LETTERSPUBLISHED ONLINE: 8 AUGUST 2010 | DOI: 10.1038/NGEO931

Divergent trends in land and ocean temperature inthe Southern Ocean over the past 18,000 yearsMatt S. McGlone1*, Chris S. M. Turney2, Janet M.Wilmshurst1, James Renwick3

and Katharina Pahnke4

Over the past 18,000 years, sea surface temperatures forthe Southern Ocean do not align with those of the adjacent,high-latitude landmasses. During the late glacial period, theocean surface warmed rapidly to present-day temperatures1,whereas the land warmed only slowly, as evidenced bythe lagged response of forest and glaciers2,3. However,in the Holocene epoch, land-based records suggest strongwarming whereas marine records indicate that ocean surfacetemperatures cooled. Here we present reconstructions ofsummer temperature for Campbell Island, in the SouthernOcean, over the past 16,500 years based on fossil pollen.We find a pronounced warming 12,500–11,000 years ago, acooling until 9,200 years ago, and a rapid warming to a peakbetween 6,000 and 5,000 years ago, followed by minor coolintervals from 5,200–4,000, 3,000–1,700 and 700–100 yearsago. As expected, our temperature reconstructions show a lateglacial lag behind nearby records of sea surface temperature6,7,and opposing trends in the Holocene. We suggest that thisdiscrepancy arises because land-based reconstructions recordsummer temperatures, whereas marine proxies generallyreflect annual temperatures. We conclude that the divergenceof the records therefore reflects changes in the seasonality ofatmospheric heat transport. As atmospheric heat transport istied to the Southern Hemisphere westerly winds, we attributeour observed changes to shifts in the position and intensity ofthe southern westerlies.

The Southern Ocean region (here the ocean south of 45◦ S)experienced major changes in upwelling and westerly airflowduring deglaciation beginning 18 kyr ago and these coincide withabrupt shifts in sea surface temperatures6 (SSTs). However, ice,marine and terrestrial records suggest contrasting temperaturetrends during key periods (see Supplementary Discussion). MostSouthern Ocean marine cores register temperatures at or abovepresent-day values by 16–15 kyr ago with peak SSTs 1–3 ◦C abovepresent between 12 and 9 kyr ago1,7. Similarly, Antarctic ice-coretemperature proxies reveal peak warming between 12 and 9 kyrago, followed by cooling to a low centred on 8 to 7 kyr ago8. Incontrast, vegetation cover at high latitudes in circum-Antarcticlandmasses reflected cooler or drier conditions than now until after11 kyr ago with peak development of postglacial vegetation between8 and 5 kyr ago. Although interpretation of vegetation sequencesfrom Tierra del Fuego/Patagonia, Australia and New Zealand interms of temperature is complicated by contemporaneous changesin precipitation9,10, tree lines in these regions achieved theirhighest altitudes in themidHolocene2,11, providing strong evidencefor warmest summer temperatures at this time. Low-altitude

1Landcare Research, PO Box 40, Lincoln 7640, New Zealand, 2Climate Change and Sustainable Futures, School of Geography, University of Exeter, ExeterEX4 4RJ, UK, 3NIWA, Private Bag 14901, Wellington, New Zealand, 4Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii96822, USA. *e-mail: McGloneM@landcareresearch.co.nz.

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and low-latitude vegetation, however, seems to have respondedmore strongly to warming winters and tracks SSTs (refs 12,13),whereas glaciers in southern South America, on the sub-Antarcticislands and southern New Zealand continued to advance until atleast 10 kyr ago14–17.

Resolving these apparently conflicting temperature trendsis difficult because marine, ice core, glacier and terrestrialreconstructions are usually from locations distant from oneanother, have different response times and most probably reflectdifferent seasons. The sub-Antarctic islands provide uniqueadvantages in addressing this issue. As pinpricks of land in theSouthern Ocean, they are thermally coupled to the surroundingsea. Unlike continental locations, they have had continuously moistclimates since deglaciation and thus vegetation change is muchmore likely to reflect temperature alone.

