Odile Peyron

Contrasting patterns of hydrological changes in Europe in response to Holocene climate cooling phases.

The quantitative reconstruction of climatic parameters from pollen and lake-level data obtained at Saint-Jorioz, Lake Annecy (eastern France), gives evidence for cooler and wetter conditions during the 8.2 ka cold event. A comparison of these regional data with other hydrological records reconstructed in Europe for the same period suggests, as a working hypothesis, that mid-latitudes between ca 50° and 43° underwent wetter conditions in response to the cooling, whereas northern and southern Europe were marked by drier climate, in the latter case leading to an interruption of the sapropel 1 formation in the Mediterranean. A similar hydrological tri-partition of Europe can be observed during other Holocene cooling phases associated with North Atlantic IRD events. Data indicate, that the middle zone characterised by wetter climate conditions could have had a more extended latitudinal amplitude during phases of climate cooling weaker than the 8.2 ka event. The differences in expansion of the wet mid-European zone depending on Holocene climate cooling phases could reflect variations in the strength of the Atlantic Westerly Jet in relation with the thermal gradient between high and low latitudes.

Holocene seasonality changes in the central Mediterranean region reconstructed from the pollen sequences of Lake Accesa (Italy) and Tenaghi Philippon (Greece)

This study presents pollen-based climate reconstructions of Holocene temperature and precipitation seasonality for two high-resolution pollen sequences from the central (Lake Accesa, central Italy) and eastern Mediterranean (Tenaghi Philippon, Greece) regions. The quantitative climate reconstruction uses multiple methods to provide an improved assessment of the uncertainties involved in palaeoclimate reconstructions. The multimethod approach comprises Partial Least Squares regression, Weighted Average Partial Least Squares regression, the Modern Analogues Technique, and the Non-Metric-Multidimensional Scaling/Generalized Additive Model method. We find two distinct climate intervals during the Holocene. The first is a moist period from 9500 to 7800 cal. BP characterised by wet winters and dry summers, resulting in a strongly seasonal hydrological contrast (stronger than today) that is interrupted by a short-lived event around 8200 cal. BP. This event is characterised by wet winters and summers at Accesa whereas at Tenaghi Philippon the signal is stronger, reversing the established seasonal pattern, with dry winters and wet summers. The second interval represents a later aridification phase, with a reduced seasonal contrast and lower overall precipitation, lasting from 7800 to 5000 cal. BP. Present-day Mediterranean conditions were established between 2500 and 2000 cal. BP. Many studies show the Holocene to have a complex pattern of climatic change across the Mediterranean regions. Our results confirm the traditional understanding of an evolution from wetter (early Holocene) to drier climatic conditions (late Holocene), but highlight the role of changing seasonality during this time. Our data yield new insights into the aspect of seasonality changes, and explain the apparent discrepancies between the previously available climate information based on pollen, lake-levels and isotopes by invoking changes in precipitation seasonality.

Was the climate of the Eemian stable? A quantitative climate reconstruction from seven European pollen records

The aim of the present study is to estimate the range of the climatic variability during the Eemian interglacial, which lasted about 10,000 years (marine isotopic stage 5e). The modem pollen analogue technique is applied to seven high resolution pollen records from France and poland to infer the annual precipitation and the mean temperature of the coldest month. The succession of pollen taxa and the reconstructed climate can be interpreted coherently. The warmest winter temperatures are centred in the first three millennia of the Eemian interglacial, during the mixed oak forest phase with Quercus and Corylus as dominant trees. A rapid shift to cooler winter temperatures of about 6 degrees to 10 degrees C occurred between 4000 and 5000 years after the beginning of the Eemian, related to the spread of the Carpinus forest. This shift is more obvious for the reconstructed temperatures than for precipitation and is unique and irreversible for the whole Eemian period. Following this climatic shift of the Eemian, variations of temperature and precipitation during the fast 5000 years were only slight with an amplitude of about 2 degrees to 4 degrees C and 200 to 400 mm/yr. The estimated temperature changes were certainly not as strong as those reconstructed for the stage 6/5e termination or the transition 5e/5d. This is consistent with the constantly high ratio of tree pollen throughout the Eemian, indicative of a succession of temperate forest types. This gradual transition between different forest landscapes can be related to intrinsic competition between the species rather than to a drastic climatic change

