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High Arctic Holocene Temperature Record from the Agassiz Ice Cap and Greenland Ice Sheet Evolution

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High Arctic Holocene Temperature Record from the Agassiz Ice Cap and Greenland Ice Sheet Evolution High Arctic Holocene temperature record from the Agassiz ice cap and Greenland ice sheet evolution Benoit S. Lecavaliera,1, David A. Fisherb, Glenn A. Milneb, Bo M. Vintherc, Lev Tarasova, Philippe Huybrechtsd, Denis Lacellee, Brittany Maine, James Zhengf, Jocelyne Bourgeoisg, and Arthur S. Dykeh,i aDepartment of Physics and Physical Oceanography, Memorial University, St. John’s, Canada, A1B 3X7; bDepartment of Earth and Environmental Sciences, University of Ottawa, Ottawa, Canada, K1N 6N5; cCentre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark, 2100; dEarth System Science and Departement Geografie, Vrije Universiteit Brussel, Brussels, Belgium, 1050; eDepartment of Geography, University of Ottawa, Ottawa, Canada, K1N 6N5; fGeological Survey of Canada, Natural Resources Canada, Ottawa, Canada, K1A 0E8; gConsorminex Inc., Gatineau, Canada, J8R 3Y3; hDepartment of Earth Sciences, Dalhousie University, Halifax, Canada, B3H 4R2; and iDepartment of Anthropology, McGill University, Montreal, Canada, H3A 2T7 Edited by Jeffrey P. Severinghaus, Scripps Institution of Oceanography, La Jolla, CA, and approved April 18, 2017 (received for review October 2, 2016) We present a revised and extended high Arctic air temperature leading the authors to adopt a spatially homogeneous change in reconstruction from a single proxy that spans the past ∼12,000 y air temperature across the region spanned by these two ice caps. 18 (up to 2009 CE). Our reconstruction from the Agassiz ice cap (Elles- By removing the temperature signal from the δ O record of mere Island, Canada) indicates an earlier and warmer Holocene other Greenland ice cores (Fig. 1A), the residual was used to thermal maximum with early Holocene temperatures that are estimate altitude changes of the ice surface through time. These 4–5 °C warmer compared with a previous reconstruction, and reg- so-called thinning curves provide a valuable constraint on model ularly exceed contemporary values for a period of ∼3,000 y. Our reconstructions of the Greenland ice sheet (5). A key conclusion results show that air temperatures in this region are now at their of the study was that the current generation of 3D thermo- warmest in the past 6,800–7,800 y, and that the recent rate of tem- mechanical ice-sheet models fail to capture the large thinning perature change is unprecedented over the entire Holocene. The inferred at sites located closer to the ice margin, particularly in warmer early Holocene inferred from the Agassiz ice core leads to northwest Greenland (Camp Century drill site; Fig. 1A). How- an estimated ∼1 km of ice thinning in northwest Greenland during ever, the veracity of the results in ref. 5 have been brought into SCIENCES the early Holocene using the Camp Century ice core. Ice modeling question due to the possible influence of the Innuitian ice sheet ENVIRONMENTAL results show that this large thinning is consistent with our air tem- across the Canadian Arctic on the altitude correction required to perature reconstruction. The modeling results also demonstrate the infer temperature from Agassiz ice during the early Holocene broader significance of the enhanced warming, with a retreat of the (6). A second issue is that the temperature record estimated northern ice margin behind its present position in the mid Holocene from Agassiz ice using two different proxies [ice melt percent (7) and a ∼25% increase in total Greenland ice sheet mass loss (∼1.4 m and oxygen isotope content (5); see next section] gives incon- sea-level equivalent) during the last deglaciation, both of which have sistent results in the early Holocene. Here, we address these is- implications for interpreting geodetic measurements of land uplift sues by considering the influence of Innuitian ice sheet thinning δ18 and gravity changes in northern Greenland. on the O temperature reconstruction from Agassiz ice, and applying the revised reconstruction to force a model of the ice core | temperature reconstruction | Holocene climate | Greenland ice sheet Greenland ice sheet. Results and Discussion nstrumented records of temperature and environmental change Reconstructing Holocene Air Temperatures. Previous air temperature Iextend for a few centuries at most. Although these records reconstructions inferred from Agassiz ice using observations of the provide evidence of climate warming, the time span covered is relatively short compared with the centuries to millennia response Significance times of some climate system components (1). In this respect, re- constructions of temperature and environmental changes obtained from climate proxies (e.g., sediment cores, ice cores) play a com- Reconstructions of past environmental changes are important