Coral records of tropical Pacific climate:Past, present, and future

Kim CobbIntan Suci NurhatiLaura ZaunbrecherHussein SayaniJulien Emile-GeayJud PartinGeorgia Inst. of Technology

Chris CharlesNiko WestphalScripps Inst. of Oceanography

Larry Edwards, Hai ChengUniversity of Minnesota

with thanks to NSF, NOAA, NCL, NGS, ACS-PRF, PARC, Cobb lab undergrads

Motivation: How is the tropical Pacific climate systemresponding to anthropogenic forcing?

Approach: Use well-dated, high-resolution paleoclimaterecords from the tropical Pacific to assess its response toknown natural and anthropogenic climate forcings.

I.The last millennium

II.The last forty years

III.The mid-Holocene

Dai and Wigley, 2000

El Niño Temperature

El Niño Precipitation

“El Niño-Southern Oscillation”(ENSO)

ENSO is a climate phenomenon in the tropical Pacific which arisesfrom coupled interactions betweenthe atmosphere and ocean

ENSO impacts global climate every2-7 years

tropical Pacific climate variabilityover decades to centuries to millennia poorly constrained;20th century trends uncertain

Why tropical Pacific climate?

Vecchi et al, 2008

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

Climate modelresponse to

greenhouse forcing

Vecchi et al, 2008

- instrumental SST datasets contain trends of different signs no help

Instrumental SST trends

18 19 20

Bunge & Clarke, 2009

# SST observations in central tropical Pacificover last 150 years

CORALS from the tropical Pacific record El Niño’s in the geochemistry of their skeletons monthly resolution

Living Porites corals provide recordsfor the last 200 years; band-counted

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

Coral archives of tropical Pacific climate

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 δ18Orain = lower seawater δ18O

warmer, wetter conditions duringEl Niño events cause lower coral δ18O

Palmyra coral oxygen isotopes vs. NINO3.4 SSTS

ST

Ano

mal

y (°

C)

SS

T A

nom

aly

(°C

)

Cobb et al., 2003

SS

T A

nom

oly

(°C

)

3

2

1

0

-1

-2

δ18O

(‰

)

-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

δ18O

(‰

)

δ1

8O δ

18O

Palmyra50 cores U/Th dated18 cores undated

Christmas25 cores U/Th dated51 cores undated

Fanning17 cores U/Th dated19 cores undated

The Line Islands Coral Collection

modern coresfrom threeislands

splice overlappingcores in lastmillennium

many coresin mid-Holocene

1 23

Mann et al., 2005

NINO34 warms inresponse to volcaniceruption…

and cools in response toincreased solar forcing

Cane-Zebiak modeland natural forcing

the ocean’s“dynamical thermostat”?

but coupled GCMsdon’t do this

Mann et al., in press, Science

GISS-ER

CSM1.4

Plots of SST difference between:

Medieval Climate Anomaly (~1000AD)(cool tropical Pacific?)

and

Little Ice Age (~1700AD)(warm tropical Pacific?)

315 years of ENSO variability-12,000 δ18O values

-variability agrees well; average δ18O values differ

Cobb et al., 2003Cobb et al., in prep

-10

-5 W/m

2

Tropical Volcanic Forcing

0.3

0.1 to 0.3

-0.1 to -0.3

W/m

2

Line Islands Fossil Coral δ18O Reconstruction

raw

2-7y bandpassed

Solar Forcing

0

Cobb et al., in prep

Crowley et al., 2000

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

NONO

Multi-proxy reconstruction of tropical Pacific climate

Furtado et al., 2009Emile-Geay et al., in prep

- use network of tropical SST and precipitation proxies in statistical reconstruction

- extract common signals from network

- generate quantitative error bars in SST

Emile-Geay et al., in prep

-10

-5 W/m

2

Tropical Volcanic Forcing

0.3

0.3

-0.3

W/m

2

Solar Forcing

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

I.The last millennium (corals & multi-proxy reconstruction)

external climate forcing has little effect on tropical Pacificclimate (ENSO characteristics nor mean state)

internal variability dominates (e.g. Wittenberg, 2009)

EXCEPT: unprecedented warming/freshening trend since ~1970AD anthropogenic response?

How much of the coral δ18O trend is warming? and how much is freshening?

What is the nature of coral trends across Line Islands?

II. The last forty yearsunprecedented trend towards warmer, wetter conditions in Palmyra and Christmas corals

Nurhati et al., submitted

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.

Coral Sr/Ca ratios good SST proxyin Line Islandcorals

Nurhati et al., submitted

combine Sr/Ca (SST)

with δ18O (SST + δ18Osw)

to obtain δ18Osw

2 proxies7 different cores3 islands…

-warming greatest at Christmas less upwelling

-freshening greatest at Palmyra increase ITCZ

-Fanning in between Palmyra and Christmas

Nurhati et al., submitted

Coral results agree with majority of AR4 GCM projections

Held & Soden, 2006

di Nezio et al., in press

increase in ITCZ strength inferred on theoretical grounds;observed in models

equatorial enhancement of warming observed inmodels

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)

A composite of all available Holocene coral data

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

-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?

?

Fossil coral climate reconstructions: the next generation

pristine modern coral

altered fossil 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., submittedZaunbrecher et al., submitted

I.The last millennium: NO, except last 40 years

II.The last forty years: YES, trend towards “El Nino-like” conditions

III.The mid-Holocene: NO?, pending more and better data

External Forcing Scorecard

Take-homes

Paleoclimate data can provide quantitative constraints for testing GCMresponses to known external climate forcing, across a range of timescales (including late 20th century).

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