Coral Records of Climate Change

Kim M. CobbGeorgia Inst. of Technology

Oceanography class, Oct 21, 2011

Research Goal:

To reconstruct tropical Pacific climate change of the recent past, so that we might better predict future climate change, and its regional signatures.

Fanning2005-?

Palmyra1997-?

Christmas1998-?

Research Funded by:NOAANSF

Fieldwork funded by:NCLThe Nature ConservancyPrince Khaled Bin Sultan

Bin Abdulaziz

El Niño-Southern OscillationAn ocean-atmospherephenomenon that originates in the tropical Pacific but affectsglobal climate patterns

December 1997 ocean temperature 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

Palmyra, Fanning, Christmas Islands

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 El Niño’s

Are severe El Niño events becoming more frequentas global temperatures increase?

The instrumental record of El Niño’s 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?

A well-placed rope swing in the Palmyra lagoon

Corals: The geologic record of El Niño

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 fossil corals leads to the Northern Line Islands

Palmyra Atoll

Research Objective: To generate >100-yr-long, high-resolution, high-fidelity climate proxy records from the tropical Pacific Ocean; to extend the record of El Niño 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

Generating climate reconstructions from the Palmyra corals:

1) Recover the corals, both modern (~10) and fossil (~100).

2) Prove that the coral geochemistry tracks large-scale climate.ie. Calibrate the modern coral record against the instrumental record of climate.

3) Apply geochemistry to fossil corals and date them (U/Th dating).

Aerial view of Palmyra

Palmyra40 cores U/Th dated28 cores undated

Christmas18 cores U/Th dated63 cores undated

Fanning33 cores undated

The Line Island Coral Collection:A work in progress…

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 δ18O (‰)

1995

1990

1985

26 27 28 29 301980

1000

/

)/(/1618

1618161818 x

OO

OOOOO

std

stdspl

δ

Coral δ18O is primarily a function of sea-surface

temperature

BUT

It also will record changesIn the δ18O of seawater

(i.e. salinity)

Climate Proxy: Coral oxygen isotopes (δ18O)

Drilled inMay 1998

Sa

mp

lin

g t

ran

se

ct

SS

T A

nom

oly

(°C

)

3

2

1

0

-1

-2

δ18 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

δ18 O

(‰

)

How well do Palmyra corals record El Niños?

Red = instrumental record of El NiñosBlack = modern coral δ18O

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

δ18 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 δ18O measurements

= 1 year of work

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

δ18 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 δ18O measurements

= 3 years of work

Single records

1°C

Palmyra Coral δ18O Sequences

warmer

colder

Date A.D.

5-coral splice

3-coral splice Modern

Single records

1°C

2-coral splice

Cobb et al., Nature, 2003

What does this coral reconstruction of tropical Pacific climate tell us?

What about that Late 20th centuryTrend?

Approach: use coral Sr/Ca ratios as an SST-onlyproxy

Nurhati et al., 2009

combine Sr/Ca (SST)

with δ18O (SST + δ18Osw)

to obtain δ18Osw (salinity)

Answer: the late 20th century trend is mostly salinity!So climate change is affecting rainfall in this area.

Nurhati et al., 2011

930 960

δ18

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

Are late 20th century El Nino events unprecedented in the last millennium?

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

Conclusions

Climate change is changing precipitation patterns in the tropicalPacific more rain (What are the implications for restof globe?)

Present-day El Nino events are not unusual. (What caused thestrong El Nino events in the 17th century, if anything?)

Food for Thought

Coral reefs are disappearing at alarming rates worldwide,due to the combined influence of rising ocean temperaturesand human disturbances (sediment runoff, over-fishing, dynamite fishing, etc).

Reef ecosystems have weakened to the point that natural climate variations, such as a large El Niño event, may cause widespreadbleaching and coral mortality (ex: 16% of world’s coral died during1997 El Nino event (WMO report #1063))

Web Resources

My homepage: http://shadow.eas.gatech.edu/~kcobb

General El Niño info: http://www.pmel.noaa.gov/tao/elnino

NOVA El Niño page: http://www.pbs.org/wgbh/nova/elnino/