Abstract. The Mount Brown South (MBS) ice core in East Antarctica (69° S, 86° E) has produced records of sea salt concentration and snow accumulation for examining past climate. In a previous study, the sea salt concentration, but not snow accumulation, showed a significant, positive relationship with the El Niño–Southern Oscillation (ENSO) from June to November. Here, we use observations and reanalysis data to provide insights into the mechanisms modulating the sea salt concentration record in the ice core, and the previously identified relationship with the tropical Pacific. The MBS site is a wet deposition site, dominated by austral winter and spring snowfall. We find stronger regional circulation anomalies during austral winter (June–August) and focus our analysis on this season. We show that sea salt is likely transported from northeast of MBS via synoptic-scale storms that accompany high precipitation events. These storms and their associated precipitation, show no substantial differences between years of high and low sea salt concentration, so we suggest it is the source of sea salt that differs, rather than the transport mechanism. A teleconnection between the tropical Pacific and high-latitude winds in the vicinity of MBS is identified. Specifically, El Niño events are related to strengthened westerly winds ∼ 60° S, leading to more local sea ice via anomalous Ekman transport in an area to the northeast of the MBS site. Impacts from La Niña are less obvious, showing that there is a non-linear component to this relationship. El Niño–associated strengthened westerly winds in the MBS region could enhance sea salt availability by increasing ocean aerosol spray and/or by increasing sea ice formation, both of which can act as sources of sea salt. This may explain why sea salt concentration, rather than snow accumulation, is most closely related to ENSO variability in the ice core record. Identifying the mechanisms modulating key variables such as sea salts and snow accumulation at ice core sites provides further insights into what these valuable records can decipher about climate variability in the pre-instrumental period.

Abstract. This study presents a statistical time-domain approach for identifying transitions between climate states, referred to as breakpoints, using well-established econometric tools. Our approach offers the advantage of constructing time-domain confidence intervals for the breakpoints, and it includes procedures to determine how many breakpoints are present in the time series. We apply these tools to a 67.1 million-year-long compilation of benthic foraminiferal oxygen isotopes (δ18O), which signify global temperature and ice volume throughout the Cenozoic. This foundational dataset is presented in Westerhold et al. (2020), where the authors use recurrence analysis to identify five breakpoints that define six climate states. Fixing the number of breakpoints to five, our procedure results in breakpoint estimates that closely align with those identified by Westerhold et al. (2020). By allowing the number of breakpoints to vary, we provide statistical justification for more than five breakpoints in the time series. Our method adds to our understanding of Cenozoic climate history in terms of the timing and rate of transitions between climate states and provides a tool for robustly assessing breakpoints in many other paleoclimate time series.

Abstract. The tax liability of peasants significantly influenced their lives and the total monetary income of the country. The damaging effects of weather extremes on crop yields were considered grounds for tax relief. Administrative documentation connected with requests for tax relief can serve as an important source of data for historical climatology, as demonstrated by the example of the Prácheň Region in southwestern Bohemia during the 17th–19th centuries. Based on the first land registry system, only hailstorm damage to crops and fires qualified peasants for tax relief from 1655 CE, while the subsequent land registry system from 1748 CE extended this to include water damage from 1775 CE. Taxation data made it possible to analyse the spatiotemporal variability of significant hailstorms, water torrents, and lightning-caused fires, together with their impacts on agriculture and the lives of peasants during the 1655–1707 and 1748–1827 CE periods in the Prácheň Region, for which summary data at governmental and regional levels were preserved. Data related to weather damage were further supplemented with other documentary sources to create a chronology of significant hailstorms, water torrents, and additional weather extremes for the analyzed region. This study and its results clearly demonstrate the potential of taxation records – available in the Czech Lands as well as in many other countries – for historical-climatological research into past damaging weather events and their human impacts.

