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1. Last Millennium Reanalysis with an expanded proxy database and seasonal proxy modeling

   https://doi.org/10.5194/cp-15-1251-2019  

   Tardif, Robert ; Hakim, Gregory J. ; Perkins, Walter A. ; Horlick, Kaleb A. ; Erb, Michael P. ; Emile-Geay, Julien ; Anderson, David M. ; Steig, Eric J. ; Noone, David ( January 2019 , Climate of the Past)

   null (Ed.)

   Abstract. The Last Millennium Reanalysis (LMR) utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records and the forward models that map climate variables to proxy measurements. Results based on an updated proxy database and seasonal regression-based forward models are compared to the LMR prototype, which was based on a smaller set of proxy records and simpler proxy models formulated as univariate linear regressions against annual temperature. Validation against various instrumental-era gridded analyses shows that the new reconstructions of surface air temperature and 500 hPa geopotential height are significantly improved (from 10 % to more than 100 %), while improvements in reconstruction of the Palmer Drought Severity Index are more modest. Additional experiments designed to isolate the sources of improvement reveal the importance of the updated proxy records, including coral records for improving tropical reconstructions, and tree-ring density records for temperature reconstructions, particularly in high northern latitudes. Proxy forward models that account for seasonal responses, and dependence on both temperature and moisture for tree-ring width, also contribute to improvements in reconstructed thermodynamic and hydroclimate variables in midlatitudes. The variability of temperature at multidecadal to centennial scales is also shown to be sensitive to the set of assimilated proxies, especially to the inclusion of primarily moisture-sensitive tree-ring-width records. 

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2. The feasibility of reconstructing hydroclimate over West Africa using tree-ring chronologies in the Mediterranean region

   https://doi.org/10.1088/1748-9326/ac79c0  

   Fosu, Boniface O ; Cook, Edward R ; Biasutti, Michela ; Buckley, Brendan M ; Nicholson, Sharon E ( June 2022 , Environmental Research Letters)

   Dendrochronology in West Africa has not yet been developed despite encouraging reports suggesting the potential for long tree-ring reconstructions of hydroclimate in the tropics. This paper shows that even in the absence of local tree chronologies, it is possible to reconstruct the hydroclimate of a region using remote tree rings. We present the West Sub-Saharan Drought Atlas (WSDA), a new paleoclimatic reconstruction of West African hydroclimate based on tree-ring chronologies from the Mediterranean Region, made possible by the teleconnected climate relationship between the West African Monsoon and Mediterranean Sea surface temperatures. The WSDA is a one-half degree gridded reconstruction of summer Palmer Drought Severity indices from 1500 to 2018 CE, produced using ensemble point-by-point regression. Calibration and verification statistics of the WSDA indicate that it has significant skill over most of its domain. The three leading modes of hydroclimate variability in West Africa are accurately reproduced by the WSDA, demonstrating strong skill compared to regional instrumental precipitation and drought indices. The WSDA can be used to study the hydroclimate of West Africa outside the limit of the longest observed record and for integration and comparison with other proxy and archaeological data. It is also an essential first step toward developing and using local tree-ring chronologies to reconstruct West Africa’s hydroclimate.

    

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3. A 1200+ year reconstruction of temperature extremes for the northeastern Mediterranean region

   https://doi.org/10.1002/joc.5955  

   Klippel, Lara ; Krusic, Paul J. ; Konter, Oliver ; St. George, Scott ; Trouet, Valerie ; Esper, Jan ( December 2018 , International Journal of Climatology)

   Proxy evidence is necessary to place current temperature and hydroclimatic changes in a long‐term context and to assess the full range of natural and anthropogenic climate forcings. Here, we present the first millennium‐length reconstruction of late summer (August–September) temperature variability for the Mediterranean region. We compiled 132 maximum latewood density (MXD) tree‐ring series of living and relictPinus heldreichiitrees from a network of four high‐elevation sites in the Pindus Mountains of Greece. Forty series reach back into the first millennium and the oldest sample dates to 575 CE. At annual to decadal scales, the record correlates significantly with August–September temperatures over the Balkan Peninsula and northeastern Mediterranean (r_1950–2014= 0.71,p< 0.001). We produce two reconstructions emphasizing interannual and decadal scale variance over the past millennium. Analysis of temperature extremes reveals the coldest summers occurred in 1035, 1117, 1217, 1884 and 1959 and the coldest decades were 1061–1070 and 1811–1820. The warmest summers occurred in 1240 and 1474, and the warmest decades were 1141–1150 and 1481–1490. Comparison of this new reconstruction with MXD‐based summer temperature reconstructions across Europe reveals synchronized occurrences of extreme cool summers in the northeastern Mediterranean, and an antiphase‐relationship with warm summer temperatures over the British Isles and Scandinavia. This temperature dipole is related to anomalies in the latitudinal position of the North Atlantic Jet. Despite the representation of common atmospheric forcing patterns, the occurrence of warm extremes is limited to few events, suggesting potential weaknesses of MXD to record warm temperature anomalies. In addition, we acknowledge problems in the observational data to capture local temperature variability due to small scale topographic differences in this high‐elevation landscape. At a broader geographical scale, the occurrence of common cold summer extremes is restricted to years with volcanically induced changes in radiative forcing.

    

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4. The Historical Development of Large‐Scale Paleoclimate Field Reconstructions Over the Common Era

   https://doi.org/10.1029/2022RG000782  

   Smerdon, Jason E. ; Cook, Edward R. ; Steiger, Nathan J. ( October 2023 , Reviews of Geophysics)

   Climate field reconstructions (CFRs) combine modern observational data with paleoclimatic proxies to estimate climate variables over spatiotemporal grids during time periods when widespread observations of climatic conditions do not exist. The Common Era (CE) has been a period over which many seasonally‐ and annually‐resolved CFRs have been produced on regional to global scales. CFRs over the CE were first produced in the 1970s using dendroclimatic records and linear regression‐based approaches. Since that time, many new CFRs have been produced using a wide range of proxy data sets and reconstruction techniques. We assess the early history of research on CFRs for the CE, which provides context for our review of advances in CFR research over the last two decades. We review efforts to derive gridded hydroclimatic CFRs over continental regions using networks of tree‐ring proxies. We subsequently explore work to produce hemispheric‐ and global‐scale CFRs of surface temperature using multi‐proxy data sets, before specifically reviewing recently‐developed data assimilation techniques and how they have been used to produce simultaneous reconstructions of multiple climatic fields globally. We then review efforts to develop standardized and digitized databases of proxy networks for use in CFR research, before concluding with some thoughts on important next steps for CFR development.

    

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5. A data assimilation approach to last millennium temperature field reconstruction using a limited high-sensitivity proxy network

   https://doi.org/10.1175/JCLI-D-20-0661.1  

   King, Jonathan M. ; Anchukaitis, Kevin J. ; Tierney, Jessica E. ; Hakim, Gregory J. ; Emile-Geay, Julien ; Zhu, Feng ; Wilson, Rob ( May 2021 , Journal of Climate)

   null (Ed.)

   Abstract We use theNorthern Hemisphere Tree-RingNetwork Development (NTREND) tree-ring database to examine the effects of using a small, highly-sensitive proxy network for paleotemperature data assimilation over the last millennium. We first evaluate our methods using pseudo-proxy experiments. These indicate that spatial assimilations using this network are skillful in the extratropical Northern Hemisphere and improve on previous NTREND reconstructions based on Point-by-Point regression. We also find our method is sensitive to climate model biases when the number of sites becomes small. Based on these experiments, we then assimilate the real NTREND network. To quantify model prior uncertainty, we produce 10 separate reconstructions, each assimilating a different climate model. These reconstructions are most dissimilar prior to 1100 CE, when the network becomes sparse, but show greater consistency as the network grows. Temporal variability is also underestimated before 1100 CE. Our assimilation method produces spatial uncertainty estimates and these identify treeline North America and eastern Siberia as regions that would most benefit from development of new millennial-length temperature-sensitive tree-ring records. We compare our multi-model mean reconstruction to five existing paleo-temperature products to examine the range of reconstructed responses to radiative forcing. We find substantial differences in the spatial patterns and magnitudes of reconstructed responses to volcanic eruptions and in the transition between the Medieval epoch and Little Ice Age. These extant uncertainties call for the development of a paleoclimate reconstruction intercomparison framework for systematically examining the consequences of proxy network composition and reconstruction methodology and for continued expansion of tree-ring proxy networks. 

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