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date: 20 September 2017

Poland’s Climate in the Last Millennium

Summary and Keywords

The reconstruction of climate in Poland in the past millennium, as measured by several kinds of proxy data, is more complete than that of many other regions in Europe and the world. In fact, the methods of climate reconstruction used here are commonly utilized for other regions. Proxy data available for Poland (whether by documentary, biological, or geothermal evidence) mainly allow for reconstructions of three meteorological variables: air temperature, ground-surface temperature, and precipitation. It must be underlined however, that air temperature reconstructions are possible only for certain times of the year. This is particularly characteristic of biological proxies (e.g., tree rings measure January–April temperature, chironomids provide data for August temperature, chrysophyte cysts identify cold seasons, etc.). Potentially, such limitation has no corresponding documentary evidence. In Poland these data are available only for climate reconstructions covering mainly the last 500 years because the number of historical sources pre-1500 is usually too small. Geothermal data allow for reconstruction of mean annual ground surface temperature generally for the last 500 years. Reconstructions of air temperature that cover the entire, or almost the entire, millennium and have high time resolution are only available from biological proxies (tree rings, chironomids, diatoms, etc.).

At present, the best source of information about climate in Poland in the last millennium is still documentary evidence. This evidence defines a Medieval Warm Period (MWP), which was present in the 11th century and probably ended in the 14th or early 15th century. Air temperature in the MWP was probably about 0.5–1.0°C warmer than contemporary conditions on average, and the climate was characterized by the greatest degree of oceanity throughout the entire millennium. A Little Ice Age (LIA) can be also distinguished in Poland’s climate history. Data show that it clearly began around the mid-16th century and probably ended in the second half of the 19th century. In this LIA, winters were 1.5–3.0°C colder than present conditions, while summers tended to be warmer by about 0.5°C. As a result, the continentality of the climate in the LIA was the greatest for the entire millennium. Mean annual air temperature was probably lower than the modern temperature by about 0.9–1.5°C. The average rise of air temperature since the mid-19th century, which is often called the Contemporary Warming Period (CWP), is equal to about 1°C and is in line with the results of reconstructions using geothermal and dendrochronological methods. The reconstruction of precipitation in Poland is much more uncertain than the reconstruction of air temperature. There was probably considerably higher average precipitation in the 12th century (and particularly in the second half of this century), in the first half of the 16th century, and also in the first half of the 18th century. The second half of the 13th century and the first half of the 19th century were drier than average. In other periods, precipitation conditions were close to average, including for the entire CWP period.

Keywords: Poland, climate reconstruction, historical climatology, climate change, climate variability

Introduction

In recent years, the issues of climate change and variability, and their consequences for the environment (be it natural or social), have become dominant themes in research above all espoused by climatologists and meteorologists. The direct cause of this is global climatic warming, observable since the second half of the 1970s, and constituting the so-called second phase of contemporary warming, the first having taken place in the years 1920–1940.

Nevertheless, as paleoclimatic studies make clear, changes in and variability to climate on all temporal and spatial scales are a universal feature of the climate system. Indeed, they may be regarded as an inherent property of them. Why then has it been only since the late 20th century that such an explosion of work on climate change has taken place? The reason, as studies (like the reports from the IPCC from 1990, 1996, 2001, 2007, and 2013) make clear, is that the rate of change is far greater now than in the last several centuries, or even ten or more centuries. The climatic models widely applied for several decades now in simulating future climate change are almost unanimous in stating that a further rise in emissions of greenhouse gases will intensify the rate of climate change, with air temperature anticipated to have been elevated by around 1.1–3.1°C (for two of the middle-of-the-road scenarios, RCP4.5 and RCP6.0, IPCC, 2013) at the end of the 21st-century world in which carbon dioxide concentrations are up to twice the historical levels.

While paleoclimatic research is carried out for different geological epochs and periods, both the assessment of the causes of the contemporary climate change referred to, and the formulation of future climate scenarios (mainly for the 21st century), are most dependent on a detailed understanding of the climate over the last millennium. This reflects the need for the scope of the so-called “natural variability” of climate to be established. And this of course requires climatic information collected in the preindustrial period (from before 1850), the influence of anthropogenic factors on the climate back then being limited enough to neglect.

A full familiarity with the climate in recent centuries is also essential if climatic models used most often in determining future climate-change scenarios are to be verified. If the model is well simulated in respect to the climate of past periods, there is a higher probability that these will simulate the future climate correctly.

How then to gain insights into past climates when instrument-based observations of different meteorological elements cover the globe satisfactorily only in the case of the last 100–200 years? In general, widespread use has been made of relationships between different natural phenomena (systems) (e.g., annual increments in the growth of trees, stalagmites, and stalactites, as well as of coral reefs) and climate, as well as an assumption regarding stationarity over longer periods of time (via the so-called principle of geological uniformitarianism).

