European Temperature Research Articles

Non‐linearity in statistical downscaling: does it bring an improvement for daily temperature in Europe?

AbstractSeveral linear and non‐linear statistical downscaling methods are compared for winter daily temperature at eight European stations. The linear methods include linear regression of gridpoint values (pointwise regression) and of predictors' principal components (PC regression). The non‐linear methods are represented by artificial neural networks. The non‐linearity is also achieved by a stratification of data by classification of circulation patterns and a linear regression conducted separately within each class. As predictors, gridded 500 hPa heights and 850 hPa temperature are used. The verification is conducted in the cross‐validation framework. The downscaling methods are evaluated according to four criteria: (1) fit to observations (quantified by the correlation coefficient), (2) shape of the statistical distribution, namely its skewness and kurtosis, (3) temporal autocorrelations with 1 day lag, and (4) interstation correlations. Considering all the criteria together, the pointwise linear regression appears to be the best method. It achieves the best fit with the observations and possesses the best temporal structure. The deviations of statistical distributions from normality are only captured by the neural networks, while the classification methods yield the best spatial correlations. Copyright © 2007 Royal Meteorological Society

Climate change and timing of spring migration in the long-distance migrant Ficedula hypoleuca in central Europe: the role of spatially different temperature changes along migration routes

Coinciding with increasing spring temperatures in Europe, many migrants have advanced their arrival or passage times over the last decades. However, some species, namely long-distance migrants, could be constrained in their arrival dates due to their largely inherited migratory behaviour and thus a likely inflexibility in their response to exogenous factors. To examine this hypothesis for pied flycatchers (Ficedula hypoleuca), we tested the effects of the temperature regimes along their migration routes north of the Sahara on their arrival times in central Europe. To do so, we developed a site-independent large-scale approach based on temperature data available on the Internet. Temperature regimes along the migration routes of pied flycatchers within Europe convincingly correlate with their first arrival times. It can be concluded that the progression of spring migration in this species is strongly influenced by temperature en route. Because of the recent inconsistent climatic changes in various parts of Europe, we hypothesize that individuals migrating along different routes will be unequally affected by further climatic changes.

Anthropogenic greenhouse forcing and strong water vapor feedback increase temperature in Europe

Europe's temperature increases considerably faster than the northern hemisphere average. Detailed month‐by‐month analyses show temperature and humidity changes for individual months that are similar for all Europe, indicating large‐scale weather patterns uniformly influencing temperature. However, superimposed to these changes a strong west‐east gradient is observed for all months. The gradual temperature and humidity increases from west to east are not related to circulation but must be due to non‐uniform water vapour feedback. Surface radiation measurements in central Europe manifest anthropogenic greenhouse forcing and strong water vapor feedback, enhancing the forcing and temperature rise by about a factor of three. Solar radiation decreases and changing cloud amounts show small net radiative effects. However, high correlation of increasing cloud‐free longwave downward radiation with temperature (r = 0.99) and absolute humidity (r = 0.89), and high correlation between ERA‐40 integrated water vapor and CRU surface temperature changes (r = 0.84), demonstrates greenhouse forcing with strong water vapor feedback.

Bjerknes’ hypothesis on the coldness during AD 1790–1820 revisited

The aim of this paper is to re-examine and quantify a hypothesis first put forward by J. Bjerknes concerning the anomalous coldness during the AD 1790–1820 period in western Europe. Central to Bjerknes’ hypothesis is an anomalous interaction between ocean and atmosphere studied here using an ocean-atmosphere coupled climate model of intermediate complexity. A reconstruction of the sea-level pressure pattern over the North Atlantic sector averaged over the period 1790–1820 is assimilated in this model, using a recently developed technique which has not been applied to paleoclimatic modelling before. This technique ensures that averaged over the simulation the reconstructed pattern is retrieved whilst leaving atmospheric and climatic variability to develop freely. In accordance with Bjerknes’ hypothesis, the model results show anomalous southward advection of polar waters into the northeastern North Atlantic in the winter season, lowering the sea-surface temperatures (SSTs) there with 0.3–1.0°C. This SST anomaly is persistent into the summer season. A decrease in western European winter surface air temperatures is found which can be related almost completely to advection of cold polar air. The decrease in summer surface air temperatures is related to a combination of low SSTs and anomalous atmospheric circulation. The modelled winter and summer temperatures in Europe compare favourably with reconstructed temperatures. Enhanced baroclinicity at the Atlantic seaboard and over Baffin Island is observed along with more variability in the position of the North Atlantic storm tracks. The zone of peak winter storm frequency is drawn to the European mid-latitudes.

