Assessing the efficiency of iron fertilization on atmospheric CO2 using an intermediate complexity ecosystem model of the global ocean

Transcription

1 The Ocean in a High CO2 World Assessing the efficiency of iron fertilization on atmospheric CO2 using an intermediate complexity ecosystem model of the global ocean Olivier Aumont 1 and Laurent Bopp 2 1 IPSL / LODyC, Paris, France 2 IPSL / LSCE, Gif s/ Yvette, France

2 Introduction : The HNLC regions The Iron hypothesis Fe

3 The Iron fertilization experiments A: IronexI E H G B: IronexII C: SOIREE D: EisenEx B A E: SEEDS F: SOFEX C F D G: Planktos H: SERIES Main results Chlorophyll From a 3 to a 40-fold increase, generally as diatoms pco 2 A 30 to 90 atm drawdown in surface pco 2 Export Production DMS Contrasting results, generally an increase An increase

4 Mitigation of atmospheric CO2 Large scale iron fertilization Iron fertilization can be used as a means of offsetting the anthropogenic carbone dioxide emission (Martin et al., 1991) Previous estimates (modeling studies) Peng and Broecker, 1991 Joos et al., 1991 Box models Southern Ocean Preindustrial: -17 to -59 atm Anthropogenic : -64 to -107 atm Sarmiento and Orr, 1991 OGCM Global Ocean Preindustrial: -3 to -72 atm Nutrient restoring Six and Maier-Reimer, 1993 OGCM Southern Ocean Preindustrial: -34 atm HAMOCC Anthropogenic : -50 atm Archer et al., 2000 OGCM Global Ocean Preindustrial: -50 atm HAMOCC Ganadesikan et al., 2003 OGCM Nutrient restoring Patchy, equatorial Pacific Low efficiency, < 10% of increase in export production as atmospheric CO2

5 Questions Iron fertilization Can the model simulate the main features of the iron fertilization experiments? What is the spatial and temporal variability of the response to fertilization? What is the long-term efficiency of the fertilization? Outline 1. Model description 2. Patchy iron fertilization 3. long-term iron fertilization on the global scale

6 Tools : Models OPA PISCES NH 4 + NO 3 - PO 4 3- Si Iron Diatoms Nano-phyto D.O.M MicroZoo Meso Zoo P.O.M Small Big Euphotic Layer (10-200m)

7 Chlorophyll surface concentrations June January Seawifs (98-03) PISCES

8 Iron distribution 5 3 Annual mean, surface Annual mean, 1000m

9 What limits diatoms growth? NO 3 +NH 4 PO 4 Fe Si

10 Iron Fertilization Patchy Iron Fertilization in the three main HNLC regions Experimental design - Iron concentration set to 2 nm in the mixed layer at day 2 and 5 - Fertilization applied over only one grid box - The model is integrated for 31 days

11 Iron Fertilization : The Southern Ocean (1) Chla Blooming conditions (Chl > 1.5 mg Chl m -3 ) 2. Small response ( Chl < 0.7 mg Chl m -3 ) 3. Moderate response (0.7 < Chl< 2.5 mg Chl m -3 ) 4. Strong response (2.5 mg Chl m -3 < Chl ) Diatoms relative abundance

12 The Southern Ocean pco2 ( atm) 0 Why such responses? Stratification, ice retreat Si limitation, Si initial < 6 umol L Mixed layer depth > 30 m, macronutrient replete 120 Export ( ) 4. Favorable conditions, strongly iron limited

13 The Southern Ocean : Comparison with data Chla (mg Chl m -3 ) Diatoms relative abundance SOFEX South SOFEX North SOIREE pco2 ( atm)

14 Seasonal evolution January February July November

15 Iron Fertilization : The equatorial Pacific pco2 ( atm) Chla (mg Chl m -3 ) export (%) Diatoms relative abundance IRONEX II

16 Iron Fertilization : everywhere & 50 yr long Changes in Chla (mg Chl m -3 ) +4 Changes in Diatoms Relative Abundance Increase of Export Production -4 (gc/m2/yr) +50 Fe Fertilization Export Production (GtC/yr) Years

17 Impact on atmospheric pco2 Preindustrial conditions Fe Fertilization Fe Fertilization 1 Years Carbon Flux (PgC/yr) 8 Export (PgC/yr) Atmospheric pco2 ( atm) -8 atm in 50 yr >80% due to Southern Ocean atm in 10 yr Atmospheric pco2 ( atm) Carbon Flux (PgC/yr) -1.8 atm in 50 yr

18 Why such a small efficiency? Nutrient Limitation of Diatoms Growth Light limitation Control NO 3 / NH 4 PO 4 Fe Si NO3 ( mol/l) NO3 ( mol/l) Fe Fert

19 Iron Fertilization : Implications for the Sulfur Cycle 8 Export Production Changes in Surface DMS Concentrations Carbon Flux +0.5 nm ppm in 50 yr Atmospheric pco % DMS Flux (TgS/yr)

20 Conclusions Patchy Iron fertilization : The model roughly captures the main features of in situ iron fertilization experiments, except in the North Pacific. In the Southern Ocean, the response depends highly on the location and the time period of the iron release. Main controlling factors are Si concentrations, the mixed layer depth, and the status of the ecosystem. The favorable season extends from November to March. Large-scale Iron fertilization : Very low efficiency : only 8 ppmv drawdown in atmospheric pco2 after 50 years. Iron fertilization should be done continuously to keep the additionally stored CO2 within the ocean. Possible drawbacks : N2O production, extension of the anoxic regions, changes in the fisheries, possible decrease in DMS production,

21 Diatoms relative abundance : vs Data Data from Gregg et al N 1 60 N N 20 N S S S Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

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23 Iron Fertilization : The North Pacific Chla (mg Chl m -3 ) Diatoms relative abundance