5
LETTERS PUBLISHED ONLINE: 21 DECEMBER 2008 DOI: 10.1038/NGEO390 Unprecedented recent warming of surface temperatures in the eastern tropical Pacific Ocean Jessica L. Conroy 1 * , Alejandra Restrepo 2 , Jonathan T. Overpeck 1,3,4 , Miriam Steinitz-Kannan 5 , Julia E. Cole 1,4 , Mark B. Bush 2 and Paul A. Colinvaux 6 Through its intimate connection with the El Niño/Southern Oscillation system, climate variability in the tropical Pacific Ocean influences climate across much of the planet. But the history of temperature change in the tropical Pacific Ocean during recent millennia is poorly known: the available annually resolved records 1,2 are discontinuous and rarely span more than a few centuries. Longer records at coarser temporal resolution suggest that significant oceanographic changes, observed at multi-year to multi-century resolution, have had important effects on global climate 3–5 . Here we use a diatom record from El Junco Lake, Galápagos, to produce a calibrated, continuous record of sea surface temperature in the eastern tropical Pacific Ocean at subdecadal resolution, spanning the past 1,200 years. Our reconstruction reveals that the most recent 50 years are the warmest 50-year period within the record. Because our diatom-based sea surface temperature index resembles Northern Hemisphere temperature reconstructions, we suggest that with continued anthropogenic warming, the eastern tropical Pacific Ocean may continue to warm. The eastern equatorial Pacific (EEP) lies under the descending limb of the atmospheric Walker Cell 6 , an area where southeasterly trade winds push warm surface water to the west, allowing colder subsurface water to rise to the surface. During an El Niño event, EEP sea surface temperature (SST) warms as trade winds slacken, upwelling weakens and a deeper mixed layer replaces the shallow thermocline. This thermal anomaly produces atmospheric teleconnections that influence climate across the globe 7 . Recent analyses suggest that Walker Circulation is weakening as a result of human-induced global warming 8 . Weaker Walker Circulation should lead to reduced upwelling and warmer SST in the EEP, yet the instrumental record of EEP SST is reliable for only a century, and this is too short to detect long-term change with confidence. To understand the full range of natural variability in EEP SST and related teleconnections, we require the longer records that palaeoclimate proxies can provide. The Galápagos Islands are located on the equator, 960 km west of the South American coast, an ideal location for investigating past changes in EEP climate (Fig. 1). Seasonally, Galápagos climate is influenced by the migration of the Intertropical Convergence Zone, which weakens trade winds from December to May, leading to warmer SST and increased convection. On interannual timescales, the El Niño/Southern Oscillation (ENSO) system dominates Galápagos climate variability. During El Niño events, the islands experience a large increase in precipitation associated with warmer 1 Department of Geosciences, The University of Arizona, Tucson, Arizona 85721, USA, 2 Department of Biological Sciences, Florida Institute of Technology, Melbourne, Florida 32901, USA, 3 Institute for Environment and Society, The University of Arizona, Tucson, Arizona 85721, USA, 4 Department of Atmospheric Sciences, The University of Arizona, Tucson, Arizona 85721, USA, 5 Department of Biological Sciences, Northern Kentucky University, Highland Heights, Kentucky 41099, USA, 6 Ecosystem Center, Marine Biology Laboratory, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. *e-mail: [email protected]. SST and increased convection; during very strong El Niño events, monthly precipitation can be an order of magnitude greater than rainfall during non-El Niño months 9 . We retrieved a 3.5 m lake sediment core from El Junco Lake, Galápagos, in September 2004 (Fig. 1). The crater holding El Junco Lake is 675 m above sea level, near the southwest summit of San Cristóbal Island (0.9 S, 89.5 W). Low-lying stratocumulus clouds typically blanket El Junco in a dense fog that limits evaporation but does not bring substantial precipitation to the lake catchment area. Local observations and simulations of lake level changes in El Junco reveal that lake level is highly sensitive to increases in precipitation related to El Niño events; lake level increases to 6 m, the height of the overflow channel, during El Niño events and gradually decreases between events 10 . We created an age model for the El Junco sediment core using radiocarbon, 210 Pb and 137 Cs radiometric dating and sampled the core at 0.5 cm resolution for diatom analysis 10 (see the Methods section). Five diatom species comprise 96% of the El Junco diatom community: Frustulia saxonica, Brachysira serians, Pinnularia biceps, Eunotia pectinalis and Encyonema minutum (Fig. 2). Multiple plankton tows indicate that the first three species comprise the open water plankton assemblage of the lake. Although they are epipelic taxa, they are constantly suspended in the water column owing to wind-driven mixing, and are considered tychoplanktonic. E. pectinalis and E. minutum are epiphytic and found attached to littoral aquatic vegetation (see Supplementary Information, Table S1). Using counts of the five dominant species in the El Junco core, we created a time series of the ratio of tychoplanktonic to epiphytic diatoms (T/E). We observe a significant (95% confidence level), positive relationship between the T/E index, Galápagos SST, precipitation, and the Niño1+2, Niño3 and Niño3.4 indices 11 (Fig. 3). When we detrend the instrumental and T/E time series, the relationships between T/E and Galápagos SST, precipitation and Niño1+2 all remain significant at the 95% confidence level. However, the detrended relationship between T/E and Niño3 is significant only at the 90% confidence level, and the detrended relationship between T/E and Niño3.4 is below 90% confidence level significance. The correlation between annual Niño3.4 and Galápagos SST is also lower, and there is no significant relationship between detrended Niño3.4 and Galápagos SST (see Supplementary Information, Table S2). We hypothesize that the strong correlations between El Junco diatoms and EEP climate is mainly a function of changing lake level. 46 NATURE GEOSCIENCE | VOL 2 | JANUARY 2009 | www.nature.com/naturegeoscience © 2009 Macmillan Publishers Limited. All rights reserved.

