Coral records of El Niño and Tropical Pacific climate change Kim M. Cobb [email protected]...
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Transcript of Coral records of El Niño and Tropical Pacific climate change Kim M. Cobb [email protected]...
Coral records of El Niño andTropical Pacific climate change
Kim M. [email protected]
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).