Climate Variability In Europe Research Articles

Tree-ring tale

Evidence of social upheaval and other adverse effects of climate variability in Europe over the past 2,500 years should give pause to people reluctant to mitigate climate change.

2500 Years of European Climate Variability and Human Susceptibility

Climate variations influenced the agricultural productivity, health risk, and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution paleoclimatic evidence. We present tree ring-based reconstructions of central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration. Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~250 to 600 C.E. coincided with the demise of the western Roman Empire and the turmoil of the Migration Period. Such historical data may provide a basis for counteracting the recent political and fiscal reluctance to mitigate projected climate change.

Compatibility between modes of low‐frequency variability and circulation types: A case study of the northwest Iberian Peninsula

Modes of low‐frequency variability have a major influence on the variability of the climate system at different temporal and spatial scales. Although the North Atlantic Oscillation explains a substantial portion of the climate variability in Europe, it is also necessary to consider other modes of low‐frequency variability, such as the Scandinavian index, the eastern Atlantic or the eastern Atlantic/western Russia indices. Furthermore, the relationship between the modes of low‐frequency variability and the climate in Europe cannot be considered to be stable over time. The aim of this paper is to assess the compatibility between the modes of low‐frequency variability in Europe (computed using a principal component analysis (PCA)) and local circulation regimes (using an automated version of the Lamb weather‐type classification), thus to observe how shifts in the positions of the modes of low‐frequency variability in Europe, could affect local circulation. Our study area (NW Iberian Peninsula) was chosen because it is characterized by the passage of cold fronts associated with the storm track in the North Atlantic Ocean and is sensitive to any variability in precipitation and temperature that is linked to the main North Atlantic modes of low‐frequency variability. The results show that there is a high degree of correlation between regional modes of low‐frequency variability derived from a statistical approach (using PCA) and real physical circulations (as represented by circulation types). Furthermore, changes in the position of modes of low‐frequency variability tend to favor some circulation types over others.

Regional sensitivities of mean and peak river discharge to climate variability in Europe

Variability of atmospheric circulation is thought to be the most important factor causing annual and decadal variability of fresh water fluxes from the continents. Previous studies, however, have rarely established the sensitivity of the basins to atmospheric circulation variability. Here we present an analysis of long‐term (>30 years) links between atmospheric forcing and winter (December–February) precipitation, and sensitivities of annual mean and maximum winter discharges observed at up to 608 stations across Europe. Links to four atmospheric indices are examined: the North Atlantic Oscillation (NAO) index, the Arctic Oscillation (AO) index, the frequency of west circulation (FWC) as described by the subjective Großwetterlagen classification, and the north to south sea level pressure difference across the European continent (SLPD). The results show that annual maximum discharges are more sensitive to variability of atmospheric circulation than mean discharges. Mean discharges vary on average between 8 and 44%, while peak discharges vary between 10 and 54% per unit index change. Discharges in Iberia and Scandinavia are more sensitive than those in central and northwest Europe. Discharge closely follows variability of atmospheric circulation. Compared with FWC and SLPD, the NAO and AO indices have only limited use for analyzing climate impacts in river basins in northwest Europe.

Seasonal and nonstationary nature of European climate variability in the twentieth century

Seasonal and nonstationary nature of European climate variability in the twentieth century

European summer climate variability in a heterogeneous multi-model ensemble

Recent results from an enhanced greenhouse-gas scenario over Europe suggest that climate change might not only imply a general mean warming at the surface, but also a pronounced increase in interannual surface temperature variability during the summer season (Schär et al., Nature 427:332–336, 2004). It has been proposed that the underlying physical mechanism is related to land surface-atmosphere interactions. In this study we expand the previous analysis by including results from a heterogeneous ensemble of 11 high-resolution climate models from the PRUDENCE project. All simulations considered comprise 30-year control and enhanced greenhouse-gas scenario periods. While there is considerable spread in the models’ ability to represent the observed summer variability, all models show some increase in variability for the scenario period, confirming the main result of the previous study. Averaged over a large-scale Central European domain, the models simulate an increase in the standard deviation of summer mean temperatures between 20 and 80%. The amplification occurs predominantly over land points and is particularly pronounced for surface temperature, but also evident for precipitation. It is also found that the simulated changes in Central European summer conditions are characterized by an emergence of dry and warm years, with early and intensified depletion of root-zone soil moisture. There is thus some evidence that the change in variability may be linked to the dynamics of soil-moisture storage and the associated feedbacks on the surface energy balance and precipitation.

