Manuel Chevalier

Who I am

I am a paleoclimatologist focusing on deepening our understanding of late Quaternary climate change in the tropics. While I worked with several climate indicators, the bulk of my work involves pollen data because of their versatility as climate indicators and strong connection with ecological principles. I analyse these pollen records using advanced statistical techniques that I develop and tailor to quantify past climate changes and their uncertainties.

The context of my research

Accurate quantitative estimates of past climate play a vital role in characterising past climate changes in a given region and assessing the performance of Earth System Models (ESMs). However, recent studies compiling climate data have revealed a significant disparity in the quality and quantity of quantified climate reconstructions between tropical and southern temperate regions, compared to the wealth of data available from the Northern Hemisphere extratropics and, to some extent, Antarctica. This glaring disparity underscores our limited understanding of past climate variability in these crucial regions, introducing considerable uncertainty into both past and future climate simulations. Beyond the evident challenges this poses for our grasp of regional climate dynamics, these pronounced “quantification data gaps” also cast a shadow over our global understanding of the climate system’s dynamics as a whole. Given the contemporary context of ongoing and projected climate change, there exists a pressing imperative to acquire more comprehensive and refined climate reconstructions from these less-studied regions. Enhanced comprehension of past climate variations in these areas will be pivotal to provide essential pieces of the puzzle and aid in unraveling the spatiotemporal connections among various elements of the global climate system.

What I do

One aspect of my research centers on accurately estimating and accounting for the inherent uncertainties in natural proxy data to reconstruct climates beyond the instrumental period. I have pioneered a specialised reconstruction method known as CREST, designed to specifically address the challenges posed by the diverse and complex vegetation of tropical regions. My work extends beyond methodology; it delves into the intricate interplay between palaeodata and climate models. I leverage the unique strengths of both to gain deeper insights into past climate dynamics. With these innovative tools at my disposal, I explore the spatial manifestations of past climate change in the tropics, spanning time scales from centuries to the broader context of glacial-interglacial cycles.

Despite the urgency of comprehensively analysing numerous records to address quantification data gaps, I hold a deep appreciation for the unique character of each individual record. My work is dedicated to the meticulous statistical examination of these records on an individual basis, thus circumventing the pitfalls associated with large-scale, indiscriminate analyses. To achieve this, I aim to collaborate closely with regional and proxy experts, fostering the highest level of accuracy in our data modelling and interpretation. By working in partnership, we can develop collaborative initiatives encompassing a wide range of topics falling within the broad scope of enhancing our comprehension of past climate change across the Global South. Drawing upon my extensive experience with various climate proxies, derived from diverse regions and spanning different time scales, I am eager to contribute to our collective pursuit of knowledge in this field.

Collaborations

Irrespective of your current career stage (student, Early-Career Researcher, or established Scientist) or location in the World, I am interested in working with you to further our understanding of past climate change in the Global South. You can reach me at chevalier.manuel@gmail.com.

Keywords

news

Sep 1, 2023	Very excited to begin my new role as Project Leader in the PalMod Phase 3 project.
Jun 19, 2023	With great honour, I am proud to announced that I was elected to be the President of the Paleoclimate Commission of INQUA for the upcoming inter-congress phase (2023-2027).
May 20, 2023	Two new publications: 1) a novel metric to do data-model comparisons in the vegetation space here and 2) a compilation of pollen-based climate reconstructions for the Northern Hemisphere here.

selected publications

Open-Access

crestr: an R package to perform probabilistic climate reconstructions from palaeoecological datasets