As sources of “indirect data,” the natural systems just referred to have inscribed in them a “climatic signal” that may be more or less distinct. The aim of paleoclimatological investigations lies in the skillful uncovering of this signal, most often by way of a process of calibration (determination of the statistical links between defined natural systems and contemporary meteorological data) and verification (checking if links established by way of calibration allow for the statistically precise reconstruction of climatic data from a period of instrumental observation other than that used in calibration). If the calibration and verification processes point to the existence of durable and statistically significant linkage between a given natural system and a defined meteorological element (e.g., air temperature), then it is possible to apply it to a period for which measurement-based observations are not available, if knowledge of the natural system back then is adequate.

More details relating to the paleoclimatic methods that have gained application are to be found in the book by Bradley (1999), the review publications by Brázdil et al. (2005) and Jones et al. (2009) and also several publications appearing at the Toruń Centre (Majorowicz et al., 2001, 2004; Przybylak et al., 2004, 2005).

When it comes to possibilities for reconstructing climate over the last millennium, Europe (including Poland) is in a privileged situation compared with most other areas, because there are written sources relating to the entire period, which are very much capable of being augmented by the climatic information obtainable from so-called “natural archives.” For this reason, many reconstructions of different climatic elements (mainly air temperature, precipitation, and atmospheric pressure) are available, as well as elements of the atmospheric circulation for Europe (e.g., Luterbacher et al., 2002, 2004; Brázdil et al., 2005; Pauling et al., 2006; Casty et al., 2007; IPCC, 2007, 2013; Glaser, 2008; Przybylak, 2008; Dobrovolny et al., 2009, 2010; Klimenko & Solomina, 2010; Majorowicz, 2010; Trachsel et al., 2012; Esper et al., 2014; Majorowicz et al., 2014). Progress is also clearly visible in the case of the reconstruction of the northern hemisphere climate, or indeed that of the world as a whole (e.g., Mann et al., 1999, 2008, 2009; Jones et al., 2001; Moberg et al., 2005; IPCC, 2007, 2013; Juckes et al., 2007; Christiansen & Ljungqvist, 2012; Ljungqvist et al., 2012; PAGES 2k Consortium, 2013; Shi et al., 2013).

For obvious reasons, knowledge on climate change in Poland over the last millennium is fullest in respect to the period of instrumental observations. Since the beginning of 1990s it has been possible for various workers to compile, and then engage in the homogenization of, 10 or more air temperature series for Poland (Górski & Marciniak, 1992; Miętus, 1996, 1998; Trepińska, 1997; Głowicki, 1998; Lorenc, 2000; Vizi et al., 2000, 2000–2001; Bryś & Bryś, 2010). The most important of these are the longest series, which relate to Warsaw (from 1779 on) and Cracow (from 1792). Furthermore, high levels of correlation observed for air temperatures across the country allow for the use of these temperature series in characterizing thermal conditions throughout Poland (Kożuchowski & Żmudzka, 2003). The longest homogeneous air temperature series (including air pressure and precipitation) for Gdańsk for the period since 1739 (J. Filipiak, personal communication, June 17, 2015) is under preparation.

In the last several decades there has also been a marked increase in the level of knowledge on the Polish climate from the so-called pre-instrumental period. Nonetheless, this does not extend back before the start of the last millennium (Maruszczak, 1988, 1991; Sadowski, 1991; Wójcik et al., 2000; Bokwa et al., 2001; Limanówka, 2001; Majorowicz et al., 2001, 2004; Kotarba, 2004; Niedźwiedź, 2004, 2010; Przybylak et al., 2004, 2005, 2008, 2010a, 2014a, 2014b; Filipiak, 2007; Przybylak, 2007, 2011; Filipiak & Miętus, 2010; Luterbacher et al., 2010; Majorowicz, 2010; Przybylak & Marciniak, 2010; Zielski et al., 2010; Koprowski et al., 2012; Opała & Mendecki, 2014; Hernández-Almeida et al., 2015; Opała, 2015).

Indirect data generally do not allow for the reconstruction of all meteorological elements typically used in describing the climate of a given area. Usually, though, it is possible to regenerate with almost full continuity the main characteristics (monthly, seasonal, and annual means or totals) for only (or as many as) three key elements:. air temperature, ground-surface temperature, and precipitation. Information on the remaining elements (e.g., on wind, cloud cover, and atmospheric phenomena) are to be found only in historical sources, mostly just those with entries concerning particular days. There are unfortunately very few such sources, and they mostly concerns short periods from the last millennium.