Unprecedented recent warming of surface temperatures in the eastern tropical Pacific Ocean

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LETTERSPUBLISHED ONLINE: 21 DECEMBER 2008 DOI: 10.1038/NGEO390

Unprecedented recent warming of surfacetemperatures in the eastern tropical Pacific OceanJessica L. Conroy1*, Alejandra Restrepo2, Jonathan T. Overpeck1,3,4, Miriam Steinitz-Kannan5,Julia E. Cole1,4, Mark B. Bush2 and Paul A. Colinvaux6

Through its intimate connection with the El Niño/SouthernOscillation system, climate variability in the tropicalPacific Ocean influences climate across much of the planet. Butthe history of temperature change in the tropical Pacific Oceanduring recent millennia is poorly known: the available annuallyresolved records1,2 are discontinuous and rarely span more thana few centuries. Longer records at coarser temporal resolutionsuggest that significant oceanographic changes, observed atmulti-year to multi-century resolution, have had importanteffects on global climate3–5. Here we use a diatom record fromEl Junco Lake, Galápagos, to produce a calibrated, continuousrecord of sea surface temperature in the eastern tropicalPacific Ocean at subdecadal resolution, spanning the past1,200 years. Our reconstruction reveals that the most recent50 years are the warmest 50-year period within the record.Because our diatom-based sea surface temperature indexresembles Northern Hemisphere temperature reconstructions,we suggest that with continued anthropogenic warming, theeastern tropical Pacific Ocean may continue to warm.

The eastern equatorial Pacific (EEP) lies under the descendinglimb of the atmospheric Walker Cell6, an area where southeasterlytrade winds push warm surface water to the west, allowingcolder subsurface water to rise to the surface. During an El Niñoevent, EEP sea surface temperature (SST) warms as trade windsslacken, upwelling weakens and a deeper mixed layer replaces theshallow thermocline. This thermal anomaly produces atmosphericteleconnections that influence climate across the globe7. Recentanalyses suggest that Walker Circulation is weakening as a resultof human-induced global warming8. Weaker Walker Circulationshould lead to reduced upwelling and warmer SST in the EEP, yetthe instrumental record of EEP SST is reliable for only a century,and this is too short to detect long-term change with confidence.To understand the full range of natural variability in EEP SSTand related teleconnections, we require the longer records thatpalaeoclimate proxies can provide.

