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

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. I.The last millennium II.The last forty years III.The mid-Holocene

Dai and Wigley, 2000 El Niño Temperature El Niño Precipitation “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; 20 th century trends uncertain Why tropical Pacific climate?

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

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

Bunge & Clarke, 2009 # SST observations in central tropical Pacific over last 150 years

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

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

Palmyra coral oxygen isotopes vs. NINO3.4 SST SST Anomaly (°C) Cobb et al., 2003 δ 18 O

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 The Line Islands Coral Collection modern cores from three islands splice overlapping cores in last millennium many cores in mid-Holocene 1 2 3

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

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

315 years of ENSO variability -12,000 δ 18 O values -variability agrees well; average δ 18 O values differ Cobb et al., 2003 Cobb et al., in prep

W/m 2 Tropical Volcanic Forcing to to -0.3 W/m 2 Line Islands Fossil Coral  18 O Reconstruction raw 2-7y bandpassed Solar Forcing 0 Cobb et al., in prep Crowley et al., 2000

Did a tropical Pacific “dynamical thermostat” play a significant role in the last millennium? Corals say NO AD mega-eruption caused no significant anomaly - solar variability poorly correlated to coral δ 18 O BUT -only one site -corals have largest errors on dec-cen timescales NO

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

Emile-Geay et al., in prep W/m 2 Tropical Volcanic Forcing W/m 2 Solar Forcing -no statistically significant response to volcanic or solar forcing -hint of MCA cooling, but error bars large

I.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?

How much of the coral δ 18 O trend is warming? and how much is freshening? What is the nature of coral trends across Line Islands? II. The last forty years unprecedented trend towards warmer, wetter conditions in Palmyra and Christmas corals

Nurhati et al., submitted 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.

Coral Sr/Ca ratios good SST proxy in Line Island corals Nurhati et al., submitted combine Sr/Ca (SST) with δ 18 O (SST + δ 18 O sw ) to obtain δ 18 O sw

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

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

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., some rare high-resolution paleoclimate data support this view (Rodbell et al., 1999; Tudhope et al., 2001; Koutavas et al., 2006)

A composite of all available Holocene coral data 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., 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? ?

Fossil coral climate reconstructions: the next generation pristine modern coral altered fossil 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

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