Joël Guiot

The temperature of Europe during the Holocene reconstructed from pollen data

We present the first area-average time series reconstructions of warmest month, coldest month and mean annual surface air temperatures across Europe during the last 12,000 years. These series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four-dimensional gridding procedure. This approach combines three-dimensional spatial gridding with a fourth dimension represented by time, allowing data from irregular time series to be ‘focussed’ onto a regular time step. We provide six regional reconstructed temperature time series as well as summary time series for the whole of Europe. The results suggest major spatial and seasonal differences in Holocene temperature trends within a remarkably balanced regional and annual energy budget. The traditional mid-Holocene thermal maximum is observed only over Northern Europe and principally during the summer. This warming was balanced by a mid-Holocene cooling over Southern Europe, whilst Central Europe occupied an intermediary position. Changes in annual mean temperatures for Europe as a whole suggest an almost linear increase in thermal budget up to 7800 BP, followed by stable conditions for the remainder of the Holocene. This early Holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west, which have continued to rise at a progressively decreasing rate up to the present day.

Reconstructing biomes from palaeoecological data: a general method and its application to European pollen data at 0 and 6 ka

Biome models allow the results of experiments with atmospheric general circulation models to be translated into global maps of potential natural vegetation. The use of biome models as a diagnostic tool for palaeoclimate simulations can yield maps that are directly comparable with palaeoecological (pollen and plant macrofossil) records provided these records are “biomized”, i.e. assigned to biomes in a consistent way. This article describes a method for the objective biomization of pollen samples based on fuzzy logic. Pollen types (taxa) are assigned to one or more plant functional types (PFTs), then affinity scores are calculated for each biome in turn based on its list of characteristic PFTs. The pollen sample is assigned to the biome to which it has the highest affinity, subject to a tie-breaking rule. Modern pollen data from surface samples, reflecting present vegetation across Europe, are used to validate the method. Pollen data from dated sediment cores are then used to reconstruct European vegetation patterns for 6 ka. The reconstruction shows systematic differences from present that are consistent with previous interpretations. The method has proved robust with respect to human impacts on vegetation, and provides a rational way to interpret combinations of pollen types that do not have present-day analogs. The method demands minimal prior information and is therefore equally suitable for use in other regions with richer floras, and/or lower densities of available modern and fossil pollen samples, than Europe.

Methodology of the last climatic cycle reconstruction in France from pollen data

Abstract A numerical method has been devised to reconstruct the climate of the last climatic cycle from pollen data. Reliable annual reconstructions have been published recently for the two French long sequences of La Grande Pile and Les Echets covering the last 140 millennia. This paper gives more details on the method used. It proposes new reconstructions based on an extended modern pollen data base. These reconstructions are consistent with the previous ones. Especially, detailed information is obtained on the seasonal characteristics of climate. Computations of GCM on the climate variations from 125 kyr to 115 kyr B.P. are confirmed for Western Europe. The insolation variations have induced at that time a decrease of seasonality accompanied by an increased cyclonic activity in winter. This has been the beginning of the first ice growth after the Last Interglacial. Other glacier growth phases are detected at the end of Substage 5c and particularly at the end of Substage 5a.

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

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.

The climate in Western Europe during the last Glacial/Interglacial cycle derived from pollen and insect remains

Using the pollen sequence of La Grande Pile XX (France), we review problems with the application of transfer functions in paleoclimatic reconstructions. One of them is to find modern analogues for the herbaceous vegetation of the cold periods. We propose a method to distinguish between steppes and tundra vegetations for which the moderns are only partial analogues of the glacial periods. Another method to solve these problems is based on constraining by insect remains. The two methods provide coherent reconstructions. The results are also compared with other paleodata. There is a good correlation with the six cold Heinrich events between 70 and 15 ka B.P. A cooling event during the Eemian period (marked by high percentages of Taxas) at about 125 ka B.P. needs still to be correlated with high resolution ocean and ice cores.

Chapter 1 Mediterranean climate variability over the last centuries: A review

Publisher Summary This chapter discusses a necessary task for assessing to which degree the industrial period is unusual against the background of pre-industrial climate variability. It is the reconstruction and interpretation of temporal and spatial patterns of climate in earlier centuries. There are distinct differences in the temporal resolution among the various proxies. Some of the proxy records are annually or even higher resolved and hence record year-by-year patterns of climate in past centuries. Several of the temperature reconstructions reveal that the late twentieth century warmth is unprecedented at hemispheric scales and is explained by anthropogenic, greenhouse gas (GHG) forcing. The chapter discusses the availability and potential of long, homogenized instrumental data, documentary, and natural proxies to reconstruct aspects of past climate at local- to regional-scales within the larger Mediterranean area, which includes climate extremes and the incidence of natural disasters. The chapter describes the role of external forcing, including natural and anthropogenic influences, and natural, internal variability in the coupled ocean–atmosphere system at subcontinental scale.

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

Climate and biomes in the West Mediterranean area during the Pliocene

A new pollen-derived method of climatic quantification, based on the mutual climatic range of plant taxa, has been applied to 17 Pliocene pollen sequences in the West Mediterranean area. The latitudinal gradient observed in the pollen data was confirmed by the climatic reconstructions: there is a gradient from north to south both for temperatures and precipitation. At the beginning of the Pliocene (5.32–5 Ma), the climate in the North Mediterranean area was, on average, warmer and more humid than today (respectively 1–4oC and 400–700 mm). In the South Mediterranean region, the climate was both warmer and drier than today (respectively equal to or 5oC higher and drier or equal humidity). The pollen-based climate estimates were then used to reconstruct biomes for the region. The results show the occurrence of three biomes: the broad-leaved evergreen=warm mixed forest, the xerophytic woods/scrub biome and the warm grass/shrub biome. These biomes are still represented today in the Mediterranean region despite different thermic and water conditions.

A method for climatic reconstruction of the Mediterranean Pliocene using pollen data

Pollen data from numerous sites around the Mediterranean Sea indicate that several important vegetation and climatic changes occurred during the Pliocene. These data are in good agreement with pollen records from northwest Europe and with δ18O curves from Mediterranean and Atlantic deep-sea cores. Quantitative palaeoclimatic reconstructions from Pliocene pollen data of the Mediterranean region cannot be based on conventional modern analogue techniques, as individual Pliocene pollen spectra contain taxa representing temperate, warm-temperate and subtropical plants that do not grow together today. Instead, we propose a new method that uses a climatic amplitude method modified to take partially into account the relative abundances of the taxa. We applied this method to the Pliocene Garraf 1 palynological sequence from Catalonia, which provides a long continuous record of climatic change from 5.3 Ma to the Early Pleistocene. We estimate that annual temperatures were 1 to 5°C higher than today and the annual precipitation 400 to 1000 mm higher than today prior to the beginning of the late Cenozoic glacial–interglacial cycles. In contrast, temperature and precipitation both fell sharply during the glacial phases of the earliest glacial–interglacial cycles.