Abstract. The complex biological and physical processes that preserve paleoclimate information over centuries or longer introduce variations in proxy time series that are unrelated to the true climate. These non-climatic variations act on different timescales and are often referred to as “noise” of a specific color, based on similarities between a time series' power spectrum and the electromagnetic spectrum of light. For example, “white noise” equally affects all timescales, where “red noise” dominates only on long timescales, similar to longwave red light. Noise spectra in proxy records have far-reaching implications in paleoclimate research, but noise characteristics are often assumed based on first principles rather than estimated directly, risking either inflating or underestimating error at particular frequencies. Here, we provide concrete definitions of the various types of timescale-dependent errors that are present in proxy data, and review the literature on methods for quantifying noise terms. We then synthesize the results of several published studies that use a common empirical approach for estimating the noise spectrum in ice-core, coral, and tree-ring data. We posit that the colors of proxy noise are archive-specific, with white noise dominating in depositional archives such as ice cores and marine sediment cores, while red noise is more common in biological archives such as tree rings and corals. Our synthesis supports assigning specific colored noise terms in proxy system models, data assimilations and other experiments.

Abstract. The Marine Isotope Stage (MIS) 9e, occurring approximately from 335 to 320 ka, represents an important period for studying the dynamics of Earth's climate. Interest in studying this interglacial period stems from the fact that it is associated with the highest atmospheric CO2 concentrations over the last 800 ka (excluding anthropogenic CO2 emissions). Numerous reconstructions of sea surface temperatures (SST) cover this time interval, yet synthesizing them into consistent regional- and global-scale climate signals is challenging because they are scattered across the globe and based on heterogeneous chronological frameworks. In this study, we present the first spatio-temporal SST synthesis over the interval 350 to 300 ka, covering this interglacial period and its preceding deglaciation (Termination IV, ∼ 350 to ∼ 335 ka). We include 98 high-resolution SST reconstructions and we establish a common temporal framework between the selected marine records, based on the latest reference ice core chronology (AICC2023). We also homogenize the proxy-calibration strategy by applying a single method for each proxy. Chronological and calibration uncertainties are quantified using Bayesian and Monte Carlo procedures. Finally, through a Monte Carlo approach, we generate global- and regional-scale SST stacks relative to Pre-Industrial Era over Termination IV and MIS 9. We highlight significant differences in terms of temporal variability, amplitude, and timing of changes in the SST records across the globe over the studied time interval. While the patterns of SST changes are homogeneous at basin-scale, heterogeneous interglacial SST peaks are observed across ocean basins. The interglacial surface temperature peaks in extra-tropic basins are similar to or warmer than the pre-industrial period (PI), while intra-tropic areas appear to be colder relative to PI during interglacial optimum. In addition, the timing in interglacial surface temperature peaks differ across the different regions. These regional temperature variations suggest that atmospheric and oceanic dynamics played a greater role than global radiative forcing in shaping the MIS 9e climate. The heterogeneous timing of changes across the different regions contribute to a smoothed global-scale response in terms of both timing and amplitude. Consequently, we find that at a global scale MIS 9e SST was as warm as the PI (∼ −0.1 ± 0.2 °C). Converted into surface air temperatures (∼ −0.3 ± 0.3 °C), this estimate agrees within the uncertainty range with previous studies based on a smaller number of records with lower temporal resolution. We also compare our results on MIS 9e and Termination IV with published SST syntheses covering more recent interglacial periods (MIS 5e and Holocene) and deglacial periods (Termination I and II). We find that the global deglacial surface air warming during Termination IV is similar in amplitude (∼ 5.7 °C) to that observed during Terminations I and II. Finally, a comparison of deglacial warming rates for these three terminations to the warming trend of the last 60 years emphasizes that the rapidity of modern climate change is unprecedented within the context of these past deglaciations.

