Coral records of tropical Pacific climate: Past, present, and future

1. Coral records of tropical Pacific climate: Past, present, and future Kim Cobb Intan Suci Nurhati Laura Zaunbrecher Hussein Sayani Julien Emile-Geay Jud Partin Georgia Inst. of Technology Chris Charles Niko Westphal Scripps Inst. of Oceanography Larry Edwards, Hai Cheng University of Minnesota with thanks to NSF, NOAA, NCL, NGS, ACS-PRF, PARC, Cobb lab undergrads

2. Motivation: How is the tropical Pacific climate system • responding to anthropogenic forcing? • Approach: Use well-dated, high-resolution paleoclimate • records from the tropical Pacific to assess its response to • known natural and anthropogenic climate forcings. • The last millennium • The last forty years • The mid-Holocene

3. Why tropical Pacific climate? “El Niño-Southern Oscillation” (ENSO) ENSO is a climate phenomenon in the tropical Pacific which arises from coupled interactions between the atmosphere and ocean ENSO impacts global climate every 2-7 years tropical Pacific climate variability over decades to centuries to millennia poorly constrained; 20th century trends uncertain El Niño Temperature El Niño Precipitation Dai and Wigley, 2000

4. Climate model response to greenhouse forcing • climate models project widely • divergent scenarios for tropical • Pacific climate under • greenhouse forcing Vecchi et al, 2008

5. Instrumental SST trends • instrumental SST datasets contain • trends of different signs  no help Vecchi et al, 2008

6. # SST observations in central tropical Pacific over last 150 years 18 19 20 Bunge & Clarke, 2009

7. Coral archives of tropical Pacific climate Living Porites corals provide records for the last 200 years; band-counted CORALS from the tropical Pacific record El Niño’s in the geochemistry of their skeletons  monthly resolution Fossil Porites corals extend the record back many centuries; U/Th-dated

8. Coral oxygen isotopes in the central tropical Pacific SST (colors) and rainfall (contours) anomalies during the 1982 El Niño Oxygen isotopes (18O/16O, δ18O) warmer water = lower coral δ18O rain = lower seawater δ18O warmer, wetter conditions during El Niño events cause lower coral δ18O

9. Palmyra coral oxygen isotopes vs. NINO3.4 SST SST Anomaly (°C) δ18O SST Anomaly (°C) δ18O Cobb et al., 2003

10. The Line Islands Coral Collection 1 splice overlapping cores in last millennium 2 modern cores from three islands 3 many cores in mid-Holocene Palmyra 50 cores U/Th dated 18 cores undated Christmas 25 cores U/Th dated 51 cores undated Fanning 17 cores U/Th dated 19 cores undated

11. Cane-Zebiak model and natural forcing NINO34 warms in response to volcanic eruption… Mann et al., 2005 and cools in response to increased solar forcing the ocean’s “dynamical thermostat”?

12. but coupled GCMs don’t do this GISS-ER Plots of SST difference between: Medieval Climate Anomaly (~1000AD) (cool tropical Pacific?) and Little Ice Age (~1700AD) (warm tropical Pacific?) CSM1.4 Mann et al., in press, Science

13. 315 years of ENSO variability • 12,000 δ18O values • variability agrees • well; average δ18O • values differ Cobb et al., 2003 Cobb et al., in prep

14. Line Islands Fossil Coral δ18O Reconstruction raw 2-7y bandpassed Cobb et al., in prep -10 Tropical Volcanic Forcing W/m2 -5 -0.1 to -0.3 0.3 W/m2 0 Solar Forcing 0.1 to 0.3 Crowley et al., 2000

15. Did a tropical Pacific “dynamical thermostat” play a significant role in the last millennium? • Corals say NO • 1258AD mega-eruption • caused no significant • anomaly • solar variability poorly • correlated to coral δ18O • BUT • only one site • corals have largest • errors on dec-cen • timescales NO

16. Multi-proxy reconstruction of tropical Pacific climate • - use network of • tropical SST and • precipitation proxies • in statistical • reconstruction • extract common • signals from network • generate quantitative • error bars in SST Furtado et al., 2009 Emile-Geay et al., in prep

17. -10 Tropical Volcanic Forcing W/m2 -5 -0.3 0.3 W/m2 Solar Forcing 0.3 • no statistically significant response to volcanic or solar forcing • hint of MCA cooling, but error bars large Emile-Geay et al., in prep

18. The last millennium (corals & multi-proxy reconstruction) • external climate forcing has little effect on tropical Pacific • climate (ENSO characteristics nor mean state) • internal variability dominates (e.g. Wittenberg, 2009) • EXCEPT: unprecedented warming/freshening trend • since ~1970AD  anthropogenic response?

19. unprecedented trend towards warmer, wetter conditions in Palmyra and Christmas corals II. The last forty years How much of the coral δ18O trend is warming? and how much is freshening? What is the nature of coral trends across Line Islands?

20. Each Line Island is climatologically unique mean annual SST in color, rainfall in contours If ITCZ is involved, might expect largest freshening signature at Palmyra. If upwelling is involved, might expect largest warming signal at Christmas. Fanning should fall in between Palmyra and Christmas. Nurhati et al., submitted

21. Coral Sr/Ca ratios good SST proxy in Line Island corals combine Sr/Ca (SST) with δ18O (SST + δ18Osw) to obtain δ18Osw Nurhati et al., submitted

22. 2 proxies 7 different cores 3 islands… • warming greatest • at Christmas •  less upwelling • -freshening greatest • at Palmyra •  increase ITCZ • -Fanning in between • Palmyra and Christmas Nurhati et al., submitted

23. Coral results agree with majority of AR4 GCM projections increase in ITCZ strength inferred on theoretical grounds; observed in models equatorial enhancement of warming observed in models Held & Soden, 2006 di Nezio et al., in press

24. III. The mid-Holocene -many models simulate reduced ENSO activity in response to different insolation forcing during the mid-Holocene (Clement et al., 1999; Otto-Bleisner et al., 2003) Clement et al., 1999 -some rare high-resolution paleoclimate data support this view (Rodbell et al., 1999; Tudhope et al., 2001; Koutavas et al., 2006)

25. A composite of all available Holocene coral data • if all corals are created • equal, it’s hard to discern • a mid-Holocene change • in ENSO variance • -are insolation-forced • changes detectable, even • if we triple the amount • of data available? ? Corals: Cobb et al., 2003, in prep; Westphal et al., in prep; Tudhope et al., 2001; Woodruffe et al., 2003; McGregor et al., 2003; Correge et al., 2001

26. Fossil coral climate reconstructions: the next generation altered fossil coral pristine modern coral • SEM screening combined • with micro-scale analyses • will improve paleoclimate • reconstructions from old • fossil corals • -work on 10 Line Island • fossil corals from ~6kybp Sayani et al., submitted Zaunbrecher et al., submitted

27. External Forcing Scorecard • The last millennium: NO, except last 40 years • The last forty years: YES, trend towards “El Nino-like” conditions • The mid-Holocene: NO?, pending more and better data Take-homes Paleoclimate data can provide quantitative constraints for testing GCM responses to known external climate forcing, across a range of timescales (including late 20th century). Reproducibility a critical, yet under-appreciated, ingredient to success.