The Galápagos Islands are located on the equator, 960 km westof the South American coast, an ideal location for investigating pastchanges in EEP climate (Fig. 1). Seasonally, Galápagos climate isinfluenced by the migration of the Intertropical Convergence Zone,which weakens trade winds from December to May, leading towarmer SST and increased convection. On interannual timescales,the El Niño/Southern Oscillation (ENSO) system dominatesGalápagos climate variability. During El Niño events, the islandsexperience a large increase in precipitation associated with warmer

1Department of Geosciences, The University of Arizona, Tucson, Arizona 85721, USA, 2Department of Biological Sciences, Florida Institute of Technology,Melbourne, Florida 32901, USA, 3Institute for Environment and Society, The University of Arizona, Tucson, Arizona 85721, USA, 4Department ofAtmospheric Sciences, The University of Arizona, Tucson, Arizona 85721, USA, 5Department of Biological Sciences, Northern Kentucky University,Highland Heights, Kentucky 41099, USA, 6Ecosystem Center, Marine Biology Laboratory, Woods Hole Oceanographic Institution, Woods Hole,Massachusetts 02543, USA. *e-mail: [email protected].

SST and increased convection; during very strong El Niño events,monthly precipitation can be an order of magnitude greater thanrainfall during non-El Niño months9.

We retrieved a 3.5m lake sediment core from El JuncoLake, Galápagos, in September 2004 (Fig. 1). The crater holdingEl Junco Lake is 675m above sea level, near the southwest summitof San Cristóbal Island (0.9◦ S,89.5◦W). Low-lying stratocumulusclouds typically blanket El Junco in a dense fog that limitsevaporation but does not bring substantial precipitation to thelake catchment area. Local observations and simulations of lakelevel changes in El Junco reveal that lake level is highly sensitiveto increases in precipitation related to El Niño events; lake levelincreases to 6m, the height of the overflow channel, during El Niñoevents and gradually decreases between events10.

We created an age model for the El Junco sediment coreusing radiocarbon, 210Pb and 137Cs radiometric dating and sampledthe core at 0.5 cm resolution for diatom analysis10 (see theMethods section). Five diatom species comprise 96% of theEl Junco diatom community: Frustulia saxonica, Brachysira serians,Pinnularia biceps, Eunotia pectinalis and Encyonema minutum(Fig. 2). Multiple plankton tows indicate that the first three speciescomprise the open water plankton assemblage of the lake. Althoughthey are epipelic taxa, they are constantly suspended in thewater column owing to wind-driven mixing, and are consideredtychoplanktonic. E. pectinalis and E. minutum are epiphytic andfound attached to littoral aquatic vegetation (see SupplementaryInformation, Table S1). Using counts of the five dominant speciesin the El Junco core, we created a time series of the ratio oftychoplanktonic to epiphytic diatoms (T/E).

We observe a significant (95% confidence level), positiverelationship between the T/E index, Galápagos SST, precipitation,and the Niño1+2, Niño3 and Niño3.4 indices11 (Fig. 3). When wedetrend the instrumental and T/E time series, the relationshipsbetween T/E and Galápagos SST, precipitation and Niño1+2 allremain significant at the 95% confidence level. However, thedetrended relationship between T/E andNiño3 is significant only atthe 90% confidence level, and the detrended relationship betweenT/E and Niño3.4 is below 90% confidence level significance.The correlation between annual Niño3.4 and Galápagos SSTis also lower, and there is no significant relationship betweendetrended Niño3.4 and Galápagos SST (see SupplementaryInformation, Table S2).

We hypothesize that the strong correlations between El Juncodiatoms and EEP climate is mainly a function of changing lake level.

46 NATURE GEOSCIENCE | VOL 2 | JANUARY 2009 | www.nature.com/naturegeoscience

© 2009 Macmillan Publishers Limited. All rights reserved.

NATURE GEOSCIENCE DOI: 10.1038/NGEO390 LETTERS

Correlation coefficient

El JuncoUrvina Bay

180° W 120° W 60° W 0° 60° E 120° E 180° E

¬0.6 ¬0.5 ¬0.4 ¬0.3 ¬0.2 0.2 0.4 0.60.3 0.5

60° N

30° N

30° S

60° S

24 °C

26 °C

25 °C

23 °C

4° S

2° S

2° N

92° W 88° W90° W

a b

Figure 1 | Maps of study area. a, Map of correlation coefficients of annual SST (ref. 11) near San Cristóbal Island (0.9◦ S,89.5◦W), with global SST (ref. 11)1910–2004. Map created in Climate Explorer (http://climexp.knmi.nl). b, Map of annual average SST around the Galápagos Islands, modified from ref. 19,including the location of El Junco and Urvina Bay.

