The Atlantic Meridional Overturning Circulation (AMOC) is at its weakest of the last millennium and is projected to further weaken over the coming decades, with the risk of a collapse over the coming century non negligible. The geologic record provides examples of rapid and large AMOC variations, with phases of AMOC weakening, preceding an AMOC shutdown. One key question is whether CO2-driven AMOC weakening is reversible, or whether it represents a tipping point, which might lead to an AMOC collapse. The study by Willeit et al. offers a new diagnostic that is simple to implement and allows for comparing AMOC stability between different models, as well as different time periods. This offers a powerful new tool in understanding the conditions for AMOC stability, and thus the AMOC future.

This paper is an excellent example of how sophisticated climate reconstructions can be made on the basis of historical documentary data. The authors have not only succeeded in analysing the rich historical descriptions in a very valuable way, they have also applied various statistical approaches very skillfully in a remarkably interdisciplinary approach. The article stands out for its deep understanding of historical sources, drought processes, and various social processes.

This study presents an interesting attempt to reconstruct global surface temperature by combining sparse SST proxy data and PlioMIP2 simulations using a state-of-the-art data assimilation framework. The work highlights the potential of this method for providing key insights into past warming patterns, and will serve as a good starting point for future investigations.

Romero et al. mapped geomorphological features on the landscape and dated moraine boulders and outwash sediments to constrain past changes of the former Patagonian Ice Sheet in southern Argentina. They show that the Lago Argentino glacier expanded during Marine Isotope Stage 6 (153.0±14.7 ka) and during Marine Isotope Stage 3, culminating at 44.5±8.0 ka and at 36.6±1.0 ka, thus preceding the Last Glacial Maximum. They hypothesize that this was a result of longer and colder winters, as well as increased precipitation resulting from an equatorward shift of the Southern Hemisphere Westerlies.

The paper entitled "Continuous synchronization of the Greenland ice-core and U-Th timescales using probabilistic inversion" presents a first and new continuously measured transfer function that quantifies the age difference between the GICC05 and the U-Th timescale during the last ice age. The transfer function was estimated using a new automated algorithm for proxy data alignment. The approach was the subject of much discussion with the various referees who evaluated it, but it is at least described clearly enough for the community to form its own opinion and henceforth use it.

This is a very interesting proxy development based on grape production. So far, grape harvest dates (ghd) are well-known proxies of past temperatures from April to July that can generate very accurate and fascinating time series (see Labbé et al. 2019, https://doi.org/10.5194/cp-15-1485-2019). However, the analysis of wine must has not yet been considered, although the present manuscript shows that it is a highly potential climate proxy when other related information is missing. This study opens the door to exciting future studies

This article presents an interesting link between climate and weather patterns, and in particular how this may have impacted recent human history.

This paper presents the first gridded reconstruction of the summer (June, July, and August) vapor pressure deficit (VPD) for the past four centuries at the European level. The reconstruction shows that from the mid-1700s, a trend towards higher summer VPD occurred in Central Europe and the Mediterranean region, a trend that has continued through the observational period. The reconstruction is based on 26 European tree-ring oxygen isotope records .

The paper submitted by Meister and colleagues presents the first comprehensive compilation and assessment of diatom oxygen isotope records in lake sediments (δ18OBSi). The authors have supplemented these data with additional lake basin parameters (such as catchment size and residence times). The manuscript first infers the spatial and temporal coverage of δ18OBSi records and then discusses common hemispheric trends on centennial and millennial time scales, with key results such as: - Common patterns for hydrologically open lakes in extra-tropical regions of the Northern Hemisphere. - Common trends during the Common Era and throughout the Holocene corresponding to known climatic epochs such as the Holocene Thermal Maximum, Neoglacial cooling, the Medieval Climate Anomaly and the Little Ice Age. This is a well-written manuscript that should make a valuable contribution to reconstructions of past climates.