Chevalier, Manuel

Climate of the Past, Apr 2022. DOI: 10.5194/cp-18-821-2022

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Statistical climate reconstruction techniques are fundamental tools to study past climate variability from fossil proxy data. In particular, the methods based on probability density functions (or PDFs) can be used in various environments and with different climate proxies because they rely on elementary calibration data (i.e. modern geolocalised presence data). However, the difficulty of accessing and curating these calibration data and the complexity of interpreting probabilistic results have often limited their use in palaeoclimatological studies. Here, I introduce a new R package (crestr) to apply the PDF-based method CREST (Climate REconstruction SofTware) on diverse palaeoecological datasets and address these problems. crestr includes a globally curated calibration dataset for six common climate proxies (i.e. plants, beetles, chironomids, rodents, foraminifera, and dinoflagellate cysts) associated with an extensive range of climate variables (20 terrestrial and 19 marine variables) that enables its use in most terrestrial and marine environments. Private data collections can also be used instead of, or in combination with, the provided calibration dataset. The package includes a suite of graphical diagnostic tools to represent the data at each step of the reconstruction process and provide insights into the effect of the different modelling assumptions and external factors that underlie a reconstruction. With this R package, the CREST method can now be used in a scriptable environment and thus be more easily integrated with existing workflows. It is hoped that crestr will be used to produce the much-needed quantified climate reconstructions from the many regions where they are currently lacking, despite the availability of suitable fossil records. To support this development, the use of the package is illustrated with a step-by-step replication of a 790 000-year-long mean annual temperature reconstruction based on a pollen record from southeastern Africa.
An uncertainty-focused database approach to extract spatiotemporal trends from qualitative and discontinuous lake-status histories

De Cort, Gijs, Chevalier, Manuel, Burrough, Sallie L., Chen, Christine Y., and Harrison, Sandy P.

Quaternary Science Reviews, Apr 2021. DOI: 10.1016/j.quascirev.2021.106870

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Changes in lake status are often interpreted as palaeoclimate indicators due to their dependence on precipitation and evaporation. The Global Lake Status Database (GLSDB) has since long provided a standardised synopsis of qualitative lake status over the last 30,000 14C years. Potential sources of un- certainty however are not recorded in the GLSDB. Here we present an updated and improved relational-database framework that incorporates uncertainty in both chronology and the interpretation of palae- oenvironmental data. The database uses peer-reviewed palaeolimnological studies to produce a consensus on qualitative lake-status histories, whose chronologies are revised and standardized through the recalibration of radiocarbon dates and the application of Bayesian age-depth modelling for strati- graphic archives. Quantitative information on absolute water-level elevation is preserved if available from geomorphological sources. We also propose a new probabilistic analytical framework that accounts for these uncertainties to reconstruct synoptic, integrated environmental signals. The process is based on a Monte Carlo algorithm that iteratively samples individual lake-status histories within the limits of their uncertainties to produce many possible scenarios. We then use Recursively-Subtracted Empirical Orthogonal Function analysis to extract dominant patterns of lake-status variability from these scenarios. As a proof of concept, we apply this framework to 67 sites in eastern and southern Africa whose lake- status histories cover part of the late Pleistocene and/or Holocene. We show that, despite the sometimes large temporal and interpretation uncertainties, and the inclusion of highly discontinuous lake-status time series, identifying the major known millennial-scale climatic phases during the last 20,000 years is possible. Our framework was also able to identify an antiphased response between the lake basins in eastern and interior southern Africa to these changes. We propose that our new database and meth- odology framework serves as a template for efficient lake-status data synthesis, encourages the incor- poration of lake-status data in palaeoclimate syntheses, and expands the possibilities for the use of such data in the evaluation of climate models.
Open-Access

Pollen-based climate reconstruction techniques for late Quaternary studies

Chevalier, Manuel, Davis, Basil A.S., Heiri, Oliver, Seppä, Heikki, Chase, Brian M., Gajewski, Konrad, Lacourse, Terri, Telford, Richard J., Finsinger, Walter, Guiot, Joël, Kühl, Norbert, Maezumi, S. Yoshi, Tipton, John R., Carter, Vachel A., Brussel, Thomas, Phelps, Leanne N., Dawson, Andria, Zanon, Marco, Vallé, Francesca, Nolan, Connor, Mauri, Achille, Vernal, Anne, Izumi, Kenji, Holmström, Lasse, Marsicek, Jeremiah, Goring, Simon, Sommer, Philipp S., Chaput, Michelle, and Kupriyanov, Dmitry