It is worth noting that the amount and quality of information concerning the aforementioned three first elements is varied, which has the effect that the reconstructions obtained on the basis of them also differ from one another. Dendrochronological and historical data may yield broad-spectrum information on air temperature, but reconstruction of precipitation data using them is much more difficult, and sometimes even impossible. In turn, geothermal data allow for nothing more than the reconstruction of ground-surface temperatures, and because these are correlated in a statistically significant way with air temperatures, they can be re-created by indirect means anyway. Biological proxies also allow for reconstruction of temperature, for both cold and warm half-years. The methods of reconstruction in the case of these elements are described in detail in more recent literature (Majorowicz et al., 2001, 2004; Przybylak et al., 2001, 2004, 2005; Koprowski et al., 2012; Hernández-Almeida et al., 2015) and so are not discussed more fully here.

It should be stated that, the further back in time examined, the lower the reliability of the presented reconstructions of climate, and the more generalized the reconstructions themselves (especially when based on historical and geothermal sources). This remark is not true of reconstructions based on dendrochronological data, however, assuming that there is stationarity in the studied relationship between climatic conditions and annual ring increments.

The current state of knowledge on changes and variability in Poland’s climate over the last millennium will be discussed.

Air Temperature

The 1001–1800 Period

In the temperate latitudes at which Poland lies, air temperature is undoubtedly the most important meteorological element conditioning the development of the biosphere. It is thus no coincidence that people since the earliest times have noted different temperature situations, but most often of all those departing markedly from the norm, and hence exerting a significant disruptive effect on life. In Poland, therefore (as in many other parts of the globe), there is most information from historical sources regarding air temperature.

Nevertheless, even in regard to temperature the amount of information is highly inadequate when set against the task of reconstructing courses over the first 500 years of the last millennium (Rojecki, 1965). The reliability of historical sources from this period is also very limited, as was already being noted as far back as in 1922 by Semkowicz, and is confirmed by today’s historians (P. Oliński, personal communication, May 31, 2015).

Equally, the dendrochronological data collected thus far extend back to 1170 in the case of northern Poland (Zielski, 1997), and to 1091 in the south (excluding the mountain regions) (Szychowska-Krąpiec, 2010). Zielski (1997) reveals statistically significant correlations between annual tree ring widths and monthly mean air temperatures, particularly in February and March, and also in January and April. Their values were equal to 0.47, 0.55, 0.26 and 0.18, respectively. This means that in the lowland and upland parts of Poland, the low temperatures occurring at the finish of winter and the beginning of spring have a strong negative influence on the width of the tree rings. Similar relationships between temperature and Scotch pine tree ring widths were also obtained for the northern-central European Lowland (see, e.g., Linderson, 1992 or von Lührte, 1992). In any case, these allow for nothing more than the reconstruction of mean values for air temperature, specifically for the January–April and February–March periods. On the other hand, in the high Polish mountains (Carpathian and Sudeten), tree ring widths depend on summer temperatures.

Recently, winter air temperature reconstructions based on biological proxies (chrysophytes) taken from laminated sediments from Lake Żabińskie (NE Poland), covering the entire millennium, have been presented by Hernández-Almeida et al. (2015).

Thus, in practice, the number of indirect data items concerning the climate in the first two centuries of the millennium is very small, which necessitates attempts to reconstruct the history of changes in air temperature in Poland by reference to available data for other areas (Maruszczak, 1991). The reliability of such analogies is limited, given that work relates to the United Kingdom, Greenland, and California, that is, very distant areas that differ from Poland in often showing different rhythms to changes in air temperature (see, e.g., Fig. 34.2 in Bradley & Jones, 1995, in which reconstructions of air temperature for Europe and North America are presented, or the recent review papers Ljungqvist et al., 2012 and Pages 2k Consortium, 2013).

Also long known is the phenomenon of the simultaneous occurrence of “positive and negative” anomalies of air temperature (i.e., departures from norms) across Europe (including Poland) and over Greenland (Kosiba, 1949) especially in winter. Contemporary climatology explains such phenomena by reference to a differential influence on thermal conditions in Europe and Greenland due to atmospheric circulation—as described with the aid of the index for the North Atlantic Oscillation (NAO)—(Hurrell, 1995, 1996).

