Coral records of El Niño and Tropical Pacific climate change Kim M. Cobb Harold Nations Symposium October 14, 2005

ENSO El Niño-Southern Oscillation A coupled ocean-atmosphere phenomenon that originates in the tropical Pacific but affects global climate patterns December 1997 SST Anomalies Why study tropical Pacific climate? -ENSO extremes carry serious economic and social costs - improved ENSO forecasts minimize the costs - the impacts are not confined to the tropical Pacific +6° +3° 0° -3° Sea Surface Temperature Anomaly (oC) El Niño impacts

“ENSO-like” Decadal Variability? (SST anomalies for proposed ~12-13yr pan-tropical climate variability, from Cobb et al, 2001) Zhang et al, 1997 Mantua et al, 1997 ENSO-like “Global Warming”?? (SST trend from , in degrees/decade, from Latif et al, 1997) “ENSO-like” low-frequency variability

El Niño La Niña 1997 El Niño 1982 El Niño The instrumental record of ENSO Are severe El Niño events becoming more frequent as global temperatures increase?

The uncertain climate future How will tropical Pacific climate change as global temperatures rise? “Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.” – IPCC 2001 Modeled global temperature rise Modeled sea level rise

The instrumental record of ENSO is too short to answer some key questions: 1. Are late 20th century El Niño events more frequent and more severe than those of the recent past? 2. Is there a correlation between average global temperature and El Niño activity? 3. How much and how fast has ENSO changed in the past? Palmyra lagoon

Corals: The geologic record of ENSO CORALS from the tropical Pacific record ENSO in the geochemistry of their skeletons Living corals provide records for the last 200 years Fossil corals enable us to extend the record (ex A.D.) COMMON RARE

The search for corals leads to the Line Islands 1997, 1998, 2000, With thanks to: HRH Khaled bin Sultan bin Abdulaziz, Norwegian Cruise Lines, Scripps Line Islands cruise

Coral oxygen isotopic ratios: a temperature and precipitation proxy reported as: Temperature dependence of seawater-carbonate oxygen isotope fractionation: coral  18 O gets more depleted as T increases (slope = -0.22‰ per 1ºC) basis: equilibrium fractionation: 18 O has different zero point energy than 16 O problems: ‘kinetic’ fractionation: growth rate, respiration vs. photosynthesis and seawater  18 O not constant! Epstein, 1953

Rainfall is depleted with respect to seawater Seawater  18 O linearly related to salinity +0.22‰ per 1psu Coral  18 O sensitive to changes in precipitation/ evaporation basis: equilibrium fractionation: water vapor more depleted than seawater problems: ‘kinetic’ fractionation: boundary layer effects change the slope of salinity vs.  18 O Fairbanks et al., 1997 Cole & Fairbanks, 1990

Coral Sr/Ca ratios: a pure temperature proxy? basis: 1)Sr conc. in seawater invariant 2) Sr 2+ incorporation into coral aragonite lattice temp. dependent problem: ‘kinetic’ effects: coral Sr/Ca depends on growth rate, other ‘biological’ overprinting

Research Objective: To generate >100-yr-long, high-resolution, high-fidelity climate proxy records from the tropical Pacific Ocean; to extend the record of ENSO back in time Materials: Modern and Fossil Corals Methods: Dating: U-Th radioactive decay series Climate proxy: Coral skeletal oxygen isotopes December 1997 SST And Rainfall Anomalies Site A baby booby at Palmyra

Modern The Palmyra Island Coral Collection Little Ice Age (LIA) canals frozen in Europe Medieval Warm Period (MWP) Greenland green

SST (°C) Palmyra Coral  18 O (‰) Drilled in May 1998 Sampling transect Calibrating the coral  18 O-temperature relationship at Palmyra Coral  18 O = -0.23(SST) R = close to inorganic slope (-0.20) -precip. plays minor role

How well does Palmyra coral  18 O record ENSO? Red = instrumental record of ENSO Black = modern coral  18 O Less smoothed More smoothed

Palmyra Island Coral Collection Turning to the fossil corals….

17 th century fossil coral-based climate reconstruction As number of overlapping corals increases splice 3 corals, 13 dates, 3,000  18 O measurements

Palmyra Island Coral Collection

splice 14 th -15 th Century Splice 5 corals, 29 dates, 14,000  18 O measurements

5-coral splice 3-coral spliceModern Single records 1°C Palmyra Coral  18 O Sequences Cobb et al., Nature, 2003 warmer colder Date A.D.

El Niño La Niña 1997 El Niño An extended history of ENSO from the Palmyra coral records Most frequent, intense El Niño events of reconstruction ENSO characteristics can change in less than a decade

Summary Coral  18 O is a sensitive, reliable proxy for tropical Pacific climate change. Most intense ENSO activity occurred during 17 th century, during “Little Ice Age”. ENSO characteristics can change dramatically from decade to decade. The late 20 th century trend towards warmer conditions in the central tropical Pacific is likely unprecedented (in the last millennium).