Abstract. The “Year Without a Summer” in 1816 was characterized by extraordinarily cold and wet periods in central Europe, and it was associated with severe crop failures, famine, and socio-economic disruptions. From a modern perspective, and beyond its tragic consequences, the summer of 1816 represents a rare opportunity to analyze the adverse weather (and its impacts) after a major volcanic eruption. However, given the distant past, obtaining the high-resolution data needed for such studies is a challenge. In our approach, we use dynamical downscaling, in combination with 3D variational data assimilation of early instrumental observations, for assessing a cold-air outbreak in early June 1816. We find that the cold spell is well represented in the coarse-resolution 20th Century Reanalysis product which is used for initializing the regional Weather Research and Forecasting Model. Our downscaling simulations (including a 19th century land use scheme) reproduce and explain meteorological processes well at regional to local scales, such as a foehn wind situation over the Alps with much lower temperatures on its northern side. Simulated weather variables, such as cloud cover or rainy days, are simulated in good agreement with (eye) observations and (independent) measurements, with small differences between the simulations with and without data assimilation. However, validations with partly independent station data show that simulations with assimilated pressure and temperature measurements are closer to the observations, e.g., regarding temperatures during the coldest night, for which snowfall as low as the Swiss Plateau was reported, followed by a rapid pressure increase thereafter. General improvements from data assimilation are also evident in simple quantitative analyses of temperature and pressure. In turn, data assimilation requires careful selection, preprocessing, and bias-adjustment of the underlying observations. Our findings underline the great value of digitizing efforts of early instrumental data and provide novel opportunities to learn from extreme weather and climate events as far back as 200 years or more.

Abstract. The mid-Holocene climate of Australia and the equatorial tropics of the Indonesian–Australian monsoon region is investigated using the Community Earth System Model (CESM) and the Weather Research and Forecasting (WRF) model. Each model is used to simulate the pre-industrial (1850) and the mid-Holocene (6000 years before 1950) climate. The results of these four simulations are compared to existing bioclimatic modelling of temperature and precipitation. The finer-resolution WRF simulations reduce the bias between the model and bioclimatic data results for three of the four variables available in the proxy data set. The model results show that temperatures over southern Australia at the mid-Holocene and in the pre-industrial period were similar, and temperatures were slightly warmer during the mid-Holocene over northern Australia and into the tropics compared to the pre-industrial. During the mid-Holocene, precipitation was generally reduced over northern Australia and in the Indonesian–Australian monsoon region, particularly during summertime. The results highlight the improved value of using finer-resolution models such as WRF to simulate the palaeoclimate.

Abstract. Investigations of past extreme climate events offer insights into the interactions between natural forces, ecosystems, and human societies. The Chongzhen drought, which occurred from 1627 to 1644 CE, stands as possibly the most severe drought in central and eastern China over the last 1500 years, remarkable for its duration and extent and the vast number of people affected. Concurrently, a widespread famine emerged, triggering peasant uprisings that are argued as having contributed to the Ming Dynasty's downfall. This study extracted 1802 drought records and 1977 famine records from Chinese historical documents to reconstruct the spatio-temporal progression of the drought and its impact on famine. The records provided drought information of season, duration, and intensity, which enabled a classification of four drought severity levels. Then kernel density estimation reconstructed the spatial pattern of drought and the drought kernel density index (DKDI) series in sub-regions. Between 1627 and 1644, the drought affected most of central and eastern China. The severe drought zone was mainly located north of 29° N, shifting from the northwestern region to the northern region and then expanding to the south. The development of drought in different regions was not synchronized. The northwestern region faced the earliest drought outbreak, which eased in the middle period and peaked in 1640. The northern region's DKDI series forms a single-peaked curve, indicating a gradual aggravation of the drought from 1633 to 1640. The Yangtze–Huai region's DKDI series shows a multi-peaked curve, with repeated cycles of worsening and easing drought, peaking in 1641. Furthermore, the reconstruction of the spatio-temporal progression of famine revealed overlapped ranges and similar development trends to that of the drought. The years marking the peak of the famine kernel density index (FKDI) in the northwestern region, northern region, and Yangtze–Huai region coincided with those of the DKDI. Regression analysis identified drought as the primary factor triggering famine, accounting for approximately 67.3 % of its occurrence. In the northern region, the contribution of drought was higher (73.4 %). Series and correlation analyses indicate a continuity in drought's impacts on famine. This paper further clarifies the dominant pathway of climate impact transmission in this case: extreme drought → declining agricultural harvest → food shortage → famine. Other natural and socio-economic factors, such as locust infestations, nomadic invasions, and economic decline, also played a role in the occurrence of famine. Human response measures were instrumental in regulating the transmission of climate change impacts.