Campbell Island (52 ◦30′ S) is a small (113 km2) landmass inthe sub-Antarctic waters south of the New Zealand mainland and

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NATURE GEOSCIENCE DOI: 10.1038/NGEO931 LETTERS

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Figure 2 | Summary pollen diagrams, inorganic matter content and summer temperature reconstructions. a,b, Data for Homestead Scarp (a) and MountHoney (b), Campbell Island. The location of accelerator mass spectrometry 14C ages is shown by arrows. Ash accumulation is a measure of the silt contentof the sediments. %TLP denotes percentage of total land pollen.

the subtropical front2 (Fig. 1). The climate is cool (mean annualtemperature 6.8 ◦C), cloudy, moist and characterized by persistent,strong westerly airflow. A dense, low (<4m tall) forest dominatedby shrubbyDracophyllum spp. forms a tree line near sea level givingway to grassland and then tundrawith increasing altitude.We inves-tigated two pollen peat sequences at temperature-sensitive locationswithin the treeline ecotone: (1) Homestead Scarp at 30m altitudein low forest; and (2) Mount Honey, at 120m altitude, close to theupper limit of continuous woody vegetation. As their reliability asproxies varies according to the prevailing vegetation, we rely pri-marily on the lower site (Homestead Scarp) for quantification of the

pre-9-kyr-ago record, and the uppermost site for the post-9-kyr-agorecord. Peat accumulation on the island began 17.5 kyr ago, im-mediately after the retreat of glaciers, and our sites provide acontinuous record of summer temperature since then (Fig. 2).

Cool, fluctuating summer temperatures averaging 2.0–2.5 ◦Clower than today prevailed between 16.5 and 12.5 kyr ago,culminating in rapid warming to an early Holocene maximumcentred on 11 kyr ago. The lowland Homestead Scarp siterecords a cooling beginning about 11 kyr ago, terminated by anabrupt temperature rise at 9.2 kyr ago that continued throughto 5.8 kyr ago, accompanied by an increase in inorganic matter

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO931

Cooling oceanWarming land

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Figure 3 | Campbell Island summer temperature records comparedagainst key palaeoclimate records from the Southern Ocean andAntarctica over the past 18 kyr. Changing summer insolation (52◦ S) is alsoshown for ref. 20. ACR and YDC denote the Antarctic Cold Reversal andYounger Dryas chronozone, respectively.

content (dominated by silt-sized particles; Fig. 2). The uplandMount Honey site shows no comparable cooling 11 kyr ago, buttemperatures also rose sharply 9.2 kyr ago, peaking between 5 and6 kyr ago (Fig. 3). Mid to late Holocene temperatures averagedabout 1 ◦C warmer (up to 2 ◦C warmer) than those in the earlyHolocene, but there were also several cool periods (5.2–4 kyr ago,3–1.7 kyr ago and 0.7–0.1 kyr ago). We compare the CampbellIsland records with the closest detailed SST marine record (MD97-2120; refs 4,5), which lies in the same sub-Antarctic water massas Campbell Island, albeit at a more northerly location (Fig. 1).

MD97-2120 shows warming by 18 kyr ago, a plateau at close topresent-day values by 14 kyr ago, then a rapid warming from 13 kyrago that peaks 2–3 ◦C warmer than now between 12 and 11 kyr ago,followed by cooling of about 1.5 ◦C by 9 kyr ago, after which therewas a steep decline to present-day SST by 8 kyr ago.

Although both island and marine records show strong warmingfrom 14 kyr ago to 11 kyr ago, there are two major differences:the marine record achieved present temperatures by 14 kyr ago,whereas the island experienced summer temperatures some 2–3 ◦Cbelow present; and the island warmed after 9 kyr ago, whereasthe ocean cooled. This contrast cannot be explained by dryconditions in the late glacial and early Holocene, as moisture-lovingvegetation was abundant; nor by thermal isolation, as no pointon the island is more than 2 km from the ocean. What thenexplains the discrepancy?

High-latitude and high-altitude vegetation is largely controlledby the summer growth season, and is much less affected bywinter18. Mean monthly SSTs vary little (annual amplitude about2 ◦C at the latitude of Campbell Island) because of the large heatcapacity of the surface waters and subsequent transfer of heat acrossthe seasons19. Marked changes in SST seasonality are thereforephysically improbable in the open Southern Ocean. Crucially, SSTsbased on the faunal composition or Mg/Ca ratios in foraminiferamainly reflect spring/summer conditions, but trend with alkenoneestimates (regarded as a proxy of mean annual temperature)from across the Southern Ocean during the Holocene1, suggestingthat marine records more closely reflect the annual mean thanany particular season.

The terrestrial bias towards the summer season in mosthigh-latitude and high-altitude temperature proxies, and the lowamplitude of the seasonal temperature cycle in the ocean, thereforehelps to explain how the observed disparities arose. Mean annualSSTs increased more rapidly during deglaciation than terrestrialsummer temperatures. The early Holocene SST peak (12–9 kyrago) therefore coincides with reduced terrestrial seasonality, coolsummers restricting forest at high altitudes and latitudes andpermitting glacier advances. After 9 kyr ago, this pattern switched:terrestrial summers warmed andmean annual SSTs fell.

