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Deglacial Temperature History of West Antarctica

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Deglacial Temperature History of West Antarctica Deglacial temperature history of West Antarctica Kurt M. Cuffeya,1, Gary D. Clowb, Eric J. Steigc, Christo Buizertd, T. J. Fudgec, Michelle Koutnikc, Edwin D. Waddingtonc, Richard B. Alleye, and Jeffrey P. Severinghausf aDepartment of Geography, University of California, Berkeley, CA 94720; bGeosciences and Environmental Change Science Center, United States Geological Survey, Lakewood, CO 80225; cDepartment of Earth and Space Sciences, University of Washington, Seattle, WA 98195; dCollege of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331; eDepartment of Geosciences, The Pennsylvania State University, University Park, PA 16802; and fScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved October 19, 2016 (received for review June 6, 2016) The most recent glacial to interglacial transition constitutes a Moreover, the low accumulation rates at East Antarctic core sites remarkable natural experiment for learning how Earth’s climate have precluded convincing quantifications of the isotopic ther- responds to various forcings, including a rise in atmospheric CO2. mometers using independent information from borehole tem- This transition has left a direct thermal remnant in the polar ice peratures, the most direct legacy of past climate. One Antarctic sheets, where the exceptional purity and continual accumulation study used such information (14), but the small accumulation of ice permit analyses not possible in other settings. For Antarc- rate at the site meant that diffusive heat transport greatly domi- tica, the deglacial warming has previously been constrained only nated advection, rendering the method imprecise. by the water isotopic composition in ice cores, without an abso- Here we present a reconstruction of West Antarctic temper- lute thermometric assessment of the isotopes’ sensitivity to tem- ature history from the site of the West Antarctic Ice Sheet perature. To overcome this limitation, we measured temperatures Divide ice core (WDC) (15, 16). This site is uniquely suitable in a deep borehole and analyzed them together with ice-core data for analyses using borehole temperatures, given the combina- to reconstruct the surface temperature history of West Antarc- tion of thick ice and high snow accumulation rate, together with tica. The deglacial warming was 11:3 ± 1:8 ◦C, approximately two the wealth of information provided by the 68-ky core record. to three times the global average, in agreement with theoretical Elsewhere, similarly favorable conditions prevail only in central expectations for Antarctic amplification of planetary temperature Greenland (10). changes. Consistent with evidence from glacier retreat in South- ern Hemisphere mountain ranges, the Antarctic warming was Reconstruction EARTH, ATMOSPHERIC, mostly completed by 15 kyBP, several millennia earlier than in the We measured temperatures in the 3.4-km-deep WDC bore- AND PLANETARY SCIENCES Northern Hemisphere. These results constrain the role of variable hole (Materials and Methods). The temperature profile reveals oceanic heat transport between hemispheres during deglaciation a direct thermal remnant of the deglacial transition and subse- and quantitatively bound the direct influence of global climate quent Holocene temperature changes (Fig. 1). forcings on Antarctic temperature. Although climate models per- Because of diffusive smoothing, borehole temperatures con- form well on average in this context, some recent syntheses of tain information about only long-term averages of climatic tem- deglacial climate history have underestimated Antarctic warming peratures (SI Reconstruction Strategy). Our analysis therefore and the models with lowest sensitivity can be discounted. incorporates two additional sources of thermometric informa- tion from the core: the deuterium isotopic composition of ice climate j paleoclimate j Antarctica j glaciology j temperature (δD) and the nitrogen isotopic composition of trapped gas (δ 15N) (SI Ice-Isotopic Data and SI Nitrogen-Isotopic Data and rom the Last Glacial Maximum (LGM) around 21 ka to the Fmiddle of the Holocene, increased greenhouse gas concen- Significance trations and reduced reflectivities of the surface and atmosphere directly increased the uptake of energy to Earth’s climate system The magnitude and timing of Antarctic temperature change −2 ◦ by about 7W·m (1–3) and warmed the surface by 3–6 C on through the last deglaciation reveal key aspects of Earth’s cli- average (4–7). Although contributing little to this global average mate system. Prior attempts to reconstruct this history relied because of the comparatively small area