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Coral records of El Niño and Tropical Pacific climate change Kim M. Cobb [email protected] Harold Nations Symposium October 14, 2005 +6° +3° 0° -3° Sea Surface Temperature Anomaly (oC) Why study tropical Pacific climate? 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 El Niño impacts - the impacts are not confined to the tropical Pacific -ENSO extremes carry serious economic and social costs - improved ENSO forecasts minimize the costs “ENSO-like” low-frequency variability “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 1949-1991, in degrees/decade, from Latif et al, 1997) The instrumental record of ENSO 1982 El Niño 1997 El Niño 3 Eastern tropical Pacific Temperature Temperature Deviation (°C) 2 El Niño 1 0 -1 La Niña -2 -3 1860 1880 1900 1920 1940 1960 1980 2000 Year 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? Modeled global temperature rise Modeled sea level 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 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 COMMON Living corals provide records for the last 200 years RARE Fossil corals enable us to extend the record (ex. 1320-1390A.D.) The search for corals leads to the Line Islands 1997, 1998, 2000, 2005 2005 2004 2005 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: 18 16 18 16 O / O ( O / O) std spl 18 O x1000 18 16 O / O std Temperature dependence of seawater-carbonate oxygen isotope fractionation: coral 18O gets more depleted as T increases (slope = -0.22‰ per 1ºC) Epstein, 1953 basis: equilibrium fractionation: 18O has different zero point energy than 16O problems: ‘kinetic’ fractionation: growth rate, respiration vs. photosynthesis and seawater 18O not constant! Rainfall is depleted with respect to seawater Coral 18O 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. 18O Cole & Fairbanks, 1990 Seawater 18O linearly related to salinity +0.22‰ per 1psu Fairbanks et al., 1997 Coral Sr/Ca ratios: a pure temperature proxy? basis: 1) Sr conc. in seawater invariant 2) Sr2+ 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 Site December 1997 SST And Rainfall Anomalies A baby booby at Palmyra The Palmyra Island Coral Collection Modern Medieval Warm Period (MWP) Greenland green 900 1000 1100 1200 Little Ice Age (LIA) canals frozen in Europe 1300 1400 1500 Date (A.D.) 1600 1700 1800 1900 2000 Building a Chronology from the Coral Oxygen Isotopic Record Palmyra Coral 18 OO (‰)(‰) Palmyra Coral Calibrating the coral 18O-temperature relationship at Palmyra - -5.1 -5.5 -5.9 Drilled in May 1998 Sampling transect Coral 18O = -0.23(SST) R = 0.81 -4.7 1995 1995 1990 1990 close to inorganic slope (-0.20) precip. plays minor role 1985 1985 19801980 26 28 29 29 30 30 26 27 27 28 SST ( C) SST (°C) How well does Palmyra coral 18O record ENSO? Red = instrumental record of ENSO Black = modern coral 18O 3 -0.6 -0.4 1 -0.2 0 0.0 -1 0.2 -2 -0.3 18O (‰) 2 -0.2 1 -0.1 0 0.0 0.1 -1 0.2 NIÑO3.4 SST Palmyra coral R = -0.84 0.3 -2 1900 1920 1940 Year (A.D.) 1960 1980 2000 18O (‰) SST Anomoly (°C) SST Anomoly (°C) More smoothed Less smoothed R = -0.66 Turning to the fossil corals…. Palmyra Island Coral Collection 900 1000 1100 1200 1300 1400 1500 Date (A.D.) 1600 1700 1800 1900 2000 17th century fossil coral-based climate reconstruction SB13/SB8 R = 0.62 SB3/SB13 R = 0.66 -5.6 splice SB13 + 0‰ SB3 - 0.05‰ SB8 + 0‰ -5.4 -5.0 18 O(‰) -5.2 -4.8 -4.6 -4.4 1640 1650 1660 1670 1680 1690 Year (A.D.) As number of overlapping corals 3increases corals, 13 dates, 3,000 18O measurements 1700 Palmyra Island Coral Collection 900 1000 1100 1200 1300 1400 1500 Date (A.D.) 1600 1700 1800 1900 2000 14th-15th Century Splice SB7 vs. CH9 R = 0.68 SB5 vs. CH5 SB6 vs. CH5 R = 0.71 R = 0.69 -5.4 splice -5.2 18 O(‰) -5.0 -4.8 -4.6 SB7 - 0.06‰ CH9 - 0.06‰ SB5 + 0.19‰ SB6 - 0.11‰ CH5 + 0.04‰ -4.4 -4.2 1320 1340 1360 1380 1400 1420 1440 Year (A.D.) 5 corals, 29 dates, 14,000 18O measurements 1460 Palmyra Coral 18O Sequences 3-coral splice Modern 5-coral splice -5.75 -5.50 Single records warmer -5.00 -4.75 1°C 18 O(‰) -5.25 colder -4.50 -4.25 900 1200 1400 1600 Date A.D. 1800 2000 Cobb et al., Nature, 2003 An extended history of ENSO from the Palmyra coral records Years 930 960 1170 1200 1320 1350 1380 1410 1440 1650 1680 1890 1920 1950 1980 1997 El Niño -0.3 18O(‰) -0.2 -0.1 El Niño 0.0 0.1 0.2 0.3 La Niña Summary Coral 18O is a sensitive, reliable proxy for tropical Pacific climate change. Most intense ENSO activity occurred during 17th century, during “Little Ice Age”. ENSO characteristics can change dramatically from decade to decade. The late 20th century trend towards warmer conditions in the central tropical Pacific is likely unprecedented (in the last millennium).