This grant will support a post-doctoral investigator to develop a major new synthesis of proxy climate records of Holocene climate variability from across the Arctic, and compare the climate reconstruction with climate model simulations. The synthesis will build on proxy climate records currently being developed by teams of researchers looking at the past 8000 years of records. The data-model comparison will draw from newly available transient climate simulations by NCAR?s Community Climate System Model (CCSM3), and from the output of new model experiments planned for 2012. The Arctic climate reconstructions generated by this project will be integrated into similar syntheses from across the globe, as envisioned by two on-going international efforts led by the IGBP-Past Global Changes (PAGES) program ? the ?Two Millennia? and the ?SynTraCE? projects.

Intellectual Merit. Depending on her/his background and career goals, the postdoctoral scientist will choose among the following research questions: (1) Was the climate transition from the Holocene thermal maximum (HTM) to the Neoglacial (5000 to 3000 years ago) uniform across the Arctic? Describing the spatial-temporal pattern of this major climate transition is necessary to assess the extent to which the climatic shift was the result of a dynamical redistribution of temperature by ocean-atmosphere circulation versus a shift in the mean state of climate across the Arctic. (2) Was Neoglacial cooling amplified in the Arctic compared with elsewhere around the world? Assessing the magnitude of cooling in the Arctic is necessary to understand the feedbacks associated with arctic amplification. (3) How does the latitudinal gradient of cooling from the HTM to the Neoglacial compare with the gradient of cooling driven by precessional forcing as simulated by CCSM3 in the SynTraCE experiment? Quantifying this gradient provides a straightforward target for a data-model comparison. (4) Are the magnitude and pattern of climate variability that are reconstructed from proxy records on timescales of decades to centuries from across the Arctic consistent with the variability simulated in CCSM3 SynTraCE experiment? Analyzing the principal modes of variability in both the proxy data and the model simulation is necessary to determine the extent to which climatic changes can reasonably be attributed to natural variability of known modes of atmospheric-ocean circulation. (5) In what ways can the output of available climate system simulations be used as boundary conditions in mechanistic models of environmental change that approximate the processes that drive the proxy indictors? Developing new ?inverse-proxy models? to predict how climate change drives proxy indicators such as glacier extent, oxygen-isotope values of precipitation, and primary productivity of Arctic lakes is necessary to avoid major assumptions that underlie classical regression-based approaches to climate reconstructions.

Broader Impact. This project will contribute to understanding climatic variability, a key challenge facing society. The results will provide benchmarks for validating climate models and for improving their ability to accurately simulate nonlinear change. Improving models for future climate projections will contribute to the ?Climate Variability and Change? major program element of the US Climate Change Science Program. The entire funding request for this proposal is to support a postdoctoral scientist (PS). This early-career scientist will receive first- hand training as part of an international, interdisciplinary team of climate scientists. S/he will learn about a broad range of both data and model-based approaches to studying Arctic and global climate change. During the fall/winter of 2011, the PS will work at the PAGES International Project Office in Bern Switzerland, where the PI is scheduled as a Guest Scientist and where they will be well situated to engage an international group in a synthesis of proxy records. The PS will also work with climate modelers at NCAR in Boulder, Colorado where s/he will learn to analyze the output of climate model simulations. At NAU, the PS will be integrated into the full breadth of the PI?s research program, which includes an active analytical laboratory and field research program. The PS also will be encouraged to help regularly teach undergraduate and graduate courses to improve his/her teaching skills.

Project Report

Large community-based syntheses are exceedingly valuable for assessing the state of scientific knowledge and motivating important discoveries. In climate science, proxy-based reconstructions of past climate changes provide insights into externally forced and intrinsic variability over regional to global scales, and are used to place recent trends in a long-term context. Comparisons between these reconstructions and the output of Earth system models provide evaluation opportunities to improve our understanding of climate forcings on time scales that are not adequately represented by the instrumental record. They also provide a tool to explore mechanisms of climate variability, with implications for future climate projections. The sole purpose of this award was to support a postdoctoral scientist for about 2 years to develop a major new synthesis of existing proxy records of Holocene (approximately the last 10,000 years) climate variability from across the Arctic. The postdoc (McKay) and the PI (Kaufman) led the Arctic Holocene Transitions (AHT) project, an international effort to investigate feedbacks within the Arctic climate system that lead to non-linear changes. This award also enabled McKay and Kaufman to integrate paleoclimate data from the Arctic into a global network of similar data, which is emerging through the IGBP Past Global Changes (PAGES) 2k project A major database of Arctic Holocene proxy climate records was published in the open-access journal, Climate of the Past. The database presents a systematic compilation of 168 sites from north of 58°N latitude where proxy time-series extend back at least to 6000 years (nearly half extend 10,000 years) and are resolved at sub-millennial scale (Fig. 1). This community resource provides ready access to a large volume of information in a coherent and logical format. It is available publically through the NOAA Paleoclimatology and is now being used to address a variety of research questions related to climate change in the Arctic. A preliminary analysis of the spatial-temporal pattern of climate change represented by the AHT dataset focuses on the transition from the Holocene thermal maximum (early Holocene) to the Neoglacial (5000 to 3000 years ago). To evaluate the spatial and temporal pattern of the onset of Neoglaciation, McKay adapted a broken-stick regression technique to objectively identify when and where cooling began or was accelerated at each of the proxy sites. The synthesis shows several distinct pulses of Neoglacial (mid Holocene) cooling across the Arctic (Fig. 2). In part, individual pulses correspond with cooling in different regions. In addition to the Holocene-long, Arctic-focused synthesis of proxy climate time series, this award enabled McKay and Kaufman to take leadership roles in the PAGES 2k synthesis of continental-scale temperature reconstructions, the most comprehensive evaluation to date of temperature change at the surface of Earth’s continents over the past one to two thousand years. McKay analyzed the proxy climate records in the PAGES 2k database to extract the most prominent trends, which were published in Nature Geosciences (Kaufman was corresponding lead author for 78 co-authors). The synthesis was featured prominently in the paleoclimate chapter of the recent (2013) report by the Intergovernmental Panel on Climate Change. The most coherent feature in nearly all of the PAGES 2k regional temperature reconstructions (including the Arctic region) is a long-term cooling trend, which ended late in the 19th century. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between 1580 and 1880 CE, punctuated in some regions by warm decades during the 18th century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling. Following the publication of the PAGES 2k global synthesis, McKay and Kaufman published an updated and expanded database of high-resolution, temperature-sensitive proxy records from the Arctic in the data-oriented outlet, "Scientific Data." This approach contributed to the "Framework for community-based climate reconstructions during the past two millennia," which was led by Kaufman on behalf of the PAGES 2k community and published in EOS Transactions. The article describes challenges and opportunities related to high-resolution climate reconstructions, and it recommends four specific directions in the near term to achieve the next synthesis of climate reconstructions globally. The PAGES 2k project was also featured in a full-length documentary film, "Taking Earth’s Temperature: Delving into Climate’s Past," funded through Northern Arizona University. Kaufman developed the storyboard for the documentary, helped to write the script, and organized the interviews with the scientists featured in the film. It is currently airing on many PBS channels across the country. Following a national search in spring 2014, McKay was selected as the top candidate for a tenure track faculty position in climate science at Northern Arizona University.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
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Neil R. Swanberg
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Northern Arizona University
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