Transcript Palmyra Coral d 18 O

Coral records of El Niño and
Tropical Pacific climate change
Kim M. Cobb
[email protected]
Harold Nations Symposium
October 14, 2005
+6°
+3°
0°
-3°
Sea Surface Temperature
Anomaly (oC)
Why study tropical Pacific climate?
ENSO
El Niño-Southern Oscillation
A coupled ocean-atmosphere
phenomenon that originates
in the tropical Pacific but affects
global climate patterns
December 1997 SST Anomalies
El Niño impacts
- the impacts are not confined to the
tropical Pacific
-ENSO extremes carry serious
economic and social costs
- improved ENSO forecasts minimize
the costs
“ENSO-like” low-frequency variability
“ENSO-like” Decadal Variability?
(SST anomalies for proposed ~12-13yr
pan-tropical climate variability, from
Cobb et al, 2001)
Zhang et al, 1997
Mantua et al, 1997
ENSO-like “Global Warming”??
(SST trend from 1949-1991, in
degrees/decade, from Latif et al, 1997)
The instrumental record of ENSO
1982 El Niño 1997 El Niño
3
Eastern tropical Pacific Temperature
Temperature
Deviation (°C)
2
El Niño
1
0
-1
La Niña
-2
-3
1860
1880
1900
1920
1940
1960
1980
Year
Are severe El Niño events becoming more frequent
as global temperatures increase?
2000
The uncertain climate future
How will tropical Pacific climate change
as global temperatures rise?
Modeled
global
temperature
rise
Modeled
sea level
rise
“Most of the observed warming over the last 50 years
is likely to have been due to the increase in
greenhouse gas concentrations.” – IPCC 2001
The instrumental record of ENSO is too short to answer
some key questions:
1. Are late 20th century El Niño events more frequent and more severe
than 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
COMMON
Living corals provide records
for the last 200 years
RARE
Fossil corals enable us
to extend the record
(ex. 1320-1390A.D.)
The search for corals leads to the Line Islands
1997, 1998, 2000, 2005
2005
2004
2005
With thanks to: HRH Khaled bin Sultan bin Abdulaziz, Norwegian Cruise Lines,
Scripps Line Islands cruise
Coral oxygen isotopic ratios: a temperature and
precipitation proxy
reported as:


18
16
18
16

O
/
O

(
O
/
O) std 
spl
18
 O
 x1000
18
16
O
/
O


std
Temperature dependence of
seawater-carbonate oxygen
isotope fractionation:
coral 18O gets more depleted as T
increases (slope = -0.22‰ per 1ºC)
Epstein, 1953


basis:
equilibrium fractionation:
18O has different zero point energy than 16O
problems:
‘kinetic’ fractionation: growth rate, respiration
vs. photosynthesis
and seawater 18O not constant!
Rainfall is depleted with
respect to seawater
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 slope
of salinity vs. 18O
Cole & Fairbanks, 1990
Seawater 18O linearly
related to salinity
+0.22‰ per 1psu
Fairbanks et al., 1997
Coral Sr/Ca ratios:
a pure temperature proxy?
basis:
1)
Sr conc. in seawater invariant
2)
Sr2+ incorporation into coral
aragonite lattice temp. dependent
problem:
‘kinetic’ effects: coral Sr/Ca
depends 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
Site
December 1997 SST
And Rainfall Anomalies
A baby booby
at Palmyra
The Palmyra Island Coral Collection
Modern
Medieval Warm Period (MWP)
Greenland green
900
1000
1100
1200
Little Ice Age (LIA)
canals frozen in Europe
1300
1400
1500
Date (A.D.)
1600
1700
1800
1900
2000
Building a Chronology from the Coral Oxygen Isotopic Record
Palmyra
Coral 18
OO
(‰)(‰)
Palmyra
Coral
Calibrating the coral
18O-temperature
relationship at Palmyra
-
-5.1
-5.5
-5.9
Drilled in
May 1998
Sampling transect
Coral 18O = -0.23(SST)
R = 0.81
-4.7
1995
1995
1990
1990
close to inorganic slope (-0.20)
precip. plays minor role
1985
1985
19801980
26
28 29
29 30
30
26 27
27 28
SST ( C)
SST
(°C)
How well does Palmyra coral 18O record ENSO?
Red = instrumental record of ENSO
Black = modern coral 18O
3
-0.6
-0.4
1
-0.2
0
0.0
-1
0.2
-2
-0.3
18O (‰)
2
-0.2
1
-0.1
0
0.0
0.1
-1
0.2
NIÑO3.4 SST
Palmyra coral
R = -0.84
0.3
-2
1900
1920
1940
Year (A.D.)
1960
1980
2000
18O (‰)
SST Anomoly (°C)
SST Anomoly (°C)
More smoothed
Less smoothed
R = -0.66
Turning to the fossil corals….
Palmyra Island
Coral Collection
900
1000
1100
1200
1300
1400
1500
Date (A.D.)
1600
1700
1800
1900
2000
17th century fossil coral-based climate reconstruction
SB13/SB8
R = 0.62
SB3/SB13
R = 0.66
-5.6
splice
SB13 + 0‰
SB3 - 0.05‰
SB8 + 0‰
-5.4
-5.0
18
 O(‰)
-5.2
-4.8
-4.6
-4.4
1640
1650
1660
1670
1680
1690
Year (A.D.)
As number of overlapping corals
3increases
corals, 13 dates, 3,000 18O measurements
1700
Palmyra Island
Coral Collection
900
1000
1100
1200
1300
1400
1500
Date (A.D.)
1600
1700
1800
1900
2000
14th-15th Century Splice
SB7 vs. CH9
R = 0.68
SB5 vs. CH5 SB6 vs. CH5
R = 0.71
R = 0.69
-5.4
splice
-5.2
18
 O(‰)
-5.0
-4.8
-4.6
SB7 - 0.06‰
CH9 - 0.06‰
SB5 + 0.19‰
SB6 - 0.11‰
CH5 + 0.04‰
-4.4
-4.2
1320
1340
1360
1380
1400
1420
1440
Year (A.D.)
5 corals, 29 dates, 14,000 18O measurements
1460
Palmyra Coral 18O Sequences
3-coral splice
Modern
5-coral splice
-5.75
-5.50
Single records
warmer
-5.00
-4.75
1°C
18
 O(‰)
-5.25
colder
-4.50
-4.25
900
1200
1400
1600
Date A.D.
1800
2000
Cobb et al., Nature, 2003
An extended history of ENSO from the Palmyra coral records
Years
930 960 1170 1200 1320 1350 1380 1410 1440
1650 1680 1890 1920 1950 1980
1997
El Niño
-0.3
18O(‰)
-0.2
-0.1
El Niño
0.0
0.1
0.2
0.3
La Niña
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 tropical
Pacific is likely unprecedented (in the last millennium).