Reconstructing large-scale climate patterns from sparse local records is crucial for understanding past climates. Yet, it remains challenging to derive those reconstructions due to the patchiness, uneven spatial distribution, and disparate nature of palaeoclimatic proxy records. In this study, Eichenseer and Jones developed a Bayesian hierarchical model to integrate ecological data with established geochemical proxies into a unified quantitative framework, which bridges the gap in the latitudinal coverage of proxy data. They showed that this framework has the potential to enhance quantitative palaeoclimatic reconstructions especially the latitudinal temperature gradient estimated from datasets with limited spatial sampling.

The Generalized Milankovitch Theory (GMT) presented and discussed in this paper provides a new view on the long-standing problem raised by the Milankovitch theory of glacial-interglacial cycles. The GMT is based on a deep insight into theory, data, and numerical modeling. It condensates the profound knowledge into a fascinatingly elegant dynamic systems theory.

The manuscript by Skinner et al. presents a unique and useful data compilation of ocean-atmosphere radiocarbon age offset (B-Atm) across the last deglaciation. The presented global average B-Atm represents a new benchmark for modelling studies seeking to constrain the ocean's role in past atmospheric CO2 change, and seeking closure of the global radiocarbon cycle. The latter is important for constraining the carbon cycle evolution over the last deglaciation (which remains to be quantitatively accounted for), as well as the history of the geodynamo and solar activity.

This paper presents two new long-term SST records from the south Atlantic Ocean. This region is marked by the strong ocean baroclinicity and associated SST gradient, and is key to understanding late Cenozoic Antarctic glaciation and deep ocean circulation. These records have the potential to significantly contribute to address those questions.

This paper presents a new proxy application of permanent gas isotopes in ice core research based on the kinetic fractionation of gases in the firn column of polar ice sheets (offset from the diffusive equilibrium profile) which may be induced by synoptic atmospheric pressure variations at the surface. Although still exploratory in nature, this new proxy would allow to reconstruct changes in mean synoptic activity for different climate states based on ice core data.

Knowledge of microparticle geometry is essential for accurate calculation of ice core volume-related dust metrics (mass, flux, and particle size distributions) and subsequent paleoclimate interpretations, yet particle shape data remain sparse in most of ice core records. The approach and results of this work are of interest for the broad geoscience community, since it potentially enables a better characterization of all data obtained from the dust in ice cores. This study of samples from South Pole ice core (SPC14) indicates that coarser particles (>5.0 μm diameter) show greater variation in measured aspect ratios than finer particles (<5.0 μm). While fine particle volumes can be accurately estimated using the spherical assumption, applying the same assumption to coarse particles has a large effect on inferred particle volumes.

The paper present results of cutting edge climate model simulations and reconstructions of volcanic eruption magnitude and timing to better understand the environmental context of the Ptolemaic era (305-30BCE). This important period of Ancient Egyptian history is known for its material and scientific advances, as well as for episodes of political and social unrest.

Understanding future changes in storm surge is key to assessing the sustainability of coastal environment to global warming and sea level rise. Past climate intervals may provide useful insights into how storm surge and sea level extremes may respond to climate forcing conditions. Focusing on the Last Interglacial (LIG, ~127,000 years ago), the study by Scussolini and co-authors applied a novel hydrodynamic modelling framework to simulate changes in sea level extremes caused by storm surges under LIG and pre-industrial climate forcing conditions. They discovered a key role of the meridional position and intensity of predominant wind bands in driving spatial distributions of sea level extremes. Their findings have broad implications for interpretations of LIG sea level from coastal archives.

This paper is providing new data about Southern Hemisphere ice sheet extent and dynamics during glaciations older than those occurring during the last climate cycle, a useful dataset that climate models require to calibrate their own reconstructions.

The manuscript presents a reconstruction of global temperature spanning the Holocene, which extends the scope of previous exercises in palaeoclimate data assimilation. The resulting reconstruction presents new insights into changes in global temperature over this period. Most notably, it confirms the results of previous studies that have shown a global cooling trend over the past 6,000 years. It also shows that a cooling trend is found even after allowing for potential biases in the proxies. These results are likely to be of considerable interest to the broader geoscience community.