Abstract. The Benguela Upwelling System (BUS), located in the southeastern Atlantic Ocean, represents one of the world's most productive regions. This system is delimited to the south by the Agulhas retroflection region. The northern boundary of the BUS is, instead, represented by the Angola–Benguela Front (ABF), which is a thermal feature separating warm waters of the Angola Basin (including the South Atlantic Central Water; SACW) from the cooler Benguela Oceanic Current (BOC). We performed statistical analyses on planktonic foraminiferal assemblages in 94 samples from Holes U1575A and U1576A, cored during International Ocean Discovery Program (IODP) Expedition 391. Drilled sites are located along the Tristan–Gough–Walvis Ridge (TGW) seamount track in the northern sector of the BUS (offshore the Namibian continental margin). The analyzed stratigraphic intervals span the Early–Late Pleistocene, marked by the Early–Middle Pleistocene transition (EMPT; 1.40–0.40 Myr), during which important glacial–interglacial sea surface temperature (SST) variabilities occurred. This work provides novel insights on the local paleoceanographic evolution of the northern BUS and associated thermocline variability based on the ecological significance of the foraminiferal assemblages. Specifically, variations in the assemblage content allowed us to characterize the different water masses (BOC, SACW, and Agulhas waters) and reconstruct their interactions during the Quaternary. The interplay of the previously mentioned water masses induced perturbations in the BUS (ABF latitudinal shifts and input of tropical waters from the Agulhas retroflection region). Furthermore, we investigated the possible link between changes in the paleoceanographic conditions and climatic events (e.g., Benguela Niño-/Niña-like phases and deglaciation stages) recorded since the EMPT.

Abstract. The winter of 1739/40 is known as one of the coldest winters in Europe since early instrumental measurements began. Many contemporary sources discuss the cold waves and compare the winter to that of 1708/09. It is less well known that the year 1740 remained cold until August and was again cold in October and that negative temperature anomalies were also found over Eurasia and North America. The 1739/40 cold season over northern mid-latitude land areas was perhaps the coldest in 300 years, and 1740 was the coldest year in central Europe in 600 years. New monthly global climate reconstructions allow us to address this momentous event in greater detail, while daily observations and weather reconstructions give insight into the synoptic situations. Over Europe, we find that the event was initiated by a strong Scandinavian blocking in early January, allowing the advection of continental cold air. From February until June, high pressure dominated over Ireland, arguably associated with frequent eastern Atlantic blocking. This led to cold-air advection from the cold northern North Atlantic. During the summer, cyclonic weather dominated over central Europe, associated with cold and wet air from the Atlantic. The possible role of oceanic influences (El Niño) and external forcings (eruption of Mount Tarumae in 1739) are discussed. While a possible El Niño event might have contributed to the winter cold spells, the eastern Atlantic blocking is arguably unrelated to either El Niño or the volcanic eruption. All in all, the cold year of 1740 marks one of the strongest, arguably unforced excursions in European temperature.

Abstract. The configuration of the Northern Hemisphere ice sheets during the Penultimate Glacial Maximum differed to the Last Glacial Maximum. However, the reasons for this are not yet fully understood. These differences likely contributed to the varied deglaciation pathways experienced following the glacial maxima and may have had consequences for the interglacial sea level rise. To understand the differences between the North American Ice Sheet at the Last and Penultimate glacial maxima (21 and 140 ka), we perform two perturbed-physics ensembles of 62 simulations using a coupled atmosphere–ice sheet model, FAMOUS-ice, with prescribed surface ocean conditions, in which the North American and Greenland ice sheets are dynamically simulated with the Glimmer ice sheet model. We apply an implausibility metric to find ensemble members that match reconstructed ice extent and volumes at the Last and Penultimate glacial maxima. We use a resulting set of “plausible” parameters to perform sensitivity experiments to decompose the role of climate forcings (orbit, greenhouse gases) and initial conditions on the final ice sheet configurations. This confirms that the initial ice sheet conditions used in the model are extremely important in determining the difference in final ice volumes between both periods due to the large effect of the ice–albedo feedback. In contrast to evidence of a smaller Penultimate North American Ice Sheet, our results show that the climate boundary conditions at these glacial maxima, if considered in isolation, imply a larger Penultimate Glacial Maximum North American Ice Sheet than at the Last Glacial Maximum by around 6 m sea level equivalent. This supports the notion that the growth of the ice sheet prior to the glacial maxima is key in explaining the differences in North American ice volume.

