Coral records of El Niño andTropical Pacific climate change

Kim M. Cobbkcobb@eas.gatech.edu

Harold Nations SymposiumOctober 14, 2005

ENSO El Niño-Southern OscillationA coupled ocean-atmospherephenomenon that originates in the tropical Pacific but affectsglobal 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

Su

rfa

ce T

empe

ratu

re

Ano

mal

y (o

C)

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, 1997Mantua et al, 1997

ENSO-like “Global Warming”??

(SST trend from 1949-1991, in degrees/decade, from Latif et al, 1997)

“ENSO-like” low-frequency variability

Year

1860 1880 1900 1920 1940 1960 1980 2000

Tem

per

atu

reD

evia

tio

n (

°C)

-3

-2

-1

0

1

2

3Eastern tropical Pacific Temperature

El Niño

La Niña

1997 El Niño1982 El Niño

The instrumental record of ENSO

Are severe El Niño events becoming more frequentas 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 yearsis likely to have been due to the increase ingreenhouse gas concentrations.” – IPCC 2001

Modeled global temperaturerise

Modeled sea levelrise

The instrumental record of ENSO is too short to answersome key questions:

1. Are late 20th century El Niño events more frequent and more severethan 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 recordsfor the last 200 years

Fossil corals enable usto extend the record(ex. 1320-1390A.D.)

COMMON

RARE

The search for corals leads to the Line Islands

1997, 1998, 2000, 2005

2005

20042005

With thanks to: HRH Khaled bin Sultan bin Abdulaziz, Norwegian Cruise Lines,Scripps Line Islands cruise

Coral oxygen isotopic ratios: a temperature andprecipitation proxy

1000

/

)/(/1618

1618161818 x

OO

OOOOO

std

stdspl

reported as:

Temperature dependence of seawater-carbonate oxygen isotope fractionation:

coral 18O gets more depleted as T increases (slope = -0.22‰ per 1ºC)

basis: equilibrium fractionation:18O has different zero point energy than 16O

problems:‘kinetic’ fractionation: growth rate, respirationvs. photosynthesis

and seawater 18O not constant!Epstein, 1953

Rainfall is depleted with respect to seawater

Seawater 18O linearlyrelated to salinity+0.22‰ per 1psu

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 slopeof salinity vs. 18O

Fairbanks et al., 1997

Cole & Fairbanks, 1990

Coral Sr/Ca ratios:a pure temperature proxy?

basis:

1) Sr conc. in seawater invariant

2) Sr2+ incorporation into coralaragonite lattice temp. dependent

problem:

‘kinetic’ effects: coral Sr/Cadepends 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 SSTAnd Rainfall Anomalies

Site

A baby booby at Palmyra

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Date (A.D.)

Modern

The Palmyra Island Coral Collection

Little Ice Age (LIA)canals frozen in Europe

Medieval Warm Period (MWP)Greenland green

1990

1995

1985

1980

Palmyra Coral O (‰)

SST ( C)

3029282726

-4.7 -5.1 -5.5 -5.9

Building a Chronology from the Coral Oxygen Isotopic Record

SST (°C)

Palmyra Coral 18O (‰)

1995

1990

1985

26 27 28 29 301980

Drilled inMay 1998

Sa

mp

lin

g t

ran

se

ct

Calibrating the coral 18O-temperaturerelationship at Palmyra

Coral 18O = -0.23(SST)R = 0.81

- close to inorganic slope (-0.20)- precip. plays minor role

SS

T A

nom

oly

(°C

)

3

2

1

0

-1

-2

18 O

(‰

)

-0.6

-0.4

-0.2

0.0

0.2

NIÑO3.4 SSTPalmyra coral

Year (A.D.)

1900 1920 1940 1960 1980 2000

SS

T A

nom

oly

(°C

) 1

0

-1

-2

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

R = -0.66

R = -0.84

18 O

(‰

)

How well does Palmyra coral 18O record ENSO?

Red = instrumental record of ENSOBlack = modern coral 18O

Les

s sm

oo

thed

Mo

re s

mo

oth

ed

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Date (A.D.)

Palmyra Island Coral Collection

Turning to the fossil corals….

Year (A.D.)

1640 1650 1660 1670 1680 1690 1700

18 O

(‰)

-5.6

-5.4

-5.2

-5.0

-4.8

-4.6

-4.4

SB13 + 0‰SB3 - 0.05‰SB8 + 0‰

SB3/SB13R = 0.66

SB13/SB8R = 0.62

17th century fossil coral-based climate reconstruction

As number of overlapping corals increases

splice

3 corals, 13 dates, 3,000 18O measurements

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Date (A.D.)

Palmyra Island Coral Collection

Year (A.D.)

1320 1340 1360 1380 1400 1420 1440 1460

18 O

(‰)

-5.4

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

SB7 - 0.06‰CH9 - 0.06‰SB5 + 0.19‰SB6 - 0.11‰CH5 + 0.04‰

SB7 vs. CH9R = 0.68

SB5 vs. CH5R = 0.71

SB6 vs. CH5R = 0.69

splice

14th-15th Century Splice

5 corals, 29 dates, 14,000 18O measurements

900 1200 1400 1600 1800 2000

18O

(‰)

-5.75

-5.50

-5.25

-5.00

-4.75

-4.50

-4.25

5-coral splice

3-coral splice Modern

Single records

1°C

Palmyra Coral 18O Sequences

Cobb et al., Nature, 2003

warmer

colder

Date A.D.

930 960

18

O(‰

)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

1170 1200Years

1320 1350 1380 1410 1440 1650 1680 1890 1920 1950 1980

El Niño

La Niña

1997El Niño

An extended history of ENSO from the Palmyra coral records

Most fr

eque

nt, in

tense

El Niño

even

ts of

reco

nstru

ction

ENSO char

acter

istics

can

chan

ge in l

ess t

han a

deca

de

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 tropicalPacific is likely unprecedented (in the last millennium).