A model‐data comparison of European temperatures in the Eemian interglacial

We present a comparison of reconstructed and simulated January and July temperatures in Europe for a time slice (∼125 kyr BP) within the last interglacial (Eemian, ∼127–116 kyr BP). The reconstructions, based on pollen and plant macrofossils, were performed on 48 European sites using a method based on probability density functions (pdf‐method). The reconstructed most probable climate values were compared with a global climate simulation which was realized with a coupled ocean‐atmosphere general circulation model. Orbital parameters and greenhouse gas concentrations have been adapted to conditions at 125 kyr BP. Reconstructions and simulation are concordant in showing higher temperatures than today over most parts of Europe in summer and in revealing a west‐east‐gradient in winter temperature differences with increasing anomalies toward eastern Europe. The results indicate that differences in the orbital parameters are sufficient to explain the reconstructed Eemian temperature patterns.

European winter temperature variability in a long coupled model simulation: the contribution of ocean dynamics

The interannual variability of the European winter air temperature is partially caused by anomalous atmospheric circulation and the associated advection of air masses, mainly linked to the North Atlantic Oscillation (NAO). However, a considerable part of the temperature variability is not linearly described by atmospheric circulation anomalies. Here, a long control simulation with a coupled atmosphere-ocean climate model is analyzed, with the goal of decomposing the European temperature (ET) anomalies in a part linked to the anomalous atmospheric circulation and a residual. The amount of interannual variability of each contribution is roughly 50%, although at subdecadal (overdecadal) time scales the variability of the residuals is dominant. These residuals are found to be linked to temperature anomalies of the same sign in the whole North Atlantic and Greenland, in contrast to the well-known temperature zonal seesaw associated with the NAO. The association between the residuals and other processes in the North Atlantic has been also analyzed. The thermohaline circulation, closely connected in the model to the intensity of the Gulf Stream, lags the evolution of the temperature residuals by several years and thus is not able to control their evolution. The variability of the oceanic convection in the Northern North Atlantic, on the other hand, correlates with the temperature residual at lags close to zero. It is hypothesized that oceanic convection produces a sea-surface temperature fingerprint that leads to the ET residuals. The implications of these results for multi-year predictability and for empirical climate reconstructions are discussed.

Bjerknes? hypothesis on the coldness during AD 1790?1820 revisited

The aim of this paper is to re-examine and quantify a hypothesis first put forward by J. Bjerknes concerning the anomalous coldness during the AD 1790–1820 period in western Europe. Central to Bjerknes’ hypothesis is an anomalous interaction between ocean and atmosphere studied here using an ocean-atmosphere coupled climate model of intermediate complexity. A reconstruction of the sea-level pressure pattern over the North Atlantic sector averaged over the period 1790–1820 is assimilated in this model, using a recently developed technique which has not been applied to paleoclimatic modelling before. This technique ensures that averaged over the simulation the reconstructed pattern is retrieved whilst leaving atmospheric and climatic variability to develop freely. In accordance with Bjerknes’ hypothesis, the model results show anomalous southward advection of polar waters into the northeastern North Atlantic in the winter season, lowering the sea-surface temperatures (SSTs) there with 0.3–1.0°C. This SST anomaly is persistent into the summer season. A decrease in western European winter surface air temperatures is found which can be related almost completely to advection of cold polar air. The decrease in summer surface air temperatures is related to a combination of low SSTs and anomalous atmospheric circulation. The modelled winter and summer temperatures in Europe compare favourably with reconstructed temperatures. Enhanced baroclinicity at the Atlantic seaboard and over Baffin Island is observed along with more variability in the position of the North Atlantic storm tracks. The zone of peak winter storm frequency is drawn to the European mid-latitudes.

The summer northern annular mode and abnormal summer weather in 2003

The summer Northern Hemisphere annular mode (summer NAM), a new mode determined through empirical orthogonal function (EOF) analysis for each individual calendar month, can describe aspects of anomalous summers such as the summer of 2003, which featured warm temperatures in Europe, Canada and Russia and cold temperature in Japan. Atmospheric circulation anomalies of the summer NAM closely resemble the anomalies in the summer of 2003 and the summer NAM index was quite large during the period from mid‐July to early August when abnormal weather took place in Europe, Canada and Russia. The index includes representations of hemispheric double‐jet streams and blockings that support extended periods of abnormal weather. The double‐jet is formed and maintained by wave forcing during the period. In contrast to the Arctic and North Atlantic oscillations, the summer NAM pattern accounts for many of the anomalous weather features observed during summer of 2003.

