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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

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slide1

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

slide2

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.
  • The last millennium
  • The last forty years
  • The mid-Holocene
slide3

Why tropical Pacific climate?

“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;

20th century trends uncertain

El Niño Temperature

El Niño Precipitation

Dai and Wigley, 2000

slide4

Climate model

response to

greenhouse forcing

  • climate models project widely
  • divergent scenarios for tropical
  • Pacific climate under
  • greenhouse forcing

Vecchi et al, 2008

slide5

Instrumental SST trends

  • instrumental SST datasets contain
  • trends of different signs  no help

Vecchi et al, 2008

slide6

# SST observations in central tropical Pacific

over last 150 years

18

19

20

Bunge & Clarke, 2009

slide7

Coral archives of tropical Pacific climate

Living Porites corals provide records

for the last 200 years; band-counted

CORALS from the tropical Pacific

record El Niño’s in the geochemistry

of their skeletons  monthly resolution

Fossil Porites corals extend the record back many centuries; U/Th-dated

slide8

Coral oxygen isotopes in the central tropical Pacific

SST (colors) and rainfall (contours) anomalies during the 1982 El Niño

Oxygen isotopes (18O/16O, δ18O)

warmer water = lower coral δ18O

rain = lower seawater δ18O

warmer, wetter conditions during

El Niño events cause lower coral δ18O

slide9

Palmyra coral oxygen isotopes vs. NINO3.4 SST

SST Anomaly (°C)

δ18O

SST Anomaly (°C)

δ18O

Cobb et al., 2003

slide10

The Line Islands Coral Collection

1

splice

overlapping

cores in last

millennium

2

modern cores

from three

islands

3

many cores

in mid-Holocene

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

slide11

Cane-Zebiak model

and natural forcing

NINO34 warms in

response to volcanic

eruption…

Mann et al., 2005

and cools in response to

increased solar forcing

the ocean’s

“dynamical thermostat”?

slide12

but coupled GCMs

don’t do this

GISS-ER

Plots of SST difference between:

Medieval Climate Anomaly (~1000AD)

(cool tropical Pacific?)

and

Little Ice Age (~1700AD)

(warm tropical Pacific?)

CSM1.4

Mann et al., in press, Science

slide13

315 years of

ENSO variability

  • 12,000 δ18O values
  • variability agrees
  • well; average δ18O
  • values differ

Cobb et al., 2003

Cobb et al., in prep

slide14

Line Islands Fossil Coral δ18O Reconstruction

raw

2-7y bandpassed

Cobb et al., in prep

-10

Tropical Volcanic Forcing

W/m2

-5

-0.1 to -0.3

0.3

W/m2

0

Solar Forcing

0.1 to 0.3

Crowley et al., 2000

slide15

Did a tropical Pacific “dynamical thermostat” play a significant role in the last millennium?

  • Corals say NO
  • 1258AD mega-eruption
  • caused no significant
  • anomaly
  • solar variability poorly
  • correlated to coral δ18O
  • BUT
  • only one site
  • corals have largest
  • errors on dec-cen
  • timescales

NO

slide16

Multi-proxy reconstruction of tropical Pacific climate

  • - use network of
  • tropical SST and
  • precipitation proxies
  • in statistical
  • reconstruction
  • extract common
  • signals from network
  • generate quantitative
  • error bars in SST

Furtado et al., 2009

Emile-Geay et al., in prep

slide17

-10

Tropical Volcanic Forcing

W/m2

-5

-0.3

0.3

W/m2

Solar Forcing

0.3

  • no statistically significant response to volcanic or solar forcing
  • hint of MCA cooling, but error bars large

Emile-Geay et al., in prep

slide18

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?
slide19

unprecedented trend towards warmer, wetter conditions in Palmyra and Christmas corals

II. The last forty years

How much of the coral δ18O trend is warming?

and how much is freshening?

What is the nature of coral trends across Line Islands?

slide20

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.

Nurhati et al., submitted

slide21

Coral Sr/Ca ratios

good SST proxy

in Line Island

corals

combine Sr/Ca (SST)

with δ18O (SST + δ18Osw)

to obtain δ18Osw

Nurhati et al., submitted

slide22

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

slide23

Coral results agree with majority of AR4 GCM projections

increase in ITCZ strength inferred on theoretical grounds;

observed in models

equatorial enhancement of warming observed in

models

Held & Soden, 2006

di Nezio et al., in press

slide24

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., 1999

-some rare high-resolution paleoclimate data support this view

(Rodbell et al., 1999; Tudhope et al., 2001; Koutavas et al., 2006)

slide25

A composite of all available Holocene coral data

  • 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?

?

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., 2001

slide26

Fossil coral climate reconstructions: the next generation

altered fossil coral

pristine modern 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

slide27

External Forcing Scorecard

  • The last millennium: NO, except last 40 years
  • The last forty years: YES, trend towards “El Nino-like” conditions
  • The mid-Holocene: NO?, pending more and better data

Take-homes

Paleoclimate data can provide quantitative constraints for testing GCM

responses to known external climate forcing, across a range of

timescales (including late 20th century).

Reproducibility a critical, yet under-appreciated, ingredient to success.