Plenary speakers

Pascale Braconnot, Lab. des Sciences du Climat et de l’Environnement, CEA-CNRS, Gif-Sur-Yvette, France.

Thoughts on the role of paleoclimate model-data comparisons in assessing climate projection levels of confidence

The Paleoclimate Modelling Intercomparison Project (PMIP) was launched 30 years ago, with the aim to understand the climate response to various external forcings and to assess the ability of state-of-the-art climate models to represent climate states different from the modern one. Since the beginning, this project was endorsed by PAGES and WCRP, and has fostered paleoclimate reconstructions from different paleoclimate archives and model-data comparisons for key periods in the past. Paleoclimate simulations are now recognized as essential steps in the overall assessment of the realisms and progresses of climate models used for future climate projections. They offer a unique way to test model performances over a wide range of climate situations that have similar magnitude than the one expected in the future.

This presentation will address several aspects of model-data comparisons, and of the evolution of the underlying scientific questions. Models have evolved from only including atmospheric components to fully coupled Earth system models that account for the interplay between atmosphere, land ocean, and ice through the energy and water cycle, as well as the coupling between these cycles and the carbon cycle. New possibilities are offered by forward modeling of the way paleoclimate archives record environmental changes, new paleoclimate indicators, or statistical methods. They open method questions and new challenges to tell if model content in terms of physics and biogeochemisty is sufficient to represent different climate states and the transitions between them. Examples will consider climate sensitivity, the water cycle, and interannual to centennial variability to illustrate different levels of model evaluation – from process understanding, to full benchmarking considering the different sources of uncertainties in model and climate reconstructions. They will also be used to question how to further provide more direct constraints on the quality of climate projections.

About the speaker:

Pascale Braconnot is a former member of the PAGES Scientific Steering Committee, and a climate modeling specialist. She is now a member of the WCRP Joint Scientific Committee. Her scientific activities range from coupled ocean-atmosphere model development with a focus on air-sea coupling, to the use of coupled Earth system models in different climatic contexts. Her main scientific interests are the role of insolation, ocean feedbacks, and the hydrological cycle and its variability in past and future climate changes. She has been the PMIP coordinator since 2001 and has contributed to or led several multi-disciplinary projects to understand past or future environmental changes. In 2009, she was awarded the EGU Milankovic Medal for her work on past monsoons and analyses of ocean and vegetation feedbacks. She was a Lead Author of the IPCC AR4 and AR5 and Review Editor for Chapter 8 on the water cycle in the IPCC AR6 report.

Rachid Cheddadi, CNRS, University of Montpellier, France.

Glacial refugia and future microrefugia: an effective plan to save plant species?

In Europe and North America, plant species survived the last glacial period in refugial areas that provided them with a suitable climate that differed from the global climate until the Holocene warming. Today, long-lived plant species are threatened by global warming, and even if the rate of migration or dispersal of some species can follow the velocity of climate change, there are many barriers that prevent them from adapting their range, which in may lead to their extinction. One of the ways to protect these plant species from a potential extinction is to assess the ability of some of their populations to survive locally and determine if the areas where they occur today are suitable as future microrefugia to accommodate them in the long term, as was the case during the unfavorable last ice age. Defining tandems of suitable microrefugia and populations requires historical data on past changes in the species' range, its resilience, its genetic adaptability, the topography of the landscape, and a prediction of future climate.

In this talk, I will summarize what fossil data collected in Morocco over the past 30 years or more have revealed about past vegetation and climate change since the last ice age, and link this to an extended genetic study of an endemic and endangered mountain tree species to show that microrefugia may be an effective option for protecting species from ongoing global warming.

About the speaker:

Rachid Cheddadi is research director at the CNRS, University of Montpellier, France. His research topics include paleoecology, paleobiogeography, and quantitative past climate reconstructions from fossil pollen records. He has studied the role of glacial refugia and postglacial recolonization processes in the distribution of modern plant species, and how these past changes may help to manage future threats to plant species. His longstanding collaborations with vegetation modelers and geneticists have led him into the field of protecting endangered mountain tree species in Morocco and Lebanon and prioritizing effective protected areas in the face of ongoing climate change.

Hai Cheng, School of Human Settlement and Civil Engineering, Xi'an Jiaotong University, Xi'an, China.

