in prep.)

75 The biological efficiency depends on C:Fe ratio of organic matter and on the retention of iron Fe e export stimulation = ΔΦ Org Fe export Fe ΔΦ retention Fe ΔΦ retention Fe ΔΦ fertilization

76 Contribution of iron retention: ratio of model with no iron regeneration to model with normal iron cycle The atmospheric uptake efficiency is insensitive to iron retention The biological export response to a given iron addition plummets to as little as 6% of the model with iron retention

77 Key unknowns in models As regards the physical efficiency (air-sea CO 2 uptake divided by CO 2 export) Depth of enhanced DIC removal by phytoplankton As regards the biological efficiency (CO 2 export divided by Fe addition) Long term fate of added Fe!! Magnitude and C:Fe ratio of enhanced uptake, export, and remineralization (not shown) Depth of remineralization (not shown)

78 Equatorial fertilization is a particularly bad idea Jin & Gruber (2003) have shown that N 2 O production and release to the atmosphere can almost cancel the greenhouse gas reduction Global biological productivity begins to drop after a while because nitrate is depleted by denitrification in the expanded oxygen minimum zone

79 Atmospheric Reservoir Effect

80 Deep ocean sequestration efficiencies Mignone et al. (2004)