Interactions between ocean biogeochemistry and climate

1. Interactions between ocean biogeochemistry and climate Guest presentation for AT 762 Taka Ito • How does marine biogeochemistry interact with climate? • What can we learn from past climate changes? • What are the implications for future climate change?

2. Outline • Introductions • Coupling mechanisms • Two-way interactions • Insights from past climate changes • Climate of the past 60 million years • Glacial-interglacial climate change • Present and future climate • The future of oceanic carbon uptake • The Southern Ocean

3. How does marine biology impacts on climate? Greenhouse effect Clouds Air-sea CO2 exchange SST DMS emission Ocean carbon cycle SW absorption Photosynthesis by phytoplankton Modified from Sarmiento and Gruber (2006)

4. How does climate impact on marine biology? Cold, nutrient-rich coastal water High biological production Ocean circulation, nutrient transport Eddy, frontal scale ~ 100km Low biological production Warm open ocean Coastal upwelling

5. Ocean currents control marine biology Upwelling Downwelling Basin, planetary scale ~ 10,000 km Large-scale atmospheric wind patterns Upwelling Downwelling Upwelling

6. Marine Biology impacts on CO2 fluxes • 90+% of the global carbon is stored in the deep ocean • Each year, ~ 2 PgC of fossil fuel CO2 is taken up by ocean • Each year, ~ 11 PgC is sequestered into the deep ocean by marine organism Air-sea CO2 flux Biological CO2 uptake N. Gruber

7. Ocean carbon pumps • Vertical gradient of DIC in the ocean (Volk and Hoffert, 1985) • Simple 1D model (single column view) • Vertical temperature gradient (solubility pump) • Sinking organic material (biological pump) Current oceanic inventory of biologically sequestered DIC ~ 1700 PgC CaCO3 burial is the ultimate sink of CO2 from weathering, volcanism and human emission (10K+ years) N. Gruber

8. Evolution of the Earth’s climate for the last 60 million years Zachos et al. (2001) Science

9. Long-term trend and abrupt transitions • Long-term CO2 sequestration • Gradual cooling of the climate • Silicate weathering: inbalance between volcanic CO2 emission and CaCO3 burial • Abrupt transitions • Ocean bathymetry (gateway hypothesis) • Climate threshold and feedbacks ex. ice sheet and ice-albedo feedback • Ocean circulation and biogeochemistry are involved in both hypothesis

10. Evolution of the Earth’s climate for the last 400,000 years • Orbital pacing of climate Vostok ice core About 50% of glacial cooling is due to the change in CO2 and the rest comes from planetary albedo

11. Known perturbations in the global carbon cycle Sigman and Boyle (2000) Nature The net effect is O(10 ppm) and is not enough to account for the 100 ppmv change

12. Hypotheses for the glacial CO2 problem

13. Southern Ocean productivity Surface phosphate climatology (WOA01) • Idealized box model study (Toggweiler and Sarmiento, 1984) • Atmospheric pCO2 is proportional to the high lat surface nutrient (PH) • 50% reduction of surface P can reproduce glacial pCO2 • Large amount of unutilized surface nutrient (P) • - Why phosphorus is not used up in the Southern Ocean?

14. Iron limitation in the Southern Ocean • Iron is necessary nutrient for marine phytoplankton • Major source is atmospheric dust deposition • Southern Ocean, far from continental dust sources, is depleted in Fe • SOIREE (Boyd et al. 2001) • Purposeful iron addition experiment • Increased marine productivity after iron addition • It is not yet clear its long-term impact on the oceanic carbon uptake

15. Antarctic ice core data • Increased dust deposition during cold and arid ice age climates (Vostok ice core data) • Iron hypothesis (Martin, 1990)

16. Climate, iron and N2 fixation • Nitrogen cycle (NO3, NO2, NH4, Organic nitrogen) • With NO3:PO4 ratio is slightly less than 16, ocean is slightly depleted in N • Some organisms can utilize N from dissolved N2 gas (N2 fixers) • Increased Fe input to the ocean can stimulate N2 fixation (Falkowski 1997)