Mid-Holocene vegetation history of the central Mediterranean

This paper provides a synthesis of available palynological data from central Mediterranean lacustrine records, studying the causes of the important vegetation changes which occurred in the mid Holocene. They are illustrated by seven well-dated records, lakes Accesa, Mezzano, Vico, and Pergusa in Italy, Maliq, Voulkaria and Malo J. in the Balkans. A good tool for disentangling climate change and human impact is the combined interpretation of pollen percentage and concentration/influx data. Pollen concentration/influx drops are an indication of increased erosion in the catchment, mainly ascribable to forest reduction. There is no doubt that major synchronous changes could not have been caused by humans alone. Many southern European records show substantial differences between early- and late-Holocene vegetation, suggesting a general evolution from wetter to drier climatic conditions, in agreement with arid phases recognised by other, independent palaeoclimatic methods. Other pollen records show a different trend. The role of changing seasonality seems important for this region of the Mediterranean. Precipitation seasonality increased during the early to mid Holocene with winter precipitation attaining a maximum, and summer precipitation a minimum. At least three rapid climate events with changes in plant biomass are in evidence: an abrupt and short change around 8200 yr BP, another one centred around 6000 yr BP, and one soon after 3000 yr BP. From the beginning of the Bronze Age (c. 4400 yr BP in this region) human impact overlapped with a climate change, probably bipartite, towards dryness. Our results show that this aridification trend began around 8000 yr BP, and culminated around 4000 yr BP.

Massive perturbation in terrestrial ecosystems of the Eastern Mediterranean region associated with the 8.2 kyr B.P. climatic event

The climatic perturbation at ca. 8.2 kyr B.P. is the strongest short-term climate anomaly within the Holocene. It is generally attributed to a meltwater-induced slowdown of the thermohaline circulation in the North Atlantic. Model simulations and available proxy data suggest that it was strongest in the high to middle latitudes around the North Atlantic. Based on new pollen data from Tenaghi Philippon, northeastern Greece, we provide evidence for a massive climate-induced turnover in terrestrial ecosystems of the Aegean region associated with the 8.2 kyr B.P. event. The reconstructed winter temperature decline of >4 °C is much stronger than suggested by model simulations and proxy data from more northern latitudes of Europe, although the latter provide a direct downstream response to a North Atlantic thermohaline circulation slowdown. We attribute this discrepancy to mesoclimatic effects; a stronger influence of the Siberian High during the 8.2 kyr B.P. event may have enhanced the katabatic air flow from the mountains bordering the study site via a larger, longer persisting snow cover. Our data demonstrate that high-amplitude temperature anomalies and increased seasonality connected to the 8.2 kyr B.P. event may also have occurred in the lower mid-latitudes, much farther south than previously thought. The magnitudes of these anomalies appear to have been strong enough to have seriously affected Neolithic settlers in the northeastern Mediterranean region.

Inverse vegetation modeling by Monte Carlo sampling to reconstruct palaeoclimates under changed precipitation seasonality and CO2 conditions: application to glacial climate in Mediterranean region

Abstract Atmospheric CO 2 concentration has greatly fluctuated during the Quaternary. These variations have influenced the vegetation changes. The assumption that the relationship vegetation–climate sensu stricto was constant through time should be reconsidered taking into account the impact of the atmospheric CO 2 content on the plants. Here we propose to use a process-based vegetation model (BIOME3) in an inverse mode to reconstruct from pollen data the most probable climate under precipitation seasonality change and under lowered CO 2 concentration in the biosphere. Appropriate tools to match the model outputs with the pollen data are developed to generate a probability distribution associated with the reconstruction (Monte Carlo sampling and neural network techniques). The method is validated with modern pollen samples from Greece and Italy: it proves to be able to reconstruct modern climate with a more or less large error bar from pollen data. The error bar depends in fact on the tolerance of the vegetation to the corresponding climatic variable. The application to six pollen assemblages from Greece and Italy, representing the last glacial maximum (LGM: 18 000 14 C-year B.P.), is done into three experiments: (1) modern CO 2 concentration; (2) LGM CO 2 concentration; (3) LGM CO 2 concentration and high winter precipitation. The latter experiment is motivated by evidence of high lake-levels in Greece during the LGM which has been attributed to winter rainfall. These experiments show that winter was ca. 15–20°C colder than the present, in agreement with previous climate reconstruction. The apparent discrepancy between the high lake-levels and the steppe vegetation during the LGM, can be explained by an increase of the winter precipitation (which leads to high lake level) while the summer season is mild and dry (which affects the vegetation). The summer temperature has three stable states (−16°C, −10°C, −2°C), but the warmest one is the most probable if we take into account the lowered CO 2 and the high lake-levels.