Abstract. Mg/Ca ratios measured in benthic foraminifera have been explored as a potential palaeothermometry proxy for bottom water temperatures (BWT). Mg/Ca-BWT calibrations from the Indian Ocean are rare and comprise conflicting results. Inconsistencies between studies suggest that calibrations may need to be region specific. The aim of this study was to develop Mg/Ca-BWT calibrations based on species-specific benthic foraminifera (Uvigerina peregrina, Cibicidoides wuellerstorfi, and Cibicidoides mundulus) in the tropical western Indian Ocean and to optimize the chemical cleaning procedure by Barker et al. (2003) applied to samples analysed in this study. The majority of samples of C. mundulus and U. peregrina, however, remained contaminated, rendering those data unusable for Mg/Ca core-top calibrations. Only Mg/Ca ratios in C. wuellerstorfi allowed a tentative Mg/Ca-BWT calibration with the relationship being: Mg/Ca=0.19±0.02⋅BWT+1.07±0.03, r2=0.87 and n=4). While this result differs to some degree from previous studies, it principally suggests that existing core-top calibrations from the wider Indian Ocean can be applied to core-tops in the western Indian Ocean. The agreement of Mg/Ca ratios at lower temperatures in C. wuellerstorfi, C. mundulus, and U. peregrina with Mg/Ca ratios reported for these species at low temperatures in other studies supports this conclusion. The clear difference in contamination between Cibicidoides spp. and U. peregrina, despite using the same cleaning procedure, supports the findings of previous studies that suggest different rigour might be required for different species. Many other uncertainties surrounding the Mg/Ca proxy exist and more calibration studies are required to improve this method.

Abstract. Quantifying past ocean nitrate concentrations is crucial for understanding the global nitrogen cycle. Here, we reconstruct deglacial bottom-water nitrate concentrations ([NO3-]BW) in the oxygen-deficient zones of the Sea of Okhotsk, the Gulf of California, the Mexican Margin, and the Gulf of Guayaquil. Using the pore density of denitrifying benthic foraminifera as a nitrate proxy, differences in [NO3-]BW are observed at the study sites spanning the Last Glacial Maximum to the Holocene. Changes in water-column denitrification, water-mass ventilation, primary productivity, and sea surface temperatures may account for nitrate differences at the study sites. The [NO3-]BW in the Sea of Okhotsk, the Gulf of California, and the Gulf of Guayaquil are influenced by the intermediate water masses while, the [NO3-]BW at the Mexican Margin is likely influenced by deglacial changes in the Pacific Deep Water. The comparison of past and present [NO3-] shows that the modern Gulf of Guayaquil and the Gulf of California currently have stronger oxygen-deficient zones with higher denitrification than during the Last Glacial Maximum. In contrast, the modern Mexican Margin and the Sea of Okhotsk may have higher oxygen than during the Last Glacial Maximum, indicated by low modern denitrification.

Abstract. Data assimilation (DA) has been successfully applied in paleoclimate reconstruction. DA combines model simulations and climate proxies based on their error sizes. Therefore, error information is crucial for DA to work optimally. However, little attention has been paid to observation errors in previous studies, especially when proxies are assimilated directly. This study assessed the feasibility of innovation statistics, a method developed for numerical weather prediction, for estimating observation errors in climate reconstruction and its impact on the reconstruction skills. For this purpose, we conducted offline-DA experiments over 1870–2000. Here, we assimilated stable water isotope records from ice cores, tree-ring cellulose, and corals. We found that the innovation-statistics-based approach correctly estimated observation errors, even with the offline-DA scheme. Although the accuracy of the estimation depended on the sample size and accuracy of the prior error covariance, the estimation generally improved the reconstruction skills. The reconstruction skills with the estimated observation errors were comparable to those with errors defined differently in the previous studies. In contrast with those methods used in previous studies, however, the innovation-statistics-based approach offers an objective and systematic way to estimate observation errors with light computational cost. As such, the innovation-statistics-based approach should contribute to improving the reconstruction skills and observation networks.

