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Coral Records of Climate Change
Kim M. Cobb
Georgia 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.
Palmyra
1997-?
Fanning
2005-?
Research Funded by:
NOAA
NSF
Fieldwork funded by:
NCL
The Nature Conservancy
Prince Khaled Bin Sultan
Bin Abdulaziz
Christmas
1998-?
Palmyra, Fanning,
Christmas Islands
Sea Surface Temperature
Anomaly (ºC)
Why study tropical Pacific climate?
+6°
+3°
0°
El Niño-Southern Oscillation
An ocean-atmosphere
phenomenon that originates
in the tropical Pacific but affects
global climate patterns
-3°
December 1997 ocean temperature
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
The instrumental record of El Niño’s
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
2000
Year
Are severe El Niño events becoming more frequent
as global temperatures increase?
The instrumental record of El Niño’s 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?
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
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 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
Site
December 1997 SST
And Rainfall Anomalies
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
The Line Island Coral Collection:
A work in progress…
Palmyra
40 cores U/Th dated
28 cores undated
Christmas
18 cores U/Th dated
63 cores undated
Fanning 33 cores undated
6000
5500
5000
4500
4000
3500
3000
Years B.P.
2500
2000
1500
1000
500
0
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
18(‰)
Palmyra
Coral O
Palmyra
Coral
O (‰)
-4.7
Climate Proxy:
Coral oxygen isotopes (18O)



Coral 18O is primarily a
function of sea-surface
temperature
-5.5
-5.9
Drilled in
May 1998
Sampling transect

 18O /16O spl (18O /16O) std 
 O
 x1000
18
16
O
/
O


std
18
-5.1
1995
1995
1990
1990
BUT
It also will record changes
In the 18O of seawater
(i.e. salinity)
1985
1985
19801980
26
28 29
29 30
30
26 27
27 28
SST ( C)
SST
(°C)
How well do Palmyra corals record El Niños?
Red = instrumental record of El Niños
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
= 1 year of work
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
= 3 years of work
1460
Palmyra Coral 18O Sequences
Single records
Single records
3-coral splice
2-coral splice
Modern
5-coral splice
1°C
warmer
1°C
colder
Cobb et al., Nature, 2003
Date A.D.
What does this coral reconstruction
of tropical Pacific climate tell us?
What about that
Late 20th century
Trend?
Approach: use
coral Sr/Ca ratios
as an SST-only
proxy
combine Sr/Ca (SST)
with δ18O (SST + δ18Osw)
to obtain δ18Osw (salinity)
Nurhati et al., 2009
Answer: the late 20th century trend is mostly salinity!
So climate change is affecting rainfall in this area.
Nurhati et al., 2011
Are late 20th century El Nino events unprecedented
in the last millennium?
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
Conclusions
Climate change is changing precipitation patterns in the tropical
Pacific  more rain (What are the implications for rest
of globe?)
Present-day El Nino events are not unusual. (What caused the
strong 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 temperatures
and 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 widespread
bleaching and coral mortality (ex: 16% of world’s coral died during
1997 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/