Extreme climate of the global troposphere and stratosphere in 1940–42 related to El Niño

Although the El Niño/Southern Oscillation phenomenon is the most prominent mode of climate variability and affects weather and climate in large parts of the world, its effects on Europe and the high-latitude stratosphere are controversial. Using historical observations and reconstruction techniques, we analyse the anomalous state of the troposphere and stratosphere in the Northern Hemisphere from 1940 to 1942 that occurred during a strong and long-lasting El Niño event. Exceptionally low surface temperatures in Europe and the north Pacific Ocean coincided with high temperatures in Alaska. In the lower stratosphere, our reconstructions show high temperatures over northern Eurasia and the north Pacific Ocean, and a weak polar vortex. In addition, there is observational evidence for frequent stratospheric warmings and high column ozone at Arctic and mid-latitude sites. We compare our historical data for the period 1940-42 with more recent data and a 650-year climate model simulation. We conclude that the observed anomalies constitute a recurring extreme state of the global troposphere-stratosphere system in northern winter that is related to strong El Niño events.

Postscpript: Migration, migrants, and biological anthropology

Anthropology is the study of people. Allpeople are migrants. Therefore, anthropologyisthe studyof migrants. Biological anthropol-ogystudiesthe effects of migration on humanphysiology, morphology, population geneticstructure, demography, health and well-being, and more. Human, or at least hominid,migration began when our ancestors leftAfrica in one or more waves of populationdispersal. Even earlier, population movementwithin Africa churned the biocultural diver-sity of that continent for more than one mil-lionyearsandstilldoessotoday.Theflowoutof Africa had biological effects on peoplealmost immediately. Colder temperatures inEurope seem to have altered tropical bodyproportions by shortening legs relative tototal stature. It is still debated if this was adirect consequence of the cold or if it wasan effect mediated by changes in diet andphysical activity (Bogin and Rios, 2003).Latitudinal variation in the intensity of solarradiation, combined with new diets, newclothing, and outdoor exposure, intensifiedselection for various shades of skin pigmenta-tion (Jablonski and Chaplin, 2000). Old dis-ease vectors were left behind, but new oneswere confronted and the selection of newgenotypes and behavioral phenotypes resist-ant to disease continued apace.Astute observers took note of bioculturaldifferences between human groups since atleast the time of the ancient Egyptians. Ittook European Colonialism to spur an effortto systematically organize and hierarchicallyarrange human variation. Those efforts cul-minated in the racial pseudo-science of the18th and 19th centuries. One major counter-attack to ‘‘race science’’ came from severalmigrants to the New World, especially FranzBoas, in the late 19th and early 20th centur-ies (Mascie-Taylor and Little, this volume).World Wars, holocausts, industrialization,urbanization, and other socio-political up-heavals in the 20th century served to inten-sify the process of human migration. By mid-century, migration studies were the bread-and-butter of a coterie of anthropologists,including Gabriel Lasker and his wifeBernice Kaplan. Lasker, Kaplan, Goldstein,Shapiro, and others were able to show,perhaps more fully than Boas may have sus-pected, the nature of human biological plas-ticity in response to migration.Today, we who continue with the researchare the intellectual descendants of Boas andLasker. We still discuss the merits and mean-ing of population biological markers rangingfrommorphologyoftheskull(Relethford,thisvolume) to stature and chest size (Weitz andGarruto, this volume). We add to the mix ofmigration research some population biologymarkers that were unknown to Boas andlargely unavailable to Lasker, such as nu-clear and mitochondrial DNA (Schurr andSherry; Cann and Lum, this volume). We havenot forgotten the broad and deep anthropo-logical roots of our past, as we remind every-one of that most anthropological of alltopics, kinship, and its role in migrationand human diversity (Fix, this volume).Earlyinthe21stcentury weliveunderfearand suspicion of migrants and migration.Refugees, immigrants, drifters, tourists, andstudents from abroad may bring disease,death, and destruction. Foreign militaryforces fight and fraternize with local popu-lations around the globe. Globalization ofindustrial and agricultural production on theone hand and markets for the distribution ofgoods on the other hand bring all humanbeings within contact. This interchange andinterdependence is both biological and cul-tural at the same time. Anthropology has aunique focusonthebioculturalnature ofpeo-ple and a formidable history of migrationresearch. Because of this, biological anthro-pologists have both the skills and the respon-sibility to replace the fears of migration andmigrants with tempered understanding.LITERATURE CITED