Abstract. Investigating climatic and environmental changes during past interglacials is crucial to improve our understanding of the mechanisms that govern changes related to current global warming. Among the numerous proxies that can be used to reconstruct past environmental and climatic conditions, pollen allows quantitative reconstructions of annual, warmest month and coldest month air temperatures as well as precipitation sums, and Chironomidae larvae are widely used to infer past summer air temperature. Chironomidae have mostly been used for reconstructing Holocene and Late Weichselian summer temperatures whilst there are only four sites in Europe with chironomid-based summer air temperature reconstructions for the Late Pleistocene and no such records for any Middle Pleistocene warm period as of the writing of this paper. In this study we present the first quantitative palaeoclimate reconstruction for the post-Holsteinian (Marine Isotope Stage – (MIS) 11b) in Central Europe based on both pollen and fossil chironomid remains preserved in palaeolake sediments recovered from Krępa, southeastern Poland. Besides being used for the palaeoclimatic reconstruction, pollen analysis provides the biostratigraphic framework and a broader perspective of climate development at the end of Holsteinian Interglacial. Fossil Chironomidae assemblages at Krępa consist mainly of oligotrophic and mesotrophic taxa (e.g. Corynocera ambigua, Chironomus anthracinus-type) while eutrophic taxa (e.g. Chironomus plumosus-type) are less abundant. The chironomid-based summer temperature reconstruction indicates July air temperatures between 15.3 and 20.1 °C during the early post-Holsteinian, while pollen-based temperature reconstructions (using MAT and WA-PLS methods) indicate temperature values from 15 to 19 °C. Pollen-derived mean temperature of the coldest month (MTCO) and mean annual precipitation sum vary from −13.2 to −9.6 °C and between 500 and 900 mm respectively. In any case, results from Krępa prove that conducting Chironomidae analysis is feasible for periods as early as the Middle Pleistocene, improving our understanding of the mechanisms that control present-day climatic and environmental changes.

Abstract. Volcanic eruptions are one of the most important drivers of climate variability, but climate model simulations typically show stronger surface cooling than proxy-based reconstructions. Uncertainties associated with eruption source parameters, aerosol–climate modelling, and internal climate variability might explain those discrepancies, but their quantification using complex global climate models is computationally expensive. In this study, we combine a reduced-complexity volcanic aerosol model (EVA_H) and a climate model (FaIR) to simulate global-mean surface temperature from 6755 BCE to 1900 CE (8705 to 50 BP) accounting for volcanic forcing, solar irradiance, orbital, ice sheet, greenhouse gases, land-use forcing, and anthropogenic aerosols and ozone forcing for the historical period (1750–1900 CE). The negligible computational cost of the models enables us to use a Monte Carlo approach to propagate uncertainties associated with eruption source parameters, aerosol and climate modelling, and internal climate variability. Averaging over the last 9000 years, we obtain a global-mean volcanic forcing of −0.15 W m−2 and an associated surface cooling of 0.12 K. Averaged over the 14 largest eruptions (injecting more than 20 Tg of SO2) of 1250–1900 CE, the mean temperature response in tree-ring-based reconstructions is in good agreement with the our simulations, scaled to Northern Hemisphere summer temperature. For individual eruptions, discrepancies between the simulated and reconstructed surface temperature response are almost always within uncertainties. At multimillennial timescales, our simulations reproduce the Holocene global warming trend typically derived from simulations and data assimilation products but exhibit some discrepancies on centennial to millennial timescales. In particular, the Medieval Climate Anomaly to Little Ice Age transition is weaker in our simulations, and we also do not capture a relatively cool period between 3000 and 1000 BCE (5000 and 3000 BP), visible in climate reanalyses. We discuss how uncertainties in land-use forcing and model limitations might explain these differences. Our study demonstrates the value of reduced-complexity volcanic aerosol–climate models to simulate climate at annual to multimillennial timescales.