Ocean iron fertilization for CO 2 sequestration

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1 Ocean iron fertilization for CO 2 sequestration Jorge L. Sarmiento Princeton University with contributions from Rick Slater, Anand Gnanadesikan, John Dunne, and Irina Marinov; also Nicolas Gruber, Xin Jin, and Francisco Chavez I. A brief tutorial on ocean biogeochemistry and the role of iron II. Previous research on iron fertilization for CO 2 sequestration III. Why has interest revived?

2 The biological pump removes surface nutrients and CO 2 and adds them to the deep ocean Photosynthesis (upper ~100 m) converts dissolved inorganic nutrients & carbon into organic matter. Remineralization converts organic matter back into dissolved inorganic nutrients and carbon Sarmiento & Gruber (2006)

3 Consequence: dissolved inorganic carbon (DIC) is higher at depth than at the surface. Mechanisms: (1) Two-thirds is due to the biological pump (2) Remainder is due to the solubility pump (CO 2 is more soluble in cold deep waters than in warm surface waters) Models suggest that if the biological pump were shut off, atmospheric CO 2 would rise by ~200 ppm. If the biological pump efficiency were increased, atmospheric CO 2 would drop by ~100 ppm

4 The biological pump depletes nutrients everywhere except in three major regions where the iron supply is insufficient QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. 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)

5 Evidence for iron limitation from short term (~1 month) 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

6 Biological production is proportional to aerosol iron input in the Antarctic QuickTime and a decompressor are needed to see this picture. QuickTime and a decompressor are needed to see this picture. Cassar, Bender, et al. (2007)

7 Subantarctic (E11-2) diatom-bound 15 N/ 14 N: Increase through the last ice age diatom-bound δ 15 N ( v. air) age (ka) More complete nitrate consumption in glacial Subantarctic? Appears consistent with history of atmospheric iron inputs May cause ~40 ppm reduction in atmospheric CO δ 18 O N. pachyderma (sin) ( v. PDB) Robinson et al., 2005 (δ 18 O + age: Ninneman and Charles, 1997)

8 II. PREVIOUS RESEARCH

9 Effect of large scale iron fertilization Region on CO 2 CO 2 response to nutrient depletion and iron addition (new) over 100 years CO 2 drawdown (ppm) Nutrient depletion Relief of iron stress + Southern Ocean (90 S to 30 S) (6.2) Tropics (18 S to 18 N) (1.6) North Pacific (30 N to 67 N) (0.5) + In parenthese s is uptake with a fixed atmos p here (not considerin g the retur n flux) Sarmiento et al. (in preparation)

10 Effect of Southern Ocean nutrient depletion on global biological productivity Nutrient depletion south of 30 S normal Marinov et al. (2006)

11 Effect of patch fertilization on CO 2 Year 1 Years 2-9 Gnanadesikan et al. (2003), macronutrient fertilization, flux to bottom.

12 Effect of patch fertilization on Nitrate (Figure shows global horizontal mean of nitrate perturbation. Note: diazotroph C:N:P = 366:50:1) QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. PAPA S. Oc. QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Eq Pac Ross

13 Problems with patch fertilization 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 (Gnanadesikan et al., 2003) Indirect verification by models requires understanding both the physical and biological efficiency and there are many uncertainties (cf. Gnanadesikan et al., 2003) 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 reduces biological productivity in other regions Fundamental alteration of nutrient ratios and the limiting nutrients: factors that structure surface ecosystems

14 III. WHY HAS INTERESTED REVIVED?

15 Patch iron fertilization scenarios in GFDL/Princeton model (based on Dutkiewicz et al., 2006) Four patch locations ~Patch size (10 3 km 2 ) PAPA (N. Pac) 50 N, 145 W 64 EqPac 3.5 S, 104 W 97 S. Ocean 60 S, 170 W 48 Ross Sea 76 S, 176 E 21 Five fertilization scenarios: 1x = 1 mo, 1 time 10x = 1 mo/yr for 10 yrs 100x = 1 mo/yr for 100 yrs 1200x = Continuous for 100 yrs Includes atmosphere with pco 2 set initially at 278 ppm Flux of 0.02 mmol m -2 y -1 bio-available iron

16 Ten year CO 2 uptake from atmosphere (gc m -2 ) Princeton/GFDL model

17 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

18 Buesseler et al. (2008) policy forum on ocean iron fertilization we do not understand the intended and unintended biogeochemical and ecological impacts. Despite these uncertainties in the science, private organizations are making plans to conduct larger-scale iron releases to generate carbon offsets. We are convinced that, as yet, there is no scientific basis for issuing such carbon credits for OIF. Adequate scientific information to enable a decision regarding whether credits should be issued could emerge from reducing uncertainties; this will only come through targeted research programs