Advection from the North Atlantic as the forcing of winter greenhouse effect over Europe

In winter, large interannual fluctuations in the surface temperature are observed over central Europe. Comparing warm February 1990 with cold February 1996, a satellite‐retrieved surface (skin) temperature difference of 9.8 K is observed for the region 50–60°N; 5–35°E. Previous studies show that advection from the North Atlantic constitutes the forcing to such fluctuations. The advection is quantified by Index Ina, the average of the ocean‐surface wind speed over the eastern North Atlantic when the direction is from the southwest (when the wind is from another direction, it counts as a zero speed to the average). Average Ina for February 1990 was 10.6 m s−1, but for February 1996 Ina was only 2.4 m s−1. A large value of Ina means a strong southwesterly flow which brings warm and moist air into central Europe at low level, producing a steeper tropospheric lapse rate. Strong ascending motions at 700 mb were observed in association with warm and moist advection from the ocean in February 1990, producing clouds and precipitation. Total precipitable water and cloud‐cover fraction have larger values in February 1990 than in 1996. The difference in the greenhouse effect between these two scenarios, this reduction in the heat loss to space, can be translated into a virtual radiative heating of 2.6 W m−2 above the February 1990 surface/atmosphere system, which contributes to a warming of the surface on the order of 2.6 K. Accepting this estimate as quantitatively meaningful, we evaluate the direct effect, the rise in the surface temperature in Europe as a result of the maritime‐air inflow, as 7.2 K (9.8 K–2.6 K). Thus, fractional reinforcement by the greenhouse effect is 2.6/7.2, or 36%, a substantial positive feedback.

A Statistical Analysis of the North Atlantic Oscillation and its Impact on European Temperature

In this study the behaviour of the North Atlantic Oscillation (NAO) and its impact on the surface air temperature in Europe 1891-1990 is analysed using statistical time series analysis techniques. For this purpose, both the NAO index (NAOI) and the surface air temperature time series from 41 European stations are split up into typical variation components. Various measures of correlation indicate that the NAOI-temperature relationships are approximately linear and most pronounced in winter. The spatial correlation patterns show a correlation decrease from North West to South East (winter) exceeding correlation coefficients of 0.6 in the Scotland-South Norwegian area. In summer, these correlations are very weak, in spring and autumn stronger but smaller than in winter. These correlations change significantly in time indicating increasing correlations in Central and North Europe and decreasing correlations in the North West. Low-frequent episodic components represented by related polynomials of different or...

Trends of extreme temperatures in Europe and China based on daily observations

Ten of the longest daily temperature series presently available in Europe and China are analysed, focusing on changes in extremes since pre-industrial times. We consider extremes in both a relative (with respect to the time of year) and an absolute sense. To distinguish changes in extremes from changes affecting the main part of the temperature distribution, a percentile smaller than 10 (and/or larger than 90) is recommended for defining an extreme. Three periods of changes in tem- perature extremes are identified: decreasing warm extremes before the late 19th century; decreasing cold extremes since then and increasing warm extremes since the 1960s. The early decreases and recent increases of warm extremes dominate in summer, while the decrease of cold extremes for winter persists throughout the whole period. There were more frequent combined (warm plus cold) extremes during the 18th century and the recent warming period since 1961 at most of the ten stations, especially for summer. Since 1961, the annual frequency of cold extremes has decreased by about 7% per century with warm extremes increasing by more than 10% per century but with large spatial variability. Compared with recent annual mean warming of about 2-3 ◦ C/century, the coldest winter temperatures have increased at three times this rate, causing a reduced within-season range and therefore less variable winters. Changes in the warmest summer temperatures since 1961 exhibit large spatial variability, with rates of change ranging from slightly negative to 6 ◦ C/century. More extensive station observations since 1961 indicate that the single site results are representative of larger regions, implying also that the extremes studied are the result of large-scale changes. Recent circulation changes in daily gridded pressure data, used as an indicator of wind speed changes, support the results by explaining some of the trends.