involved, the warming on isotopic indicators without absolute calibration. To over- in polar regions holds particular interest. In addition to driving come this limitation, we combined isotopic data with mea- changes of ice sheets, permafrost, and hydrology and modulating surements of in situ temperatures along a 3.4-km-deep bore- oceanic and atmospheric circulations, polar warming partly con- hole. Deglacial warming in Antarctica was two to three times trolled both the evolution of surface reflectivity and the trans- larger than the contemporaneous global temperature change, fer of carbon dioxide from ocean to atmosphere and hence the quantifying the extent to which feedback processes amplify climate forcing itself. Further, reconstructions of polar warming global changes in polar regions, a key prediction of climate during deglaciation permit quantification of one key prediction models. Warming progressed earlier in Antarctica than in the of climate theory—that feedback processes amplify temperature Northern Hemisphere but coincident with glacier recession changes in polar regions relative to the global average (4, 8, 9), in southern mountain ranges, a manifestation of changing a phenomenon referred to as polar amplification. Arctic data oceanic heat transport, insolation, and atmospheric CO2 that reveal a warming three to four times the global average based can further test models. on a wide variety of indicators (6), including combined analyses of ice-core data and borehole temperatures (10, 11). Limited Author contributions: K.M.C. and C.B. designed research; K.M.C., G.D.C., E.J.S., C.B., T.J.F., available constraints suggest a smaller but still amplified Antarc- M.K., E.D.W., R.B.A., and J.P.S. performed research; K.M.C., G.D.C., E.J.S., C.B., T.J.F., M.K., tic warming, roughly 1.5 to 2.5 times greater than the global and E.D.W. analyzed data; and K.M.C. wrote the paper. average (6, 12). This Antarctic estimate, however, derives from The authors declare no conflict of interest. the isotopic composition of ice measured in cores. The scaling This article is a PNAS Direct Submission. between ice isotopic composition and temperature depends on a Freely available online through the PNAS open access option. great many factors (section 15.5 in ref. 13), such as the seasonal 1To whom correspondence should be addressed. Email: [email protected]. timing of snowfall and rate-dependent fractionations in clouds, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. that remain poorly known and are expected to vary with time. 1073/pnas.1609132113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1609132113 PNAS Early Edition j 1 of 6 Downloaded by guest on September 27, 2021 3500 3500 mid- to late Holocene (SI Basis Functions and Fig. S2). Magni- tudes are within ±0:5 ◦C in the final reconstruction. 3000 2011 15 3000 2014 To incorporate δ N data, the temperature history can be 2014 measured model adjusted further by an amount ∆TN determined by densification 2500 2500 physics to reconcile the accumulation rate histories derived two independent ways (Materials and Methods and SI Firn Thickness 2000 2000 Model and Use of Nitrogen Isotope Data): one from Ts (t) and 15 1500 δ N using a model for the evolving firn density and thickness 1500 and a second one from the ice core’s observed annual layer thick- 1000 nesses, corrected for strain using glaciological parameters deter- 242.6 243.0 243.4 mined in the optimization of Eq. 2. The multiplier !(t) ranges Elevation above bed (m) 500 between 0 and 1, allowing us to control the relative importance of ice and gas isotopic records. It safeguards against possible prob- 0 245 255 265 -0.02 0 0.02 lems with the nitrogen isotope method; the accuracy of ∆TN as Temperature (K) Mismatch (K) a thermometer is not well established, especially given imperfec- tions of firn densification models and irregularities of the gas- Fig. 1. Observed and modeled temperatures in the WAIS Divide borehole. trapping process at the firn base. We examined reconstructions (Left) Ice temperature profile observed in 2014. At this scale the model and with various ! values and used borehole temperatures to iden- measurements are indistinguishable, as are the profiles measured in 2011 tify the range of admissible scenarios (SI Calculation of Limits and 2014. Left Inset expands the upper portion and compares the model to and Tolerance). the 2014 observation. The cold region centered around 1,700 m is a rem- ! > 0:5 nant of the last glacial climate. (Right) Difference between observed tem- Our final reconstruction (Fig. 2) uses for all but peratures and optimal models (model minus observed), for both the 2011 the late Holocene (Materials and Methods and SI Firn Thick- and 2014 measurements. Fig. S1).
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