Earth-Science Reviews, Nov 2020. DOI: 10.1016/j.earscirev.2020.103384

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Fossil pollen records are well-established indicators of past vegetation changes. The prevalence of pollen across environmental settings including lakes, wetlands, and marine sediments, has made palynology one of the most ubiquitous and valuable tools for studying past environmental and climatic change globally for decades. A complementary research focus has been the development of statistical techniques to derive quantitative estimates of climatic conditions from pollen assemblages. This paper reviews the most commonly used statistical techniques and their rationale and seeks to provide a resource to facilitate their inclusion in more palaeoclimatic research. To this end, we first address the fundamental aspects of fossil pollen data that should be considered when undertaking pollen-based climate reconstructions. We then introduce the range of techniques currently available, the history of their development, and the situations in which they can be best employed. We review the literature on how to define robust calibration datasets, produce high-quality reconstructions, and evaluate climate reconstructions, and suggest methods and products that could be developed to facilitate accessibility and global usability. To continue to foster the development and inclusion of pollen climate reconstruction methods, we promote the development of reporting standards. When established, such standards should 1) enable broader application of climate reconstruction techniques, especially in regions where such methods are currently underused, and 2) enable the evaluation and reproduction of individual reconstructions, structuring them for the evolving open-science era, and optimising the use of fossil pollen data as a vital means for the study of past environmental and climatic variability. We also strongly encourage developers and users of palaeoclimate reconstruction methodologies to make associated programming code publicly available, which will further help disseminate these techniques to interested communities.
Open-Access

Temperature change in subtropical southeastern Africa during the past 790,000 yr

Chevalier, Manuel, Chase, Brian M., Quick, Lynne J., Dupont, Lydie M., and Johnson, Thomas C.

Geology, Jan 2021. DOI: 10.1130/G47841.1

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Across the glacial-interglacial cycles of the late Pleistocene (∼700 k.y.), temperature variability at low latitudes is often considered to have been negligible compared to changes in precipitation. However, a paucity of quantified temperature records makes this difficult to reliably assess. In this study, we used the Bayesian method CREST (Climate REconstruction SofTware) to produce a 790,000 yr quantified temperature reconstruction from a marine pollen record from southeast Africa. The results reveal a strong similarity between temperature variability in subtropical Africa and global ice volume and CO2 concentrations, indicating that temperature in the region was not controlled by local insolation, but followed global trends at these time scales, with an amplitude of ∼4 °C between glacial minima and interglacial maxima. The data also enabled us to make an assessment of the impact of temperature change on pollen diversity, with results showing there is no link between glacial-age temperatures/CO2 and a loss of diversity in this record.
Southeast African records reveal a coherent shift from high- to low-latitude forcing mechanisms along the east African margin across last glacial–interglacial transition

Chevalier, Manuel, and Chase, Brian M.

Quaternary Science Reviews, Oct 2015. DOI: 10.1016/j.quascirev.2015.07.009

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Late Quaternary climate variability in the southern African subtropics is still only poorly resolved, with significant complexity and apparent contradictions in the regional dataset. To more effectively interpret and synthesize key regional records, we reanalysed the data from 13 pollen sequences from the summer rainfall zone of South Africa spanning the last 45,000 years, obtaining directly comparable quantitative reconstructions of mean annual temperature and summer rainfall. Temperature reconstructions from across the region provide consistent results, with all sites reflecting trends observed in southwest Indian Ocean sea-surface temperatures in the adjacent Mozambique Channel. Precipitation reconstructions are more heterogeneous, with two distinct subregions being identified. In the northeast, long-term trends in precipitation are determined by sea-surface and continental temperature trends, revealing a positive relationship between temperature and rainfall. This long-term pattern appears to be primarily driven by high northern latitude mechanisms, with direct local insolation being subordinate. Their relative impact reversed during terminal glacial period/early Holocene, at which time direct insolation forcing became the main driver of rainfall variability. Further south, in central South Africa, precipitation variability appears also to be influenced by the latitudinal position of the Southern Hemisphere westerlies, which combine with tropical flow to create tropical-temperate trough, advecting moisture into the interior. In this region, periods of maximum precipitation coincide with periods of elevated SSTs and equatorward expansions of the westerly storm track. This study allows for a fully constrained understanding of climate dynamics along the eastern African margin for the last 45,000 years, linking dynamics to drivers and describing how the climate systems evolved across the last glacial–interglacial transition.