Transcript Document

As the world warms: coral records of climate change
Kim Cobb
EAS, Georgia Inst. of Technology
Acknowledgements
Lab members:
Intan Suci Nurhati
Julien Emile-Geay
Laura Zaunbrecher
James Herrin
Hussein Sayani
EAS undergrads
Chris Charles
Scripps Inst of Oceanography
Larry Edwards, Hai Cheng
University of Minnesota
with special thanks to:
Prince Khaled bin Sultan bin Abdulaziz
Norwegian Cruise Lines
Palmyra Research Consortium
NOAA, NSF
Research Goal: constrain tropical Pacific response to anthropogenic
global warming
Approach: reconstruct tropical Pacific climate at high-resolution for
the last millennium
El Niño Temperature
WHY?
“El Niño-Southern Oscillation”
(ENSO)
ENSO is a climate pattern in the
tropical Pacific which arises
from coupled interactions between
the atmosphere and ocean
El Niño Precipitation
ENSO impacts global climate every
2-7 years (huge impact on rainfall)
Tropical Pacific climate variability
over decades to centuries to
millennia poorly constrained
Dai and Wigley, 2000
Why study the El Niño-Southern Oscillation?
1. It is the largest source of year-to-year global climate variability.
2. It carries negative societal consequences, both economic and humanitarian.
3. Improved forecasts minimize negative societal consequences.
4. It’s a fascinating, mysterious, and complex natural phenomenon.
What is the El Niño-Southern Oscillation?
A natural climate cycle in the tropical Pacific that alternates between
warm (El Niño) and cold (La Niña) phases every 2-7 years.
A bird’s-eye view of two of the
largest El Nino events of the century
And the last year of tropical
Pacific sea-surface temperatures
ENSO affects temperatures and rainfall in Georgia
Tropical Pacific climate and global warming
ENSO std in control run
ENSO std Observed
ENSO std in GHG run
La Niña El Niño
Collins et al., 2004
Timmermann et al., 1999
Cane et al., 1997
We don’t know how the tropical
Pacific will respond to global
warming, if at all.
Studies are contradictory because:
1) climate data are scarce prior to
1950
2) climate models do not simulate
ENSO
accurately
Research Questions
How has the tropical Pacific climate system responded to CO2 forcing?
What aspects of present tropical
Pacific climate are unprecedented?
compare last several
decades to recent centuries
Palmyra
1997-?
Fanning
2005-?
Christmas
1998-?
Corals: The geologic record of El Niño
Living Porites corals provide records
for the last 200 years
Fossil Porites corals enable us to
extend the record back many centuries
CORALS from the tropical Pacific
record El Niño’s in the geochemistry
of their skeletons
ENSO and coral oxygen isotopes (18O)
in the Central Tropical Pacific (CTP)
SST and rainfall anomalies during the 1982 El Nino
During El Niño events, positive SST and precipitation
anomalies both contribute to negative coral d18O anomalies
in the CTP,
so we can measure coral d18O continuously back in time
to reconstruct past ENSO activity, compare it to today’s
activity
Generating climate reconstructions from the Line Islands 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 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
How well do corals track temperature?
3
R = -0.66
2
T Anomoly (°C)
More smoothed
Less smoothed
Red = instrumental record of tropical Temp.
Black = modern coral 18O
1
0
Overlapping fossil corals: ancient El Niño events
Good reproducibility between coral geochemical records
increases confidence in coral climate reconstructions.
A millennium-long reconstruction of tropical Pacific temperature
warmer,
wetter
cooler,
drier
Key climate observations:
1) late 20th century warming is unprecedented in the last millennium
2) no cooling during the Northern Hemisphere’s “Little Ice Age”
3) significant cooling implied during the NH’s “Medieval Warm Period”
4) strongest ENSO activity during 1630-1650AD, not 1980-1998
Conclusions
-paleoclimate data have an important role to play in
global climate research
-corals provide quantitative reconstructions of temperature
-present-day El Nino events not unprecedented; no evidence
for anthropogenic influence
- evidence for ongoing tropical Pacific climate change that could
shape future global temperature and precipitation patterns
A climate scientist’s plea:
Get your climate information from a climate scientist
(not the media, politicians, etc)
Use flourescent light bulbs, don’t drive SUVs, and eat your broccoli