Abstract. Paleoclimatological field reconstructions are valuable for understanding past hydroclimatic variability, which is crucial for assessing potential future hydroclimate changes. Despite being as impactful on societies as temperature variability, hydroclimatic variability – particularly beyond the instrumental record – has received less attention. The reconstruction of globally complete fields of climate variables lacks adequate proxy data from tropical regions like South America, limiting our understanding of past hydroclimatic changes in these areas. This study addresses this gap using low-resolution climate archives, including speleothems, previously omitted from reconstructions. Speleothems record climate variations on decadal to centennial timescales and provide a rich dataset for the otherwise proxy-data-scarce region of tropical South America. By employing a multi-timescale paleoclimate data assimilation approach, we synthesize climate proxy records and climate model simulations capable of simulating water isotopologs in the atmosphere to reconstruct 2000 years of South American climate. This includes surface air temperature, precipitation amount, drought index, isotopic composition of precipitation amount and the intensity of the South American Summer Monsoon. The reconstruction reveals anomalous climate periods: a wetter and colder phase during the Little Ice Age (∼ 1500–1850 CE) and a drier, warmer period corresponding to the early Medieval Climate Anomaly (∼ 600–900 CE). However, these patterns are not uniform across the continent, with climate trends in northeastern Brazil and the Southern Cone not following the patterns of the rest of the continent, indicating regional variability. The anomalies are more pronounced than in previous reconstructions but match trends found in local proxy record studies, thus highlighting the importance of including speleothem proxies. The multi-timescale approach is essential for reconstructing multi-decadal and centennial climate variability. Despite methodological uncertainties regarding climate model biases and proxy record interpretations, this study marks a crucial first step in incorporating low-resolution proxy records such as speleothems into climate field reconstructions using a multi-timescale approach. Adequately extracting and using the information from speleothems potentially enhances insights into past hydroclimatic variability and hydroclimate projections.

Abstract. The Late Pliocene marks the intensification of Northern Hemisphere glaciation (iNHG), offering a unique opportunity to study climate evolution and ice-sheet-related feedback mechanisms. In this study, we present high-resolution Mg / Ca-based sea surface temperatures (SSTs) and subsurface temperatures (SubTs) derived from the foraminiferal species Globigerinoides ruber and Globorotalia hirsuta, respectively, at the Integrated Ocean Drilling Program (IODP) Expedition 306 Site U1313 in the mid-latitudinal North Atlantic during the early Late Pliocene, 3.65–3.37 million years ago (Ma). We find distinct differences between our new G. ruber Mg / Ca-based SST record and previously published alkenone-based SST records from the same location. These discrepancies in both absolute values and variations highlight distinctly different seasonal influences on the proxies. The G. ruber Mg / Ca-based SST data were primarily influenced by local summer insolation, showing a dominant precession cycle. Conversely, the variations in alkenone-based SST, dominated by the obliquity and lacking the precession cycle, are found to be more indicative of cold-season changes, despite previous interpretations of these records as reflecting annual mean temperatures. A simultaneous decline in Mg / Ca-based SST and SubT records from 3.65 to 3.5 Ma suggests a diminished poleward oceanic heat transport, implying a weakening of the North Atlantic Current (NAC). A comparison with Early Pleistocene G. ruber Mg / Ca-based SST records shows a shift in the dominant climatic cycle from precession to obliquity, alongside a marked increase in amplitude, indicating an enhanced influence of obliquity cycles correlated with the expansion of Northern Hemisphere ice sheets.