A method to determine warm and cool steppe biomes from pollen data; application to the Mediterranean and Kazakhstan regions

An objective method for the assignment of pollen spectra to appropriate biomes has been published recently. The aim of this paper is to improve the distinction between warm and cool steppes, thus refining vegetation and climate reconstruction, particularly during the Last Glacial Maximum. A set of modern pollen spectra from the Mediterranean and Kazakhstan regions, dominated today by open vegetation types, has been analysed statistically in order to relate pollen taxa abundances to warm acid cool grass/shrub plant functional types (PFTs). A statistical test using modern pollen data shows that the method is able to distinguish between cool and warm steppe biomes with a high degree of confidence. The method has been applied to two fossil pollen records. The results of this exercise showed that cool steppe dominated in central Greece between 18 000 and 13 000 yr BP, while in western Iran the vegetation was at the boundary between cool and warm steppes. These vegetation types were replaced by warm mixed forest in Greece and warm steppe in Iran after that time span

European climate optimum and enhanced Greenland melt during the Last Interglacial

The Last Interglacial climatic optimum, ca. 128 ka, is the most recent climate interval signifi cantly warmer than present, providing an analogue (albeit imperfect) for ongoing global warming and the effects of Greenland Ice Sheet (GIS) melting on climate over the coming millennium. While some climate models predict an Atlantic meridional overturning circulation (AMOC) strengthening in response to GIS melting, others simulate weakening, leading to cooling in Europe. Here, we present evidence from new proxy-based paleoclimate and ocean circulation reconstructions that show that the strongest warming in western Europe coincided with maximum GIS meltwater runoff and a weaker AMOC early in the Last Interglacial. By performing a series of climate model sensitivity experiments, including enhanced GIS melting, we were able to simulate this confi guration of the Last Interglacial climate system and infer information on AMOC slowdown and related climate effects. These experiments suggest that GIS melt inhibited deep convection off the southern coast of Greenland, cooling local climate and reducing AMOC by ~24% of its present strength. However, GIS melt did not perturb overturning in the Nordic Seas, leaving heat transport to, and thereby temperatures in, Europe unaffected.

Quantitative reconstructions of annual rainfall in Africa 6000 years ago: Model‐data comparison

precipitation are based on 85 pollen sites 14 C dated at 6000 ± 500 years B.P and distributed over the whole of Africa. To improve the reliability of the pollen-based climate reconstruction, two methods are used: the ‘‘modern analogues technique’’ (MAT) and the ‘‘plant functional types’’ (PFT) methods. We then conduct a model-data comparison for five distinct regions, allowing an evaluation of model outputs (the Sahara-Sahel, the eastern Sahara, western equatorial Africa, East Africa, and Madagascar). The pollen-inferred reconstructions are compared with 21 mid-Holocene simulations yielded by Atmospheric General Circulation Models (AGCMs), and coupled ocean-atmosphere-vegetation models (OAVGCMs). The large-scale feature of the hydrological changes is shown to be well captured by most of the models. Data show that during the mid-Holocene, the Sahara was considerably wetter than today (+200 to +700 mm/yr). The results reinforce the conclusion that the AGCMs significantly underestimate this precipitation increase in the Sahara whereas the OAVGCM simulations are in accordance with the data. Our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in East Africa and Madagascar. The model-data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen-based reconstruction shows no change or only slight drying there.

Sensitivity of the European LGM climate to North Atlantic sea‐surface temperature

Recent reconstructions of Sea-Surface Temperatures (SSTs) for the Last Glacial Maximum (LGM, 21kyr BP) based on foraminifera and dinoflagellate proxies suggest that the north Atlantic may have been warmer than estimated by CLIMAP [1981]. To better understand the impact of such a warm north Atlantic on the global LGM climate, we used two different AGCMs to perform sensitivity studies. With the new, warmer SSTs, both models simulate a hydrological cycle and temperatures very different from those obtained with the CLIMAP boundary conditions. The most noticeable differences occur in winter over North America and Siberia whereas southern Europe is only weakly affected at all seasons. Whichever the conditions prescribed over the north Atlantic, both models underestimate the large cooling recorded by continental proxy data over the Mediterranean Basin.