for placing recent climate change in context and testing climate plementary role to the instrumented records by providing a longer models. Periods of past climates warmer than today provide temporal context within which to interpret the magnitude and rate insight on how components of the climate system might re- of recent changes (2). Furthermore, the relatively large spatial and spond in the future. Here, we report on an Arctic climate record temporal variability captured in these reconstructions represents a from the Agassiz ice cap. Our results show that early Holocene useful dataset to test models of the climate system (3). Of par- ’ air temperatures exceed present values by a few degrees Cel- ticular interest are periods during Earth s history when the climate sius, and that industrial era rates of temperature change are was warmer than at present, as these provide information that is unprecedented over the Holocene period (∼12,000 y). We also potentially more relevant to changes in the future. demonstrate that the enhanced warming leads to a large re- In this study, we focus on the reconstruction of past climate sponse of the Greenland ice sheet; providing information on using ice cores from the Agassiz ice cap, located on Ellesmere ’ A the ice sheet s sensitivity to elevated temperatures and thus Island in the Canadian Arctic Archipelago (Fig. 1 ). This site is helping to better estimate its future evolution. of particular interest as it is located in the high Arctic, and temperature reconstructions can be compared with those from Author contributions: B.S.L., D.A.F., G.A.M., and B.M.V. designed research; B.S.L., D.A.F., more southerly locations to estimate polar amplification of cli- and L.T. performed research; B.S.L., L.T., P.H., J.Z., J.B., and A.S.D. contributed new re- mate in the past (4). Furthermore, it is located proximal to the agents/analytic tools; B.S.L., D.A.F., D.L., B.M., and J.Z. analyzed data; and B.S.L., D.A.F., Greenland ice sheet, and so can be used to better constrain the G.A.M., and D.L. wrote the paper. climate forcing used to model the past evolution of this ice sheet. The authors declare no conflict of interest. 18 In a recent study (5), δ O measurements in ice from the This article is a PNAS Direct Submission. Agassiz (81°N) and Renland (70°N) ice caps (Fig. 1A) were used 1To whom correspondence should be addressed. Email: [email protected]. to estimate temperature records for these locations throughout This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the Holocene. The two time series were remarkably similar, 1073/pnas.1616287114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1616287114 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 −80˚ −60˚ 80˚ −40˚ −20˚ 0˚ substantially higher temperatures during the early Holocene 9 9 ABAgassiz 8 8 compared with preindustrial values. Although there are few 80˚ 7 7 quantitative reconstructions of high Arctic air temperatures for C) o Camp Century 6 6 5 5 4 4 GRIP/GISP2 3 3 70˚ HTM 2 2 70˚ Renland 1 1 0 0 A Temperature anomaly ( 7 C) −1 −1 6 o −2 −2 5 60˚ 60˚ −12 −10 −8 −6 −4 −2 0 4 −60˚ −40˚ Time (kyr before present) 3 2 Fig. 1. Location map and Agassiz proxy temperature records: (A) Map 1 showing the study area with the names and locations of ice core borehole 0 sites mentioned in the text. (B) The 25-y resolution, elevation-corrected −1 Agassiz δ18O temperature reconstruction (dark red) with 2σ uncertainty −2 Temperature anomaly ( (light red) and the elevation-corrected Agassiz melt record (green), both −3 C) extended to 2009 CE. Ref. 5’s δ18O Agassiz–Renland temperature re- o −4 B construction is also shown (blue) for comparison. Each record is referenced to its preindustrial temperature value at 1750 CE. −5 −6 −7 18 melt layers (7) and the δ O record (5, 8) are inconsistent in the −8 B ) early Holocene (Fig. 1 ). The melt record indicates temperatures −9 peaking in the early Holocene (∼11 ky) with a steady decline until 188 −2 Summer temperature ( −10 C 8 ky (7, 9); whereas, the earlier reconstruction (5) shows air tem- 186 (W m peratures reaching a maximum between 8 and 9 ky. The melt- N 184 o record reconstruction is a proxy for summer (June, July, August) 80 temperatures, and is derived using a lineartransferfunctionrelating 182 melt percent to summer air temperatures along with the present- 180 Methods 14 day lapse rate correction at the surface of the ice cap ( and D 178 Fig. S1). This technique can only be used to quantify summer air 12 temperatures in the range −8 °C (no melt below this temperature) 176 10 Annual average I to about −3 °C (100% melt above this temperature). In contrast, the δ18O record is a proxy for mean annual air temperature and 8 spans a much larger temperature range. However, these differences 6 between the two proxies do not reconcile the discrepancies between Pollen (grains/L) 4 the earlier δ18O-based temperature reconstructions and the melt reconstruction shown in Fig.
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