Coral Records of 20th Century
Tropical Pacific Climate
This research constitutes my
Ph.D. dissertation at Georgia Tech with
Dr. Kim Cobb,
on investigating how anthropogenic climate change may impact
tropical Pacific climate. Tropical Pacific climate variability
such as the interannual El Nino-Southern Oscillaiton (ENSO)
strongly regulates global temperature and rainfall
patterns. Global Climate Model (GCMs) have provide differing
results on projecting the character of tropical Pacific changes
under greenhouse warming (see Vecchi et al. 2008). Studies on
quantifying tropical Pacific changes using instrumental datasets
has been challenged by the sparse availability of long,
continuous climate records from the tropical Pacific (see Deser
et al. 2010). Therefore here our approach is to reconstruct
climate proxy records derived from coral skeletal geochemistry.
A better understanding of tropical Pacific climate variability
and its secular trend under greenhouse forcing is crucial in
formulating adaptation strategies for climate change.
Kim Cobb & Jordan Watson retrieving coral core from Fanning
Island, central tropical Pacific.
Scientific Motivation: A Trend Towards Warmer and/or More
Precipitation over the 20th Century?
oxygen isotopic (d18O)
records from the central tropical Pacific exhibit late 20th
century trends towards depleted
suggesting that warming and/or freshening
have occurred in the region (Evans
et al. 1999; Urban et al. 2000; Cobb et al. 2001).
fact that this negative coral
trend is unprecedented in the last millennium (Fig.1) strongly
suggests that anthropogenic climate forcing has caused
appreciable warming and/or freshening in this region.
to better understand the
mechanism underlying this trend, I
quantify the role of
sea-surface temperature (SST) and salinity changes via paired
(a dual proxy for SST and salinity)
and Sr/Ca ratio (as a SST proxy). I derive the values of d18O
of seawater (d18Osw)
as a salinity proxy in the central tropical Pacific by
subtracting the SST contribution on coral d18O
1. Modern and fossil coral
records from Palmyra Island
spanning the last millennium. A late-twentieth-century trend
toward lower coral
values is indicative of
in the central tropical Pacific (modified after Cobb et al.
Spatial Study of Late 20th century (1972-1998) Tropical Pacific
In Nurhati et al. (2009), I
reconstructed the histories of central tropical Pacific SST and
salinity using corals from Palmyra, Fanning and Christmas
over the late 20th century (1972-1998). Collectively, the three
islands are sensitive to ENSO and decadal-scale tropical Pacific
climate variability. The islands span SST and salinity gradients
due their relative distances to equatorial ocean currents and
the Inter-Tropical Convergence Zone (ITCZ).
The late 20th century portion of this work shows warming and
freshening trends in the central tropical Pacific, in line with
climate model projections
of weakened tropical Pacific zonal SST gradient (FIg. 2).
Stronger warming trends towards the equator are consistent with
a weakening of the equatorial upwelling inferred from
instrumental data [McPhaden and Zhang, 1999]. Stronger
freshening trends towards Palmyra suggests a strengthening
and/or equatorward shift of the ITCZ that may support the
enhancement of hydrological patterns (wet area is getting
wetter, and dry area is getting drier) under global warming as
projected by a majority of IPCC climate models [Held and Soden,
2000]. See Nurhati et al. (2009) for more details on scientific
questions and findings.
2. Central tropical Pacific coral climate proxy records. (a) A
reconstruction from Palmyra Island (Cobb et al. 2003) showing
the unprecedented late 20th century trend towards lower coral
values (warmer, wetter conditions; note inverted y-axis). (b)
records from Palmyra (Cobb et al. 2001) (red), Fanning (green),
and Christmas (blue) islands (note inverted y-axis). (c) Sr/Ca-derived
SST reconstructions from Palmyra, Fanning, and Christmas. (d)
(salinity proxy) reconstructions from Palmyra, Fanning, and
Christmas. (e) The Southern Oscillation Index (SOI).
Tropical Pacific SST and Salinity over the 20th century
In Nurhati et al. (2011), I presented 20th century SST and
salinity proxy record from Palmyra Island.
visual comparison of the three coral records (Fig. 3) reveals
trend is responsible for the large trend in coral
with warming playing a secondary role.
The study highlights that central tropical Pacific SST and
salinity linkages to modes of interannual climate variability
(e.g. canonical eastern Pacific ENSO and central Pacific ENSO)
and decadal-scale variability of North Pacific phenomenon (e.g.
the Pacific Decadal Oscillation and the recently uncovered North
Pacific Gyre Oscillaion (Di Lorenzo et al. 2008).
See Nurhati et al. (2011) for more details on scientific
questions and findings.
3. Palmyra coral monthly resolved
Sr/Ca-derived SST, and
records from 1886 to 1998. (a) Palmyra coral
record (Cobb et al. 2001), (b) Sr/Ca-derived SST (black) plotted
with ERSST (gray; Smith et al. 2008), and (c)
from this work:
I. S., K.M. Cobb, and E. Di Lorenzo. (2011). Decadal-scale SST and
salinity variations in the central tropical Pacific: Signatures of
natural and anthropogenic climate change.
Journal of Climate, 24: 3294-3308, doi: 10.1175/2011JCLI3852.1.
I. S., K.M. Cobb, C.D. Charles, and R.B. Dunbar. (2011).
Correction to "Late 20th century warming and and freshening in the
central tropical Pacific." Geophysical Research Letters,
38(24), L24707, doi:10.1029/2011GL049972.
I. S. (2010).
Coral records of central tropical Pacific sea-surface temperature and
salinity variability over the 20th century. Ph.D. Dissertation. Georgia
Institute of Technology.
I. S., K.M. Cobb, C.D. Charles, and R.B. Dunbar. (2009). Late 20th
century warming and and freshening in the central tropical Pacific.
Geophysical Research Letters,
And collaborative papers from this work:
I.S. Nurhati, K.M. Cobb, H.V. McGregors, D. Sinclair, and R.M. Sherrell
Systematic ENSO-driven nutrient variability recorded by central Pacific
corals. Geophysical Research Letters, 40(1-6):
SuppInfo Chosen as a research
al. (2013). Inter-laboratory study for coral Sr/Ca and other element/Ca ratio
measurements. Geochemistry, Geophysics and Geosystems, 14(9):
R., K.M. Cobb, A.L. Cohen, W.C. Elliot, I.S. Nurhati, K.A. Rose, and
L.K. Zaunbrecher. (2011).
Effects of diagenesis on paleoclimate reconstructions from modern and
young fossil corals. Geochim. Cosmochim. Acta, 75(21): 6361-6373, doi: