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DAMP21ka.nc: NetCDF file containing the model prior, proxy values, and DAMP21ka reconstruction for lake status, precipitation, and temperature variables.\n\nclhancock/DAMP21ka-v1.0.0.zip: Notebooks used to generate figures for Hancock et al. (2024)\n\nHolocene-code_development_hydroclimate.zip: Code used to generate the DAMP21ka reconstruction \n\n \n\nHancock, C. L., Erb, M. P., McKay, N. P., Dee, S. G., and Ivanovic, R.: A global Data Assimilation of Moisture Patterns from 21,000–0 BP (DAMP-21ka) using lake level proxy records" 

Global hydroclimate significantly differed from modern climate during the mid-Holocene (6 ka) and Last Glacial Maximum (21 ka). Consequently, both periods have been described as either a partial or reverse analogue for current climate change. To reconstruct past hydroclimate, an offline paleoclimate data assimilation methodology is applied to a dataset of 216 lake status records which provide relative estimates of water level change. The proxy observations are integrated with the climate dynamics of two transient simulations (TraCE-21ka and HadCM3) using a multivariate proxy system model (PSM) which estimates relative lake status from available climate simulation variables. The resulting DAMP-21ka (Data Assimilation of Moisture Patterns 21 000–0 BP) reanalysis reconstructs annual lake status and precipitation values at 500-year resolution and represents the first application of the methodology to global hydroclimate on timescales spanning the Holocene and longer. Validation using Pearson's correlation coefficients indicates that the reconstruction (0.24) is more skillful, on average, than model simulations (0.09), particularly in portions of North America and east Africa, where data density is high and proxy–model disagreement is prominent during the Holocene. Results of the PSM and assimilation are used to evaluate climatic controls on lake status, spatiotemporal patterns of moisture variability, and proxy–model disagreement. During the mid-Holocene, wetter conditions are reconstructed for northern and eastern Africa, Asia, and southern Australia, but in contrast to the model prior, negative anomalies are observed in North America, resulting in drier-than-modern conditions throughout the Northern Hemisphere midlatitudes. Proxy–model disagreement in western North America may reflect a bias in model simulations to stronger sea level pressure gradients in the North Pacific during the mid-Holocene. The data assimilation framework is able to reconcile these differences by integrating the constraints of proxy observations with the dynamics of the model prior to produce a more robust estimation of hydroclimate variability during the past 21 000 years. 

Abstract Paleoclimate reconstructions are now integral to climate assessments, yet the consequences of using different methodologies and proxy data require rigorous benchmarking. Pseudoproxy experiments (PPEs) provide a tractable and transparent test bed for evaluating climate reconstruction methods and their sensitivity to aspects of real-world proxy networks. Here we develop a dataset that leverages proxy system models (PSMs) for this purpose, which emulates the essential physical, chemical, biological, and geological processes that translate climate signals into proxy records, making these synthetic proxies more relevant to the real world. We apply a suite of PSMs to emulate the widely-used PAGES 2k dataset, including realistic spatiotemporal sampling and error structure. A hierarchical approach allows us to produce many variants of this base dataset, isolating the impact of sampling bias in time and space, representation error, sampling error, and other assumptions. Combining these various experiments produces a rich dataset (“pseudoPAGES2k”) for many applications. As an illustration, we show how to conduct a PPE with this dataset based on emerging climate field reconstruction techniques. 

Abstract. In 2013, the Intergovernmental Panel on Climate Changeconcluded that Northern Hemisphere temperatures had reached levelsunprecedented in at least 1400 years. The 2021 report now sees global meantemperatures rising to levels unprecedented in over 100 000 years. ThisTechnical Note briefly explains the reasons behind this major change.Namely, the new assessment reflects additional global warming that occurredbetween the two reports and improved paleotemperature reconstructions thatextend further back in time. In addition to past and recent warming, theconclusion also considers multi-century future warming, which therebyenables a direct comparison with paleotemperature reconstructions onmulti-century time scales. 

Abstract. Annually laminated lake sediment can track paleoenvironmental change at high resolution where alternative archives are often not available. However,information about the chronology is often affected by indistinct and intermittent laminations. Traditional chronology building struggles with thesekinds of laminations, typically failing to adequately estimate uncertainty or discarding the information recorded in the laminations entirely,despite their potential to improve chronologies. We present an approach that overcomes the challenge of indistinct or intermediate laminations andother obstacles by using a quantitative lamination quality index combined with a multi-core, multi-observer Bayesian lamination sedimentation modelthat quantifies realistic under- and over-counting uncertainties while integrating information from radiometric measurements (210Pb,137Cs, and 14C) into the chronology. We demonstrate this approach on sediment of indistinct and intermittently laminatedsequences from alpine Columbine Lake, Colorado. The integrated model indicates 3137 (95 % highest probability density range: 2753–3375) varveyears with a cumulative posterior distribution of counting uncertainties of −13 % to +7 %, indicative of systematic observerunder-counting. Our novel approach provides a realistic constraint on sedimentation rates and quantifies uncertainty in the varve chronology byquantifying over- and under-counting uncertainties related to observer bias as well as the quality and variability of the sediment appearance. The approachpermits the construction of a chronology and sedimentation rates for sites with intermittent or indistinct laminations, which are likely moreprevalent than sequences with distinct laminations, especially when considering non-lacustrine sequences, and thus expands the possibilities ofreconstructing past environmental change with high resolution. 

Abstract. Paleoclimatic records provide valuable information about Holocene climate, revealing aspects of climate variability for a multitude of sites around the world. However, such data also possess limitations. Proxy networks are spatially uneven, seasonally biased, uncertain in time, and present a variety of challenges when used in concert to illustrate the complex variations of past climate. Paleoclimatic data assimilation provides one approach to reconstructing past climate that can account for the diversenature of proxy records while maintaining the physics-based covariancestructures simulated by climate models. Here, we use paleoclimate dataassimilation to create a spatially complete reconstruction of temperatureover the past 12 000 years using proxy data from the Temperature 12k database and output from transient climate model simulations. Following the last glacial period, the reconstruction shows Holocene temperatures warming to a peak near 6400 years ago followed by a slow cooling toward the present day, supporting a mid-Holocene which is at least as warm as the preindustrial. Sensitivity tests show that if proxies have an overlooked summer bias, some apparent mid-Holocene warmth could actually represent summer trends rather than annual mean trends. Regardless, the potential effects of proxy seasonal biases are insufficient to align the reconstructed global mean temperature with the warming trends seen in transient model simulations. 

Abstract The quantity and preservation of carbon‐rich organic matter (OM) underlying permafrost uplands, and the evolution of carbon accumulation with millennial climate change, are large sources of uncertainty in carbon cycle feedbacks on climate change. We investigated permafrost OM accumulation and degradation over the Holocene using a transect of sediment cores dating back to at least c. 6 ka, from a hillslope in the Eight Mile Lake watershed, central Alaska. We find decimeter‐scale organic‐rich (111 ± 45 kg C m⁻³) and organic‐poor (49 ± 30 kg C m⁻³) layers below an upper peat, which store 35% ± 11% and 41% ± 20% of the carbon in the upper 1 m, respectively. In organic‐poor layers, scattered¹⁴C ages of plant macrofossils and higher percentages of degradedAlnusandBetulapollen indicate reworking by cryoturbation and hillslope processes. Whereas organic carbon to nitrogen ratios generally indicate OM freshening up‐core, amino acid bacterial biomarkers, includingd‐enantiomers and gamma‐aminobutyric acid, suggest enhanced degradation prior to 5 ka. Carbon accumulation rates increased from ∼4 to 14 g C m⁻² year⁻¹from c. 8 to 0.2 ka, coinciding with decreasing temperatures and increasing moisture regionally, which may have promoted OM accumulation. Carbon stocks within the upper 1 m average 66 ± 13 kg C m⁻², varying from 77 kg C m⁻²in a buried depression on the upper slope to 48 kg C m⁻²downslope. We conclude that heterogeneity in preserved OM reflects a combination of hillslope geomorphic processes, cryoturbation, and climatic variations over the Holocene. 

Abstract The North American monsoon (NAM) is an important source of rainfall to much of Mexico and southwestern United States. Westerly winds (westerlies) can suppress monsoon circulation and impact monsoon timing, intensity, and extent. Recent Arctic warming is reducing the temperature gradient between the equator and the pole, which could weaken the westerlies; however, the implications of these changes on the NAM are uncertain. Here we present a new composite index of the Holocene NAM. We find monsoon strength reached a maximum circa 7,000 years ago and has weakened since then. Proxy observations of temperature, hydroclimate and upwelling, along with model simulations, show that the NAM was modulated by the westerlies over the Holocene. If the observed Holocene pattern holds for current warming, a weaker meridional temperature gradient and weaker westerlies could lead to a stronger future NAM. 

Abstract. The response of the hydrological cycle to anthropogenic climatechange, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics,rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annualresolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition ofseawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variabilitythrough time. To increase the utility of such reconstructions, we presentthe CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca–δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88 % of these records providing data coverage from 1800 CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the useof the database. The CoralHydro2k database tracks large-scale temperatureand hydrological variability. As such, it is well-suited for investigationsof past climate variability, comparisons with climate model simulationsincluding isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information's Paleoclimate Data Archive at https://doi.org/10.25921/yp94-v135 (Walter et al., 2022).