Geomagnetic forcing and climatic variations in Europe, North America and in the Pacific Ocean

The influence of geomagnetic forcing on the variability of key climatic systems including the North Atlantic Oscillation, fluctuations of the temperature in Europe and North America, variability of the rainfall in the Asian Monsoon region, and the El Niño-Southern Oscillation (ENSO) in the Pacific region was studied to identify possible causes. Statistically significant correlation coefficients were found between geomagnetic activity, the sea-level atmospheric pressure (s.l.p.) and temperature. Enhanced geomagnetic forcing is shown to lead to the intensification of the westerly zonal flow and to fluctuations of temperature in Europe, in the United States, and northern Asia. Correlation coefficients that were found between changes in geomagnetic activity, the All India Rainfall Index and ENSO suggest that the increase of geomagnetic activity in April also strengthens the zonal flow even in the Southern Hemisphere in the southern fall and winter. Strong southerly winds and the cross-equatorial flow extend across the entire Indian Ocean leading to the above-normal monsoon rainfall in India in summer. At a time of decreased geomagnetic activity in April, the Australian high intensifies and the southerly cross-equatorial flow in the Indian Ocean is much weaker. The monsoon rainfall is deficient and an El Niño (Warm event) is usually observed in the Pacific Ocean.

The Association between ENSO and Winter Atmospheric Circulation and Temperature in the North Atlantic Region

The association among ENSO, the Northern Hemisphere sea level pressure (SLP), and temperatures in Europe has been analyzed during the period 1873-1995. In the first part, the SST of the Nino-3 region has been used to select extreme cold and warm ENSO events and periods that can be regarded as normal. The study was carried out for winter with the constraints that the ENSO events were well developed during the winter of study, and that they are extreme events. Composites of winter SLP and temperatures have been made for the selected cold and warm events as well as for normal cases and compared with each other. In the North Atlantic area, no statistically significant SLP anomaly patterns were found associated with warm events, while during cold events a statistically significant anomaly pattern resembling the positive phase of the North Atlantic oscillation (NAO) was found. The temperature analysis shows statistically significant negative anomalies during cold events over the Iberian Peninsula and positive anomalies over the British Isles and southern Scandinavia, consistent with the SLP anomalies. The SLP and temperatures have also been analyzed for spring. The patterns resemble those found for winter but the anomalies have lower amplitudes. For the completion of the composite analysis, the consistency among events of the relationship between ENSO and SLP as well as between ENSO and temperatures was examined. The results show that the significant patterns found in the composite analysis in the North Atlantic area are not the result of a few major events, but rather because both the SLP and temperature anomalies in this area during cold ENSO events are stable and qualitatively similar to those found during the positive phase of the NAO. The possible physical basis for this association between NAO and ENSO is discussed.

Atmospheric circulation and surface temperature in Europe from the 18th century to 1995

AbstractThe relationship between surface atmospheric circulation and temperature in Europe from the 1770s to 1995 is examined using correlation analysis. The atmospheric circulation is represented by six indices: the three leading principal components (PCs) of an empirical orthogonal function (EOF) analysis of 20 European pressure series from 1822 to 1995, which represent the central tendency of European pressure (EOF 1), a zonal circulation pattern (EOF 2) and a meridional pattern (EOF 3), a North Atlantic zonal index constructed from Gibraltar and Reykjavik pressure series for 1821–1995; a Western European zonal index constructed from Madrid, Barcelona, Lund and Trondheim for 1786–1995; and an index constructed from Paris and London, 1774–1995. Eight long temperature series from northwestern and central Europe were correlated with these circulation indices. European temperatures in general had the highest correlations with the zonal circulation indices in winter, with almost 70% of the variability in the temperature records explained by variations in the zonal index. The correlation coefficients between PC 3 (representing meridional circulation) and temperatures were highest in spring and autumn, particularly for Scandinavia. Running correlation series calculated over 25‐year windows reveal significant non‐stationarities in the relationship between surface temperature and atmospheric circulation on decadal time scales, suggesting caution must be used in extrapolating current relationships between circulation and temperature for future climate predictions based on downscaling or past palaeoclimatic reconstructions. Copyright © 2001 Royal Meteorological Society

Impact of global sea surface temperature on summer and winter temperatures in Europe in a set of seasnal ensemble simulations

Impact of global sea surface temperature on summer and winter temperatures in Europe in a set of seasnal ensemble simulations

Impact of global sea surface temperature on summer and winter temperatures in Europe in a set of seasonal ensemble simulations