17. Outstanding issues • Ocean GCM coupled to marine ecosystem model (with explicit Fe cycle) still cannot reproduce glacial pCO2 • - Small response in atmospheric CO2 • Regional compensation • Other mechanisms? • Ocean circulation / seaice changes • Phytoplankton community structure changes and its impact on CaCO3 burial • Total nutrient inventory (N2 fixation) and its response to the increased iron deposition

18. What controls the air-sea CO2 flux in the present climate? • Takahashi et al. (2002)

19. What controls the air-sea CO2 flux? Biological flux Thermal flux • Patterns of uptake and outgassing for natural CO2 • CO2 uptake: cooling and net biological carbon sink • CO2 outgassing: heating and upwelling of regenerated DIC

20. Simulated anthropogenic CO2 fluxes • Difference between two simulations for preindustrial and contemporary conditions Forward model: OCMIP-2 Orr et al. (2002) Inverse model: Mikaloff-Fletcher et al. (2006) • Highlights the importance of the Southern Ocean

21. Extratropical SH is a region of net CO2 uptake • Is the SH carbon flux changing? How? Mean CO2 fluxes Natural Anthro into ocean out of ocean (mol m-2 yr-1) Lovenduski et al. (2007)

22. Extratropical atmospheric variability NAM SAM From NWS CPC • Month-to-month variability of atmospheric pressure • Contraction and expansion of polar vortex • Shift of westerly wind belt

23. SAM impacts on SH circulation • During positive phase of SAM, stronger wind over ACC increases zonal transport and upwelling in the Antarctic region Hall and Visbeck (2002) Rintoul et al. (2000)

24. Observed biological response Regression of SeaWIFS chlorophyll anomaly onto SAM index (1997-2004) • Chlorophyll responds to SAM (Lovenduski and Gruber 2005) • Antarctic region • Chl increases with SAM index • Subantarctic region • Chl decreases with SAM index • Mechanisms? • Impact on carbon fluxes?

25. Variability of SH carbon fluxes • Positive-phase of SAM leads to anomalous outgassing • Atmospheric CO2 budget: Butler et al. (2007) • GCM simulation: Lovenduski et al. (2007) • Driven by increased wind-driven upwelling of deep waters enriched in DIC Regression of CO2 flux onto SAM index

26. Multi-decadal trends Observed temperature trend : IPCC (2007) chap 3 • Linear trend (1979-2005) based on satellite observation • Relatively small temperature change in the SH - Large heat capacity of the Southern Ocean

27. Observed and modeled SAM trends Observation IPCC models G. Marshall (2003) R. Miller (2006) • Positive trend in SAM • All of the IPCC models predict positive trend • Driven by ozone depletion and global warming • Stronger westerly wind over ACC

28. Carbon flux trends driven by SAM • Positive trend in SAM leads to increased upwelling of deep waters enriched in DIC • Outgassing of natural CO2 • Atmospheric inversion (1981-2004) (Le Quéré et al. 2007) • Carbon uptake in the Southern Ocean may decline over time…

29. The future: the effect of Southern Ocean stratification Precipitation change (2095-2005) • Increased precipitation under global warming • Potential melting of Antarctic ice sheet • SST warming Can stratification counteract wind stress changes? NCAR CCSM SRES A1B scenario

30. Oceanic variability • Explicitly resolved eddies impact on MOC structure and its sensitivity to the surface winds Hallberg and Gnanadesikan (2006)

31. Toward realistic Southern Ocean carbon cycle simulation • Southern Ocean State Estimate (Mazloff, Heimbach and Wunsc) • OCMIP / ecosystem model (Dutkiewicz et al. 2005)

32. Challenges for future modeling • Model evaluation and improvements • Process-level improvements • Testing models against observational metric • Statistical analysis • Determine modes of carbon flux variability • Attribution experiments • Hierarchical modeling • Repeat calculations taking out one process at a time • Simple models help to interpret complex simulations