Causes for the Great Oxidation Event (GOE) has long been an outstanding question in Earth sciences. Yet, the exploration of this question has been limited to the 1D photochemical models. Here, the authors presented new insights to solve this question with a 3D photochemical-climate model. This model allows the investigation of influences of the atmospheric circulation on the GOE and the coupling between the climate and the dynamics of the oxidation. The results are highly interesting, and would be of wide interest in Earth science community.

Although previous studies addressed this original way of how deducing past climate conditions from color paintings, such as aerosol optical depth (AOD) the new results are critically reviewing these former publications and yield new critical elements in this original research. While concluding that reconstructions of AOD may be possible in some specific cases, the authors do warn about the strong uncertainties related to parameters not carefully considered, which restrains the field of possibilities. To assess the scientific soundness of the results, three reviewers from complementary fields of atmospheric sciences positively reviewed the original manuscript. They have contributed to improve the quality of this paper and all agreed upon the conclusions research, which is a fair guarantee that these results should broadly interest the geoscience community.

The paper presents an interesting review of deep time ecosystems and suggests that the interpretation of the evolution of ancient sedimentary systems can be refined and better compared to today’s changes in ecosystems. Concerning the rate of climate change, this study implies that anthropogenically induced global warming and subsequent sea level rise today occurs more than ten times faster than the fastest rise reconstructed for the Oxfordian (159 Ma - 154 Ma)

Huhtamaa et al. assess the socioeconomic consequences of 17th century volcanic eruptions in Fennoscandia. They find that while all the eruptions led to poor grain harvest in the region through their climatic impact, the socioeconomic response varied. They suggest that the micro-regional socioeconomic system modulated the socioeconomic response to each eruption. Such a framework should be used to further our understanding of the impact of volcanic eruptions on societal crises.

Combining a new record of bipolar volcanism from Greenland and Antarctic ice cores with records of abrupt climate change derived from the same ice cores, this study provides the most convincing evidence yet of the influence of large volcanic eruptions on long term climate variability, namely Dansgaard-Oeschger warming events. This should garner significant popular interest.

This study presents a novel Monte Carlo Empirical Orthogonal Function analysis that combines a dense isotopic proxy network with isotope-enabled climate models to study dynamics and variability of South American Monsoon System (SAMS) over past millennium. This analysis reveals that the leading modes of SAMS variability over the past millennium are regionally stable, arising independently of external forcing (solar, volcanic, orbital). Significant enhancement of the SAMS is accompanied by southward displacement of the South Atlantic Convergence Zone during the Little Ice Age, giving rise to a strengthened South American precipitation dipole. Proxy-model comparison demonstrates outstanding mismatches in centennial-scale hydroclimate departures during the Medieval Climate Anomaly and Little Ice Age from the last millennium mean state. The results of last millennium SASM variability contextualizes modern variability of SASM. The identified proxy data-model mismatch raises a key question to climate simulations of SASM variability, which may attract broad attention from paleoclimate modeling community. Moreover, the reported methodology can be applied to paleoclimate problems of various spatial and temporal scales and is of relevance for the broader geoscience community.

The alternation of vegetated and arid Sahara has long attracted wide interest in paleoclimate field. This study presents a suite of highly interesting simulations of the past 190,000 years with a state-of-the-art Earth system model of intermediate complexity. This set of experiments are rigorously designed and analyzed, which allow the authors to make an important discovery of what controls the onset of African Humid Period: i.e. a threshold in orbital forcing, which decreases at elevated CO2 concentration. Moreover, maximum rates of change in simulated vegetation extent during the onset and termination of African Humid Period correlate strongly with the rates of change of the orbital forcing. A factor separation analysis further confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past African Humid Period. These results have broad implications to future variability of Sahara climate in response to orbital forcing changes and non-linear earth system feedbacks.

Reviewer 2 of that paper made a clear statement about that paper which can be used at the present level. "This manuscript’s goal is to provide recommendations for how best to develop age models for new ice cores that are anticipated to retrieve ice from 0.8-1.5 Myr ago. As significant resources are being invested to retrieve this ice, the thoughtful advance planning for the creation of these age models is commendable. In proposing specific age modeling strategies, the manuscript also generates useful hypotheses about the types of climate responses that are expected to be found in the new ice cores."