AbstractIn the PRediction Of climate Variations On Seasonal to interannual Time‐scales (PROVOST) ensemble of seasonal simulations, performed with the European Centre for Medium‐Range Weather Forecasts (ECMWF) atmospheric general‐circulation model forced by observed sea surface temperature (SST) for the 1979–1993 period, the consistency between individual members of the ensemble of simulated temperature at 850 hPa (T850) in Europe varies considerably from year to year. This interannual variability is assumed to be a consequence of slowly varying lower‐boundary forcings, most notably the SST forcing, plus a random part from the internal, chaotic atmospheric variability. In those years when the ensemble members show a particularly consistent pattern of T850 anomalies in Europe, the consistency is significantly higher than would be expected from the interannual variability, indicating that the SST forcing has a significant impact on the model simulated temperature. Composites of global SST indicate that the El Niño/Southern Oscillation (ENSO) might be linked to the high internal consistency of some of the winter season ensembles of T850 anomaly patterns in Europe. However, in some ENSO years the ensemble shows no consistent signal in Europe, so the occurrence of an ENSO event is not sufficient to ensure high internal ensemble consistency in Europe.In a separate approach, an attempt to linearly relate global SST anomaly patterns to the ensemble mean of T850, using canonical correlation analysis, also indicates a link between simulated T850 anomaly patterns in Europe and ENSO events. Mature El Niño (La Niña) conditions are associated with positive (negative) T850 anomalies in the model in southern Europe and negative (positive) anomalies in northern Scandinavia and over the Barents Sea in the January‐February season. A weaker relation is found between onsetting ENSO conditions and T850 anomalies in the model in the south‐eastern and westernmost parts of Europe in the July‐August season.Validations against re‐analysed observations of T850 show poor skill in Europe for the ensemble mean of simulated T850, and no obvious relation is found between internal ensemble consistency and skill. The link that is found in the model between ENSO and simulated T850 in Europe, is not evident between ENSO and re‐analysed T850.

Greenhouse gases could affect ocean circulation

The buildup of greenhouse gases could precipitate an abrupt collapse of the oceans' prevailing circulation system and send temperatures in Europe plummeting about 10° Celsius or more in a span of 10 years, says Wallace Broecker, professor of Earth and environmental sciences at Columbia University's Lamont‐Doherty Earth Observatory, and author of new research that appears in the November 28 issue of Science magazine.

Permafrost Distribution and Temperatures in Europe During the Younger Dryas

Permafrost distribution and temperatures in Europe during the Younger Dryas stadial are reconstructed based on an assessment of periglacial phenomena attributed to this cold interval. The significance of immature ice-wedge casts showing characteristics of both ice-wedge casts and frost cracks is discussed. Careful evaluation of periglacial features enables quantification of the mean annual air temperature and mean temperature of the coldest month. Results suggest that, during the coldest part of the Younger Dryas, continuous permafrost existed north of ∼54°N (i.e. in Fennoscandia and the northern part of the British Isles and Ireland). Mean annual air temperatures at sea level near or below −8°C and mean temperatures of the coldest month well below −20°C are deduced for this zone. Discontinuous permafrost was present between 54°N and 50°N (i.e. in central and southern England and Ireland, The Netherlands, upland Belgium, northern Germany and Poland). It is suggested that in this zone mean annual air temperatures were between −8°C and −1°C, with mean temperatures of the coldest month above −20°C. © 1997 John Wiley & Sons, Ltd. La répartition du pergélisol et les températures de l'Europe pendant le Dryas récent sont reconstituées sur la base d'une étude critique des phénomènes périglaciaires attribués à cet intervalle froid. La signification de fentes de gel peu développées montrant des caractéristiques à la fois de fentes de gel à remplissage de glace et de fissures de gel, est discutée. Une évaluation soigneuse des phénomènes périglaciaires conduit à une quantification de la température moyenne annuelle de l'air et à celle du mois le plus froid. Les résultats suggèrent que pendant la période la plus froide du Dryas récent, un permafrost continu a existé au nord du 54° de latitude nord en Scandinavie et dans la partie septentrionale des Iles Britanniques et d'Irlande. Les températures moyennes annuelles de l'air au niveau de la mer auraient été inférieures ou égale à −8°C tandis que les températures moyennes du mois le plus froid auraient été en dessous de −20°C. Un pergélisol discontinu était présent entre 54 et 50°N au centre et au sud de l'Angleterre, aux Pays-Bas, dans les parties élevées de la Belgique, au nord de l'Allemagne et en Pologne. Il est proposé de considérer que, dans cette zone, les températures moyennes annuelles de l'air ont été abaissées d'une valeur comprise entre −8°C et −1°C, avec des températures moyennes du mois le plus froid inférieures à −20°C. © 1997 John Wiley & Sons, Ltd.