Explanations for this changing seasonality pattern must showhow a small island, surrounded by a vast ocean with a mutedseasonal cycle, can have an independent summer temperaturetrend. We believe that there are only two possible explanations:direct effects of changing insolation on the island or changing heattransport by the southern westerlies.

The early Holocene at the latitude of Campbell Island hadreduced summer insolation (minimum about 4.6% less thanpresent at 10 kyr ago; Fig. 3; ref. 20). Given the small size of theisland, it seems unlikely that this small decline in summer insolationcould cool the land relative to the ocean. Furthermore, the mostrapid Holocene change in island summer temperatures took place9 kyr ago, when summer insolation had barely changed from its10-kyr-ago low point. We therefore rule out any significant directcontribution of insolation. In the sub-Antarctic islands, summertemperatures may exceed those of the surrounding ocean by adegree or more through advection of heat during summer bysouthwards-displaced cyclones, and the concomitant reduction incool air from the south21. Our preferred explanation thereforerelies on the dynamics of wind circulation and associated surfacemovement of ocean currents.

Large shifts in the latitude and intensity of the core South-ern Hemisphere westerlies have occurred since the last glacialperiod22,23. A study of high-latitude marine records (53.2◦–61.9◦ S)south of the Antarctic Polar Front showed enhanced opal accumu-lation during the late glacial, peaking 16 kyr ago6, suggesting thecore of the westerlies migrated rapidly southwards from their LastGlacial Maximum position some 5◦ latitude north of their present

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NATURE GEOSCIENCE DOI: 10.1038/NGEO931 LETTERSposition at about 50◦ S, to sit over the Antarctic CircumpolarCurrent (Fig. 1). The strongest westerly airflow then lay to thesouth of Campbell Island, reducing poleward meridional airflowand thus decreasing transport of heat. This is consistent with thecool summers and less windy conditions recorded on the islandduring the late glacial and earlyHolocene. At the same time, reducedwind velocity at the latitude of the island would have decreasedwind-driven Ekman current transport of cool water northwards24,thus contributing to the overall ocean warming trend registeredin the marine records.

About 9 kyr ago, opal accumulation declined south of thepolar front, implying an equatorwards migration of the westerlies6(Fig. 3). Wind-blown silt began to increase in the Homestead Scarpprofile, and sand and small stones abruptly appear in sea-cliffpeat profiles about 9 kyr ago on both Campbell and Aucklandislands, indicating an increase in wind strength2 (Fig. 2). Onthe other side of the Pacific at 52◦–53◦ S in Patagonia, westerlywinds increased to reach present intensities 9.2 kyr ago9,25, withmore northerly areas (41◦ S) experiencing peak wind intensitylater in the mid Holocene23. Just as the poleward position ofthe southern westerlies reduced the marine and atmospherictemperature contrast, intensified westerlies positioned directly overCampbell Island increased it.

The Southern Annular Mode24 (SAM) provides a modernanalogue (see Supplementary Information and Fig. S1). At present,westerly circulation is stronger than normal over Campbell Islandduring the extreme positive phases of SAM, and the polar vortexcontracts, leaving Campbell Island more exposed to air from lowerlatitudes. Stronger winds imply more vigorous mixing and reduceddiurnal temperature variability. Both of these factors combine toreduce the risk of low air temperatures over Campbell Island,resulting in a positive temperature anomaly on average. Theopposite occurs when the SAM is strongly negative, with weakerwesterlies and an expanded polar vortex. During positive phasesof SAM, stronger westerly winds result in enhanced Ekman-drivendrift of surface water towards the Equator, cooling the SouthernOcean at the latitude of Campbell Island. Antarctic deuterium-excess records suggest development of more intense Equator-to-pole atmospheric transport in the course of the Holocene, possiblydriven by the increase in the latitudinal insolation gradient7, whichis consistent with our observations.

Shifts in westerlies affect precipitation23, but they must have alsohad profound effects on temperature seasonality. Our results showthat terrestrial seasonal temperatures can departmarkedly fromSSTtrends in the adjacent oceans, mainly because of atmospheric heattransport. Although contrasting land–ocean temperature trendshave been noted for the Antarctic Peninsula26, we now extend thisconcept to the whole of the Southern Ocean region. We furthersuggest that terrestrial proxies that respond to mean annual orwinter, rather than summer, temperatures should follow oceantrends. Finally, the Campbell Island summer temperature recordhas a striking similarity to summer temperature reconstructionsfrom northern and central Europe27. As the summer insolationcurves were in approximate antiphase for the two hemispheres, yetthe summer temperature trendsmatch, shifting westerlies may haveplayed a similar role in both hemispheres.