Yea

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0 2 4 6 80 5 10 15¬4 ¬2 0 2 40 20 40 60 800 20 40 600 5 10 150 20 400 20 40% F. saxonica % B. graveolens% continental

pollenln T/E % E. minutum % E. pectinalis% P. biceps% B. serians

Figure 2 | Time series of key El Junco diatom and pollen abundances. Warm SST intervals are highlighted in red.

Observational data reveal that warmer SST brings greatly enhancedprecipitation to theGalápagos through enhanced convection, whichraises El Junco lake level. With rising lake levels, the abundanceof tychoplanktonic diatoms increases relative to epiphytic diatomsas the area of open water habitat increases relative to the area ofthe littoral habitat12. With decreasing lake level, the littoral habitatencroaches on the centre of the lake13 (the core location), causingthe deposition of more epiphytic diatoms. Thus, the ratio of thenumber of tychoplanktonic to epiphytic diatoms in El Junco canbe an indicator of fluctuating lake level12. We therefore interpretlarger T/E values to reflect higher lake level, increased precipitationand warmer EEP SST. However, given the temporal resolutionof our record, we cannot separate El Niño-related changes fromlower-frequency climate variability.

The El Junco diatom record ends 1.2m down the sedimentcore, at about ad 730. Few diatoms are preserved in the sedimentbelow this depth. We present diatom-inferred SST values fromonly ad 800 to present, as before ad 800, diatom valves are scarce.We document five periods of elevated T/E values: (ad) 800–1000,1320–1470, 1650–1690, 1740–1820 and 1890 to present (Fig. 4). Thelast 50 years of the El Junco record have the highest T/E values ofthe entire record, indicating the last 50 years were very likely thewarmest 50-year period of the past 1,200 years (mean value of thelast 50 years is >2σ above the average 50-year mean for all other50-year periods from ad 800 to present).

Fossil pollen data from the El Junco core support our diatomresults (Fig. 2). Bursera graveolens, a lowland species that flowersonly with seasonal or El Niño rains14, has its highest amplitudepeaks in abundance during periods of warmer diatom-inferred SST,particularly in the last 50 years. An increase in convective airflow,which accompanies warmer SST and increased rainfall, is alsorequired to transport lowland B. graveolens pollen to the highlands.The abundance of continental pollen, carried in the trade winds

from South America, also increases during most periods of warmSST and increased rainfall, probably due to increased convectionentraining more pollen in the atmosphere and increased rainoutdeliveringmore pollen to a larger lake surface.

Periods of inferred warmer SST, stronger convection andincreased precipitation could be interpreted as periods of increasedEl Niño frequency. However, various El Niño reconstructions donot generally support this interpretation over the past 1,200 years.The grain size records of ENSO frequency from El Junco10and Laguna Pallcacocha in the Andes15 suggest more El Niñoevents from ad 1000–1300, when inferred SST, precipitation andconvection are low (Fig. 4). Another record of ENSO fromthe Peru Pacific margin4 suggests more El Niño events fromad 1300–1500, when our diatom-inferred SST and pollen dataindicate precipitation and convection are high. Only the El Juncosand record indicates more El Niño events from ad 800–1000,when our inferred EEP SST, precipitation and convection arehigh. None of the records indicates an increase in the number ofEl Niño events during the recent warming trend from ad 1890to present. Independent of interannual variability, increased SST,precipitation and convection may suggest a warmer mean stateof the EEP, perhaps due to weaker Walker Circulation8 or amore southerly Intertropical Convergence Zone16,17, which wouldweaken the trade winds, deepen the thermocline and increase localconvection and precipitation.

If we use our calibrated T/E index as a direct proxy for GalápagosSST, an increase of 0.5 ◦C is implied over the last 50 years, anda warming of 1 ◦C since the beginning of the nineteenth century.We do not create a time series of SST for the past 1,200 yearsbecause we lack a modern calibration data set from a series oflakes (El Junco is the only freshwater lake in the Galápagos), andour downcore calibration is based on only 20 data points. Yetthe observed warming trend in the El Junco diatom record is

NATURE GEOSCIENCE | VOL 2 | JANUARY 2009 | www.nature.com/naturegeoscience 47© 2009 Macmillan Publishers Limited. All rights reserved.

LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO390

SST (°C

)

Year

Precipitation (m

m yr ¬

1)ln

T/E

2

3

456

Year

r = 0.79

r = 0.79

r = 0.78

r = 0.67

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r = 0.58

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)

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¬1

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o1+

2 (°

C)

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o3.4

(°C

)ln

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a b

Figure 3 | El Junco T/E diatom values and climatic variables. a, Time series of T/E and Galápagos SST. Red (SST) and black (T/E) horizontal lines indicatethe mean of both time series. Error bars on T/E values indicate 2 sigma age model errors. Monthly SST data are averaged according to the age of each0.5 cm sediment sample interval. We used SST (ref. 11) for the two grid cells surrounding San Cristóbal (0◦, 88–90◦W) from 1910–2004. b, Time series ofT/E values, Galápagos SST, Galápagos precipitation9,28, the Niño1+2, Niño3 and Niño3.4 indices11. Correlation coefficients indicate the relationshipbetween the T/E values and instrumental data.

similar to that implied by a trend towards more negative δ18Ovalues observed in most coral records across the tropical Pacificand Indian oceans2. These increasingly negative δ18Ovalues indicateSST warming and freshening over the past two centuries. One ofthe few coral records that does not suggest a warming trend overthe past two centuries is located in the Galápagos18. However, thiscoral record was collected from an area of anomalously cool SST inthe western part of the archipelago (Fig. 1b), caused by shoaling ofthe Equatorial Undercurrent19; this coral may be insulated from alonger-term, atmosphere-forced trend owing to a continual supplyof cooler upwelled water.

The diatom-inferred SST record agrees with aspects of acoral-based SST reconstruction from Palmyra Atoll in the centraltropical Pacific (Fig. 4). Both records suggest unprecedentedwarmth in the last 50 years compared with the past 1,000 years.An abrupt increase in δ18O values in a coral segment at ad 1700also coincides with an abrupt decline in T/E values. However,our diatom data suggest warmer EEP SST during the periodad 930–960, when coral δ18O values suggest the coolest centralPacific SST of the past millennium. The discrepancy is significantbecause this period of cooler SST inferred from the coral oxygenisotopes is a key record that suggests the tropical Pacific wasmore ‘La Niña-like’ during the Medieval era (ad ∼800–1250). ALa Niña-like state of the tropical Pacific during this interval wouldbe consistent with evidence of increased solar forcing and the oceanthermostat mechanism20–22. However, our diatom-based EEP SSTindex indicates a more heterogeneous Medieval era with episodesof both warmer and cooler SST.

Our diatom-based SST index also resembles a compositeof reconstructed Northern Hemisphere temperature anomalies23(Fig. 4). The pervasiveness of recent warmth is also supportedby the only other high-resolution, tropical proxy records oftemperature, those from ice cores24,25 (Fig. 4). The similaritybetween the Northern Hemisphere temperature records, tropicalice core records and the El Junco diatom index demonstrates thatrecent unprecedented warming extends from the high northernlatitudes, through the tropics and into the Southern Hemisphere.Our observations thus support the view that as the atmospherewarms, so too does EEP SST.

Potential mechanisms for the recent warming trend includethe observed weakening of the Walker Circulation in the pasttwo centuries, which could lead to a reduction of EEP upwellingand warmer EEP SST (ref. 8). Or, the observed warming may bepan-tropical, as demonstrated by the warming trend recorded incoral records across the Pacific and Indian oceans2. It is also possiblethat periods of increased ENSO frequency could be expressed in ourlower-resolution record as periods of warmer EEP SST. Althoughour results do not address the interannual variability of ENSO, theysupport the argument that with continued global warming, EEP SSTwill also continue to increase.

MethodsWe sampled 0.25 cm3 of wet sediment for diatom analysis continuously downcoreat 0.5 cm intervals and pretreated the sediment samples with 30% H2O2 forseveral hours on low heat with cold digestion continued for two to three days.Coverslips were prepared with the Battarbee method26, in which a known volumeof sediment, suspended in a known volume of deionized water, is pipetted onto

48 NATURE GEOSCIENCE | VOL 2 | JANUARY 2009 | www.nature.com/naturegeoscience

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NATURE GEOSCIENCE DOI: 10.1038/NGEO390 LETTERS

Warmer SST

Cooler SST

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°C

%

More El Niños

Less El Niños

More El Niños

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¬0.5

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yra

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l δ18

O (‰

)