On the natural variability of the pre-industrial European climate

We suggest that climate variability in Europe for the “pre-industrial” period 1500–1900 is fundamentally a consequence of internal fluctuations of the climate system. This is because a model simulation, using fixed pre-industrial forcing, in several important aspects is consistent with recent observational reconstructions at high temporal resolution. This includes extreme warm and cold seasonal events as well as different measures of the decadal to multi-decadal variance. Significant trends of 50-year duration can be seen in the model simulation. While the global temperature is highly correlated with ENSO (El Nino- Southern Oscillation), European seasonal temperature is only weakly correlated with the global temperature broadly consistent with data from ERA-40 reanalyses. Seasonal temperature anomalies of the European land area are largely controlled by the position of the North Atlantic storm tracks. We believe the result is highly relevant for the interpretation of past observational records suggesting that the effect of external forcing appears to be of secondary importance. That variations in the solar irradiation could have been a credible cause of climate variations during the last centuries, as suggested in some previous studies, is presumably due to the fact that the models used in these studies may have underestimated the internal variability of the climate. The general interpretation from this study is that the past climate is just one of many possible realizations and thus in many respects not reproducible in its time evolution with a general circulation model but only reproducible in a statistical sense.

Northwest Africa upwelling and the Atlantic climate variability

Summer Subtropical North Atlantic (SNA) Sea Surface Temperature (SST) anomalies have recently been connected to winter atmospheric variability in the Atlantic European sector. Wind‐induced surface heat flux changes are believed to be the major cause of SNA SST anomalies. However, the impact of coastal upwelling off northwest Africa, on SNA SST variability has been overlooked. Since the area is known to have a richer dynamical behavior than other regions of the central SNA, subsurface thermal anomalies are included in our analysis. Results show that this upwelling region affects significantly the patterns of co‐variability between climatically relevant variables, suggesting that more intricate processes may be involved in generating surface anomalies that, in turn, may influence the European sector climate variability.

Evidence for cooler European summers during periods of changing meltwater flux to the North Atlantic.

We analyzed fossil chironomids (nonbiting midges) and pollen in two lake-sediment records to reconstruct and quantify Holocene summer-temperature fluctuations in the European Alps. Chironomid and pollen records indicate five centennial-scale cooling episodes during the early- and mid-Holocene. The strongest temperature declines of approximately 1 degrees C are inferred at approximately 10,700-10,500 and 8,200-7,600 calibrated 14C years B.P., whereas other temperature fluctuations are of smaller amplitude. Two forcing mechanisms have been presented recently to explain centennial-scale climate variability in Europe during the early- and mid-Holocene, both involving changes in Atlantic thermohaline circulation. In the first mechanism, changes in meltwater flux from the North American continent to the North Atlantic are responsible for changes in the Atlantic thermohaline circulation, thereby affecting circum-Atlantic climate. In the second mechanism, solar variability is the cause of Holocene climatic fluctuations, possibly triggering changes in Atlantic thermohaline overturning. Within their dating uncertainty, the two major cooling periods in the European Alps are coeval with substantial changes in the routing of North American freshwater runoff to the North Atlantic, whereas quantitatively, our climatic reconstructions show a poor agreement with available records of past solar activity. Thus, our results suggest that, during the early- and mid-Holocene, freshwater-induced Atlantic circulation changes had stronger influence on Alpine summer temperatures than solar variability and that Holocene thermohaline circulation reductions have led to summer-temperature declines of up to 1 degrees C in central Europe.

Holocene climate variability in Europe: Evidence from δ 18O, textural and extension-rate variations in three speleothems

Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ 18O trends that are almost exactly out of phase with coupled δ 18O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ 18O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ 18O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ 18O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ 18O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ 18O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ 18O, probably indicative of warm and/or dry conditions. Exceptionally low δ 18O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ 18O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ 18O and extension rate. Instead, its extension rate correlates roughly with δ 13C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ 18O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ 18O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ 18O and extension rate. Had drip-water availability been the dominant control on δ 18O at this semi-arid site then higher δ 18O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.