High and low latitude climate interactions at multiple timescales

Earth’s natural climate system varies persistently at multiple timescales with different dynamics. While we outline a sustainable future for a climate context, detailed case studies of past climate variations on various timescales are important, which however, haven’t yet been fully developed. In the last two decades, significant advances in understanding the climatic controls on d18O in atmosphere and cave environment together with landmark developments in U-Th dating techniques have propelled speleothems to the forefront of paleoclimatology and substantially improved our view of our planetary climate system on a wide range of timescales. I will present here speleothem data across multiple timescales, providing detailed case study in the context of high and low latitude climate interactions. On orbital-scale, a set of cave d18O records from different climate domains suggest that precession-induced changes in summer insolation in producing distinct climate variability in the ice-sheet proximal and distant regions via the (delayed) ice-volume forcing and in-phase CO2 feedback/forcing, and the direct summer insolation forcing and CH4 feedback in driving the climate variability at low-latitude monsoon regimes. On millennial-scale, we used a combination of well-dated speleothem and ice core records to pinpoint the timings of the Younger Dryas event onsets and terminations in various climatic domains around the world. We show that the event occurred first at high northern latitudes then propagated southward into the tropical monsoons belt within a few decades, eventually reaching Antarctica before reversing the course to its eventual termination. It appears that changes in Antarctic and/or low-latitude hydroclimate might have served as precursors of the YD termination or Holocene onset. On centennial-decadal scales, new high-resolution record of the 8.2 kyr event from the Southern Indian Ocean reinforce the double peak structure centered at ~8.2 and 8.1 kyr BP, withstanding the interhemispheric anti-phased relationship. Importantly, the significance of the 8.1 kyr BP peak is prominent in Southern than Northern Hemisphere proxy records, likely implying a different mechanism. There is a large array of hydroclimate events worldwide around 4.2-3.9 kyr BP, the so-called ‘4.2 kyr event’. These events are not representative of one synchronous event globally, and it remains unclear whether the events are indeed a manifestation of one time-transgressive event in different geographical regions. I will conclude by briefly presenting the annual resolution speleothem record across the ‘Chongzhen Drought’ event (~1637–1644 CE) and its wide social-economic impacts. The case studies are fundamental for gaining insights into the inner workings of the earth’s climate system for a sustainable future.

About the speaker:

Hai Cheng has been a professor at the Xi’an Jiaotong University since 2010. He received his PhD in geochemistry in 1988 from Nanjing University. He joined the Institute of Geology at the Chinese Academy of Sciences in 1988 and was a Senior Research Scientist at the University of Minnesota from 1993 to 2010. Hai Cheng has received many honors and awards, including appointment as a Geochemical Fellow of the Geochemical Society and European Association of Geochemistry (2015), AGU Fellow (2017), and the AGU Emiliani Lecturer (2019). He has been at the leading edge in developing U-series dating techniques and provided innumerable U-Th dates for collaborators who have made discoveries about paleoclimatic change worldwide. He played a world-leading role in paleoclimate studies of global speleothems, including the longest East Asian (640 kyr) and Indian (280 kyr) monsoon records, the longest Westerly climate records from central Asia (500 kyr) and North America (335 kyr), and the longest record from the Amazon Basin (250 kyr).

María Eugenia Ferrero, Instituto Argentino Nivologia, Glaciología y Ciencias Ambientales (IANIGLA), CCT-CONICET, Argentina.

Dendrochronology and climate in the tropical Andes and lower lands: What we know and what to expect
M. Eugenia Ferrero1, Mariano S. Morales1, Edilson J. Requena-Rojas2, Lidio López1, Duncan Christie3, Laia Andreu-Hayles4, Milagros Rodríguez-Catón5, Mathias Vuille6 and Ricardo Villalba1
1Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, CONICET, Mendoza, Argentina
2Laboratorio de Dendrocronología, Universidad Continental, Huancayo, Peru
3Laboratorio de Dendrocronología y Cambio Global, Universidad Austral de Chile, Valdivia, Chile
4Lamont-Doherty Earth Observatory of Columbia University, New York, USA
5Department of Plant Sciences, University of California, Davis, USA
6Department of Atmospheric and Environmental Sciences, State University of New York, Albany, USA

All Andean populations depend almost exclusively on rainwater for domestic use, irrigation and agricultural activities. Since these activities are directly affected by climatic conditions (high social vulnerability), it is essential to know the changes and trends in rainfall. The presence of the Andes and altitudinal gradients create very marked differences in hydroclimatic patterns; instrumental records help to understand spatial patterns but are very limited to understand long-term variability. In this case, dendrochronology allows for the development of high-resolution paleoclimatic reconstructions.

It is well known that developing ring-width chronologies in the tropics is challenging, both because of the enormous species diversity and variability of environments, as well as the forest and land use processes occurring in different regions. In tropical South America, a network of dendrochronological archives constructed from different species has been developing and growing over the last 15 years. These paleorecords are helping us to understand how climate (particularly precipitation) operates in different environments and on different timescales.

In this talk I will present advances in tropical dendrochronology, with emphasis on the Andes and adjacent lowland regions, the hydroclimatic reconstructions developed to date in different biomes and how these records are helping us to understand large-scale forcings, long-term trends and how precipitation is behaving in recent years in the South American tropics.