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ture

an

omal

y (°

C w

rt 1

961¬

1990

)La

guna

Pal

lcac

ocha

col

our

reco

rd

(Red

inte

nsity

uni

ts)

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40

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¬4

6

El Junco % sand

Tropical South A

merican

ice core δ18O

(‰)

Peru lithic flux (%

of maxim

um)

EEP

SST

inde

x (l

n T

/E)

Figure 4 | Comparison of El Junco T/E, tropical Pacific and Northern Hemisphere climate records. a, Per cent overlap of the multi-decadal timescaleuncertainty ranges of 10 reconstructed Northern Hemisphere (NH) temperature anomaly time series23. Instrumental temperature is shown with the solidblack line. b, 10-year average δ18O values (h) from the Huascarán and Quelccaya ice cores24,25. c, Annual, uncorrected Palmyra coral δ18O values (h)from the central tropical Pacific1. d, 20-year smooth of lithic flux from the Peru Pacific margin4. e, 20-point smooth of red intensity units from LagunaPallcacocha, Ecuador15. f, El Junco sand record, an indicator of El Niño events10. High sand abundance in the nineteenth century is probably due to humanland use. g, El Junco T/E index.

coverslips in a modified ‘Battarbee chamber’ of known area and then evaporated.We mounted dried coverslips in Zrax diatom mountant (refractive index >1.7).We counted at least 300 diatom valves in each sample at ×1,000 under immersionoil. Where diatoms became increasingly scarce before ad 800, we counted the slidesat×400. If>50% of the valve was present, we counted it as one valve. We analysedthe pollen using standard pollen preparation procedures27. We counted at least250 grains per sample. The age model is derived from the same age model described

in detail in ref. 10, using radiocarbon ages from bulk sediments and 210Pb, the137Cs bomb-spike and post-bomb radiocarbon measurements on the uppermostsediments. Individual sample ages were calculated by fitting a smooth curve tothe chronological data. An abrupt, 1.5-fold increase in pollen concentration from28 to 30 cm indicated a slower sedimentation rate than calculated from the agemodel, so we applied a sedimentation rate correction to the age model based on themean pollen concentration in the adjacent sample depths. We calculated a slower

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO390

sedimentation rate for this short depth interval by adding the extra centimetresrepresented by the pollen spikes. The T/E index values are skewed and are thusexpressed with a log transformation. To facilitate the comparison between T/Eand the instrumental data, we interpolated the irregularly spaced (in time) T/Evalues to 5-year resolution and averaged the instrumental data to match thesame 5-year intervals. We compare the diatom data with Galápagos SST from1910–2004, as before 1910, the number of observations per year drops off to nearzero (see Supplementary Information, Discussion). All age model and diatom dataare available online at the World Data Center for Paleoclimatology in Boulder,Colorado, USA. (http://www.ncdc.noaa.gov/paleo/paleo.html).

Received 3 October 2008; accepted 18 November 2008;published online 21 December 2008

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AcknowledgementsWe are grateful for the field assistance of M. Miller, J. Weiss, H. Barnett, T. Damassa,B. Fonseca and R. Smittenberg. Thanks to A. Cohen and J. Sachs for valuable discussionand helpful comments, W. Gosling for Galápagos climate data and M. Brenner andZ. Zhang for chronological data. Special thanks to the Galápagos National Park andthe Charles Darwin Research Station for logistic support, The University of ArizonaDepartment of Geosciences for extra funding and the University of Arizona AMS Facilityfor radiocarbon dates. This research was financially supported in part by a NationalScience Foundation Graduate Research Fellowship, as well as a grant from the ClimateProgram Office of NOAA.

Author contributionsP.A.C. first discovered El Junco, named the lake, and motivated the current research.P.A.C., J.T.O., J.E.C., M.B.B. and M.S.-K. planned the project. J.T.O., M.S.-K., J.L.C.and P.A.C. participated in the expedition to the Galápagos. J.T.O. and J.L.C. collectedthe sediment cores. J.L.C. developed the age model, collected the diatom data and wrotethe paper. A.R. collected the pollen data. All authors commented on the manuscript andhelped analyse the results.

Additional informationSupplementary Information accompanies this paper on www.nature.com/naturegeoscience.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions. Correspondence and requests for materials should beaddressed to J.L.C.

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