Late-Quaternary disruptions of the Atlantic Meridional Overturning Circulation (AMOC) have been linked to profound impacts on local and regional climate as well as vegetation. Temperature seasonality patterns during these AMOC events, most notably during the Younger Dryas, are not yet fully understood, as studies have shown two possible scenarios: overall cooling patterns in Younger Dryas, or warm summers with extreme winter cooling. Here we assess the seasonal temperature trends of late-Quaternary weak AMOC states in Northern Europe, based on new quantitative pollen- and plant macrofossil-based January and July temperature reconstructions using multi-method ensemble reconstruction approaches. For plant macrofossils, we implement a “dynamic” calibration approach, where a spatial extent of the modern calibration region varies based on an independent proxy for the past continentality regime. This allows the reconstruction to algorithmically select the most representative modern calibration region for each time slice. We find an indication that abrupt AMOC weakening is associated with winter-dominated cooling of up to 10 °C but relatively stable summers.

Proxy data for eastern North American hydroclimate indicate strong and persistent multi-millennial droughts during the Holocene, but climate model simulations often fail to reproduce the proxy-inferred droughts. Diagnosing the data–model mismatch can offer valuable insights about the drivers of hydrological variability and different regional sensitivities to hydroclimate forcing. Here we present a proxy–modeling synthesis for Holocene climates in the eastern North American mid-latitudes, including machine-learning-based water balance reconstructions and high-resolution climate simulations. These data-model results resolve prior-generation inconsistencies, show consistent spatiotemporal patterns of Holocene hydroclimate change, and enable assessment of the driving mechanisms. This agreement suggests that the secular summer insolation trend, combined with the Laurentide Ice Sheet deglaciation and its effect on atmospheric circulation, together explain the extent and duration of drier-than-present climates. In addition, our high-resolution proxy data and transient simulations reveal clear multi-centennial climate variability. In our simulations, temperature-driven increases in evapotranspiration exceed regional precipitation gains, drying much of the region during the mid Holocene. This suggests that the mid-Holocene multi-millennial drought was driven by similar processes compared to the drying trajectory projected for mid-latitude North America over this century, which is also primarily driven by warming.

A continuous three-year field study, focusing on the thermal regime and the heat budget of twelve shallow Arctic lakes in northwest Finland, was conducted between 2019 and 2022. The results reveal diverse thermal regimes among these lakes, ranging from cold monomictic to discontinuous cold polymictic and dimictic patterns, reflecting the unique lake responses to their environmental settings. The heat budget of these lakes was predominantly influenced by the strong seasonality of the radiation balance, with latent and sensible heat fluxes consistently exhibiting negative values during the ice-free period, peaking in the summer or late fall. Air temperature and solar radiation were the primary drivers affecting lake thermal structures, at both local and regional scales. The influence of wind speed and cloudiness was more significant for lakes in the treeless tundra, but their regional impact remains relatively weak, along with the impact of precipitation. Additionally, we emphasize the critical role of lake location, geography, and morphology, and particularly altitude, lake size, and water column transparency, in determining changes in stratification and mixing dynamics, overshadowing the influence of lake depth. In conclusion, this study provides new insights into the evolving thermal dynamics of lakes in the European Arctic.

The recent advancements of new quantitative tools compatible with plant macrofossil proxy data have revived its potential for paleoclimate research. Plant macrofossils are commonly used in so-called indicator-species approaches, using methodologies that are typically built on known observations linking modern plant distributions with climate. This allows complementary paleoclimate reconstructions using an approach that is not limited by the spatial availability of calibration samples obtained from surface sediments (e.g., pollen or chironomids). We aim to evaluate the impact that various methodological choices have on the plant-macrofossil based reconstructions of January and July temperature patterns for the Lateglacial (14–11 ka BP) period. We use a variety of classic and novel quantitative climate reconstruction algorithms with plant macrofossil assemblages from 13 sites of the Baltic States. We use unfiltered plant data to evaluate the ability of each method to also handle the presence of plants that might have a weak sensitivity to temperature. Additionally, we test the influence of another methodological choice – the choice of modern calibration region – on the reconstructed climate. Our findings indicate that, with no prior filtering of summer and winter-sensitive plants, temporal temperature variations can be reconstructed with methods that implement probability density functions. Although some disparities in reconstructions are seen between the tested algorithms, we note that the choice of calibration region bears a greater influence on the results. A calibration region that best represents the past environment should be chosen rather than one representing the same spatial extent as the fossil site(s). Moreover, for long-term reconstructions, a “dynamic calibration set” approach should be considered in future studies by using a range of calibration regions and mirroring the continuously changing broadscale environmental regime of the past.

Rainfall seasonality in the tropics has a substantial impact on both ecosystems and human livelihoods. Yet, reconstructions of past rainfall variability have so far generally been unable to differentiate between annual and seasonal precipitation changes. Past variations in seasonality are therefore largely unknown. Here, we disentangle hydrogen isotopic (δD) signals from terrestrial leaf waxes and algae in an 8000-year peat core from Sumatra, which reflect annual versus wet season rainfall signals, respectively. We validate these results using lipid biomarkers by reconstructing vegetation dynamics via n-alkane distributions and peatland hydrological conditions using glycerol dialkyl glycerol tetraethers (GDGTs), as well as biomass burning using levoglucosan concentrations in the core. Finally, we compare our proxy results to a transient climate model simulation (MPI-ESM1.2) to identify the mechanism for seasonality changes. We find that algal δD indicates stronger Indonesian-Australian Summer Monsoon (IASM) precipitation in the Mid-Holocene, between 8 and 4.2 cal ka BP. A period of alternating flooding, droughts and wildfires is reconstructed between 6 and 4.2 cal ka BP, implicating very strong monsoonal precipitation and drying out and burning during a longer and intensified dry season. We attribute this strong rainfall seasonality in the Mid-Holocene mainly to orbitally forced insolation seasonality and a strengthened IASM, consistent with the modeling results. In terms of annual rainfall, terrestrial plant δD, vegetation composition and GDGTs all indicate wetter conditions peaking between 3 and 4.5 cal ka BP, preceded by drier conditions, followed by drastic and rapid drying in the late Holocene from around 2.8 cal ka BP. Our multiproxy annual precipitation reconstruction thereby indicates the wettest overall conditions approximately 1500–2000 years later than a nearby speleothem δ18O record, which instead follows the seasonally biased algal δD in our record. We, therefore, hypothesize that speleothem reconstructions over the Holocene in parts of the tropics with low but significant seasonality may carry a stronger seasonal component than previously suggested. The data presented here contribute with new insights on how isotopic rainfall proxies in the tropics can be interpreted. Our findings resolve the seasonal versus annual components of Holocene rainfall variability in the Indo-Pacific Warm Pool region, highlighting the importance of considering seasonality in rainfall reconstructions.