About the speaker:

Dr. Eugenia Ferrero is a researcher at the Institute of Nivology, Glaciology and Environmental Sciences (IANIGLA, CONICET) in Mendoza, Argentina. Her research focuses on hydroclimatic reconstructions along tropical and subtropical environments in South America based on tree-ring records. Her work includes the study of instrumental data variability, and the impacts of climate on ecosystems long-term responses.

Lindsey Gillson, Dept of Biological Sciences, University of Cape Town, Rondebosch, South Africa.

A past-present-future perspective on using paleoecology to conserve African ecosystems

Africa has a wealth of biodiversity, and a long history of customary management, alongside a large rural population that is directly dependent on ecosystem services and rapidly changing socio-economic contexts. These circumstances demand sustainable strategies that balance development and conservation for the future. Conservation of biodiversity and social-ecological systems therefore requires a past-present-future perspective that includes local knowledge as well as an awareness of future resilience and adaptability.

One of the main challenges in conservation management in Africa is to conserve biodiversity and enhance livelihoods. Central to this is knowledge of the historical range of variability in key ecological parameters such as tree density, fire regimes, and herbivore density, which were disrupted on much of the continent during the 18th and 19th centuries. Paleoecology offers a potentially valuable role in reviewing and assessing the range of these variabilities on scales of centuries to millennia. Specifically:
• Reconstructing vegetation change using fossil pollen provides insights into how landscapes looked before the intensive management that began in the 19th century.
• Analysis of charcoal can help to reconstruct fire regimes prior to policies of fire suppression and prescribed burning.
• Analysis of coprophilous spores can be used to address important conservation questions regarding the abundance and management of herbivores, both wild and domestic.
• Phytoliths, diatoms, and other climate-related proxies can help us to gain an understanding of how ecosystems responded under warmer/drier climates.

Application of such data in real-world conservation contexts requires good calibration against modern analogues that can convert paleo proxies to indicators that are useful and understandable to managers and other stakeholders. Furthermore, modeling tools can facilitate stakeholder engagement by incorporating local knowledge of ecosystem dynamics. This enables stakeholders to explore the effects of different future climate and land-use scenarios on land cover and ecosystem services, improving inclusivity of conservation in the process.

About the speaker:

Lindsey Gillson is Professor at the Plant Conservation Unit, University of Cape Town, South Africa. Her interests include landscape ecology, theoretical ecology, conservation biology and applied paleoecology. Her research focusses on long-term ecosystem dynamics, and the implications for conservation, sustainability, and ecosystem services. She has a focus on African ecosystems, especially savannas, and is interested in multidisciplinary studies of these complex socio-ecological systems. She is currently leading extensive projects on various ecosystems in southern Africa and Madagascar.

Jean-Jacques Hublin, Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.

The origin of Homo sapiens: a North African perspective

Both genetic data of extant humans and fossil remains point to an African origin of our species. For a long time, the oldest securely dated specimens of Homo sapiens fossils have only been known from sites in East Africa. According to the so-called “Garden of Eden” model, all humans living today were believed to descend from a small population that lived in East Africa around 200 thousand years ago. In this region, humans anatomically close to modern humans would have appeared quite rapidly around this date. However, the discoveries made at the Jebel Irhoud site (Morocco) have seriously challenged this model. In this cave site, deposits dated to about 300 thousand years ago yielded primitive forms of Homo sapiens associated with stone tools assemblages from the early Middle Stone Age. Not only did this discovery extend the antiquity of our species, but it also demonstrated that over a long period of time Homo sapiens evolved rather gradually. It led to the rise of a new model for Homo sapiens origin involving the entire African continent. During the late Middle and early Late Pleistocene, “Green Sahara” climatic episodes facilitated the spread of African populations to the Near-East and their contacts with Eurasian Neandertals. During this period, archeological evidence documents the increasing technical and social complexity of Middle Stone Age African groups. It is highly likely that these changes triggered the subsequent expansion of Homo sapiens into the colder mid-latitude environments of Eurasia and the subsequent replacement or absorption of local archaic hominins.

Allison Karp, Yale University, New Haven, CT, USA.

Heterogenous savanna fire response to paleo-rainfall shifts

Rainfall regimes in the tropics and subtropics are projected to change in the future. Predicting how savanna fire will respond to these shifts is non-trivial because rainfall moderates fire spread through both fuel-load (the amount of fuel) and fuel-moisture (if the fuel is dry enough to ignite), which means annual burned area may either increase or decrease as rainfall increases. The African Humid Period (AHP; ~14-6 ka), when rainfall increased substantially across tropical Africa, provides an opportunity to examine how fire activity across sites responds to long-term climate forcings. In this talk, I will preset two new fire records, and synthesize them alongside paleofire and hydroclimatic data from several African sites that span the savanna rainfall gradient. We find that during the AHP, fire patterns differ, with elevated fire activity at arid to semi-arid sites and decreasing fire activity at mesic-to-humid sites. Results emphasize that changes in precipitation can elicit different fire responses depending on the absolute mean annual precipitation at a site, and that accounting for climatic heterogeneity within biomes may improve predictions of how fire activity will respond to future changes in rainfall.