MethodsPeat soils cover the whole island and preserve excellent pollen and spore records.We selected two to span the treeline ecotone. (1) Homestead Scarp is a 4-m-deepsoil peat profile at 30m altitude, in tall (2–3m) lowland scrub; and (2) MountHoney, is a 6-m-deep bog peat profile at 120m altitude, in low scrub and grasslandclose to the limit of woody cover. The Mount Honey profile was sampled witha Russian corer and the Homestead Scarp profile directly from the face of dugpit. Standard palynological treatment using KOH and acetolysis to concentratepollen residues was employed. A sum of 260 pollen grains or more per sample wascounted and the results presented as a percentage of a pollen sum including allpollen and spore types.

Sixty-two surface pollen samples were analysed for sites ranging in elevationfrom 1m to 545m above sea level28. Surface samples covered 11 vegetation typesincluding dwarf forest (>2m high), shrubland, scrub-tussock, tussock grassland,maritime tussock grassland, maritime turf, macrophyllous forbland, tundra,cushion bog with scrub, cushion bog and flush (sedge swamp). The pollen andspore spectra are not obscured by over-represented wind-dispersed taxa, whichis common in places with more complex vegetation communities. As treelinealtitudes are tightly linked to average growing season temperature, and have little orno relationship to the length of the growing season or winter temperatures18, shiftsin tree line should reflect summer temperatures.

Modern analogue techniques were used to estimate January (summer)temperatures (with an estimated uncertainty of 0.98 ◦C at 1σ ; see SupplementaryInformation and Table S1) from percentages of pollen and spores in modernsamples. The resulting equations were used to estimate January temperatures fromthe high-resolution pollen and spore percentage diagrams.

Campbell Island has had a continuous record of climate since 1941 fromthe New Zealand Meteorological Station at the head of Perseverance Harbour,within 1 km of Homestead Scarp. An estimate of January (warmest month) airtemperature for each sampling location was calculated using a formula based on theelevation of the site. The pollen percentages and estimated January temperaturesmade up the modern calibration data set.

The performance of a number of transfer function models was tested usingweighted averaging, partial least squares, weighted averaging partial least squaresand the modern analogue technique. Cross-validation by bootstrapping was usedto provide an estimate of prediction error for the training set (root mean squareerror 0.930053) and fossil samples (root mean square error of prediction 0.981059).Modern analogue (using chi-squared distance of dissimilarity, 10 closest analoguesand 1,000 bootstrapping cycles to calculate sample specific errors) gave the bestmodel performance and lowest prediction errors (Supplementary Table S1).Minimal adequate models were chosen by using the smallest number of usefulcomponents where a component should give a reduction in prediction error incross-validation of 5% or more of the root mean square error of prediction for thesimplest one-component model.

As a result of the limited flora of the island, once themost thermophilous taxon(Dracophyllum) has completely occupied a site, modern analogue temperaturereconstruction cannot identify further warming; likewise, once tundra speciesdominate a site, further cooling simply creates a sparser vegetation but withoutmarked changes in the percentage pollen spectrum. Temperature estimates aretherefore likely to be most accurate when the site lies in the transition zone betweencomplete woody cover and tundra.

For the chronologies, a minimum of 20 14C ages were obtained fromplant fragments and peat from each sequence. A sedimentation rate modelincorporating Markov Chain Monte Carlo sampling was generated to build upa distribution of possible age solutions, using the OxCal calibration program(http://c14.arch.ox.ac.uk). Using Bayes theorem, the algorithms employedsample possible solutions with a probability that is the product of the priorand likelihood probabilities. Taking into account the deposition model andthe actual age measurements made, the posterior probability densities quantifythe most likely age distributions. Ages that had a very small overlap with thelikelihood probability distribution returned an agreement index of<60% and werediscarded. A k parameter value of 1 cm−1 was chosen to reflect the variable natureof peat sedimentation. A calibrated chronology has been developed from the 19accelerator mass spectrometry 14C ages obtained for each sequence (SupplementaryTable S2 and Fig. S2).

Received 6 November 2009; accepted 8 July 2010;published online 8 August 2010

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AcknowledgementsM.S.M. and J.M.W. were supported by funds from the New Zealand Foundation forResearch, Science and Technology New Zealand, in the Ecosystem Resilience OutcomeBased Investment programme. C.S.M.T. is grateful for the Philip Leverhulme Prizethat helped support his contribution to this work. H. Jones and S. Rouillard kindlyprepared the figures.

Author contributionsM.S.M. and J.M.W. carried out the field work, M.S.M. the pollen analyses and J.M.W.the quantification; C.S.M.T. developed the chronology; J.R. undertook the atmosphericcirculation analyses and K.P. the interpretation of marine records. M.S.M. and C.S.M.T.wrote the Letter and all commented on the text.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed toM.S.M.

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