About the speaker:

Dr. Allison Karp is a molecular paleoecologist specializing in paleofire in grassy systems. Her research asks how wildfire has impacted terrestrial ecosystems, carbon cycling, and climate on geologic timescales. Allison completed her Ph.D. in the Department of Geosciences at Pennsylvania State University, and she is currently a Postdoctoral Associate in the Department of Ecology and Evolutionary Biology at Yale University. Her work has contributed to understanding of how feedbacks between changing fire, climate, and biotic (i.e., floral and faunal) interactions lead to abrupt landscape transitions on a variety of spatial and temporal scales.

Timothy Lenton, University of Exeter, Exeter, UK.

Learning from past climate tipping points to avoid future ones

Tipping points exist in social, ecological and climate systems and those systems are increasingly causally intertwined in the Anthropocene. Climate change and biosphere degradation have advanced to the point where we are already triggering damaging environmental tipping points, and to avoid worse ones ahead will require finding and triggering positive tipping points towards sustainability in coupled social, ecological and technological systems. To help with that Tim will outline how tipping points can occur in continuous dynamical systems and in networks, the causal interactions that can occur between tipping events across different types and scales of system – including the conditions required to trigger tipping cascades, the potential for early warning signals of tipping points, and how they could inform deliberate tipping of positive change. In particular, the same methods that can provide early warning of damaging environmental tipping points can be used to detect when a socio-technical or socio-ecological system is most sensitive to being deliberately tipped in a desirable direction. Tim will provide some example targets for such deliberate tipping of positive change.

About the speaker:

Tim Lenton is Director of the Global Systems Institute and Chair in Climate Change and Earth System Science at the University of Exeter. He has over 20 years of research experience in studying the Earth as a system, and developing and using models to understand its behaviour. He is particularly interested in how life has reshaped the planet in the past, and what lessons we can draw from this as we proceed to reshape the planet now. His accolades include the Times Higher Education Award for Research Project of the Year 2008; the Royal Society of London William Smith Fund, 2008; the Royal Society Wolfson Research Merit Award, 2013. He is a Turing Fellow and a Fellow of the Linnean Society, the Geological Society and the Society of Biology.

Pradeep Srivastava, Indian Institute of Technology and Wadia Institute of Himalayan Geology, India.

Flood history of the Himalaya: A geologist’s perspective
Pradeep Srivastava1,2, Pankaj Sharma2, Poonam Chahal3, Anil Kumar2 and Sandeep Panda2
1Department of earth Sciences, Indian Institute of Technology, Roorkee, India
2Wadia Institute of Himalayan Geology, Dehradun, India
3Fredy & Nadine Herrmann Institute of Earth Sciences, Hebrew University, Jerusalem, Israel

Rivers that drain the Himalaya experience frequent floods leading to large-scale sediment erosion, and landscape changes in the mountains and its foreland. Such natural changes at an interface with society are disastrous accounting to severe losses in terms of economy and lives. The volatility of these extreme events increases as the climate warms and in order to prepare better predictive models the long-term records of flood (1) frequency, (2) forcing factors, and (3) sediment/water routing of these events are needed as the century-long instrumental records that currently exist are not enough.

Slack Water Deposits (SWDs) are the sedimentary archives that occur along the rivers at the specific geomorphic locations represent the past floods. When chronologically constrained, SWDs have potential to extend flood records deeper in time. In this context, the Indus and the Brahmaputra rivers that cover the full climate and tectonic spectrum of Himalaya were explored, dated, and studied for their sediment provenance. In this talk, we will discuss the results obtained on these studies and attempt to answer: (1) Why are the floods in the Indus and the Brahmaputra an order of magnitude different in terms of discharge they bring? (2) Where do they erode the most and what Geology explains it? (3) What are timescales and driving factors of these floods in Himalaya?

About the speaker:

Dr. Pradeep Srivastava is a faculty in the Department of Earth Sciences, Indian Institute of Technology-Roorkee, INDIA. Dr. Srivastava is trained fluvial sedimentologist, geomorphologist and luminescence dating person. He applies his knowledge to understand the evolution of thrust and fold belts and riverine responses to climate change across Quaternary timescales. Dr. Srivastava has worked extensively on exploring paleoflood records across the Himalaya. He has 25 years of working and publishing experience in Himalaya, Ganga Foreland, Namib deserts, and southeastern America.