The 1001–1500 Period

Bearing in mind the reservations previously referred to, but not having other reconstructions available, cited here are mainly the results of the work by Maruszczak (1991). According to him, the 11th century was probably warmer than the norm determined for the period he studied, while the 12th century (especially the second half) was the warmest of the entire millennium. The reconstructed mean temperature for the months from January to April was one of the higher ones in the case of the years 1170–1200, albeit not the highest (Figure 1). In southern Poland, the temperature in February and March was high at the beginning and end of the 12th century. In contrast, it was very low in mid-century (Szychowska-Krąpiec, 2010). In the entire millennium, the number of days below 4°C (DB4°C)—which according to Hernández-Almeida et al. (2015) represents winter temperatures very well—was the highest (>104) in the first half of the 11th century (Figure 2). An abrupt drop in the 11-year mean of DB4°C, from 104 to 99, occurred from 1055–1066 to 1071–1082. Then, about 90 years of slightly warmer winters than the present norm were noted (1961–1990, DB4°C = 102.7), but nevertheless colder than the 1001–2000 mean (DB4°C = 100.3). Strongly in line with the results of other reconstructions described here, the end of the 12th century saw one of the warmest winters in the history or records (Figure 2). The air temperature changes described in Poland correspond well with the summer air temperature changes reconstructed for Germany and Switzerland (except the 11th century), central Europe, and Scandinavia (see Figs. 3.1 and 3.2 in Niedźwiedź et al., 2015) but are less agreeable to both summer and winter air temperatures for the Baltic Sea region (Schimanke et al., 2012).

For the period 1201–1500 there is slightly more information, thanks to the availability of the work by Sadowski (1991), albeit a study drawing on limited and low-reliability historical sources. Recently though, a new reconstruction of air temperature made by Przybylak et al. (2011) also covers the end of this period (15th century), despite containing many gaps (see Figure 3).

Thus, according to Maruszczak (1991), mean annual values for air temperature were above the norm in the 13th century and the first half of the 14th, only to give way to around 100 years of cooling. A warmer climate returned in the second half of the 15th century. According to the research by Sadowski (1991), the 13th century was a time of both the least severe winters and the least hot summers to a greater extent than at any other time throughout the nearly 800-year period under study. That would mean in turn that the period was one of the greatest oceanicity of climate (see Fig. 6 in Sadowski, 1991). As Figure 1 shows, mean January–April values of air temperature were relatively high, and varied only little in the 13th century, with only slightly lowered values in the first half. A similar course was characteristic for the 14th century. Research by Sadowski (1991) also gives no indication of greater changes in air temperature during that century. There is thus no confirmation of the occurrence of the cooling of climate in the second half of the 14th century that Maruszczak (1991) wrote about. DB4°C reconstruction for the analyzed period is more in agreement with the reconstruction results of Sadowski (1991) and Przybylak et al. (2001) than Maruszczak. Winters generally were very warm, except during two short periods: 1255–1283, and the second half of the 15th century. Comparisons with the air temperature reconstructions from central Europe, Germany, Switzerland, and the Baltic Sea region mentioned earlier show very good agreement, particularly for the 13th century. On the other hand, the Scandinavian summer temperature was colder than the 13th century norm (Schimanke et al., 2012).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 1. A reconstruction of mean January–April air temperature in Poland for the period 1170–1994 using a standardized chronology of tree ring widths for Scotch pine (Pinus sylvestris L.) (after Przybylak et al., 2001, modified). Key: Rek-11, Rek-50—11- and 50-year running means; reconstruction using areally averaged air temperatures from Warsaw, Bydgoszcz, and Gdańsk for calibration, Obs—mean January–April areally averaged air temperature from Warsaw, Bydgoszcz, and Gdańsk. Reconstructed air temperature variability was not corrected to the real variability shown in the instrumental series.

Poland’s Climate in the Last MillenniumClick to view larger

Figure 2. Chrysophyte-based DB4°C reconstruction for NE Poland (gray line: year-to-year, black line: 11-year filter). Based on data taken from Hernández-Almeida et al. (2015).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 3. Reconstructions of mean 10-year air temperatures (oC) in Poland from 1401 to 1800: winter (DJF) and summer (JJA). l- and 2-anomalies with respect to 1901–1960 and 1789–1850 means, respectively (after Przybylak, 2011).

In turn, the newest reconstructions of mean temperature in late winter and early spring (the February–March period) show clearly how the whole 13th century and beginning of the 14th century were cool, while a marked warming kicked in from the 3rd decade onward (Szychowska-Krąpiec, 2010; Koprowski et al., 2012). According to Sadowski, the 15th century was characterised by the greatest degree of climatic continentality at any time in the years 1201–1980. At this time, there was an abrupt increase in the number of severe winters to as many as six in the decade 1451–1460. The same result was also obtained by Przybylak (2011). Sadowski (1991) also reported the occurrence of a large number of hot summers (two to five per decade) in the period from 1470. In line with the results obtained from historical sources (Maruszczak, 1991; Sadowski, 1991; Przybylak, 2011), the last 30 years of the 15th century were apparently either within—or rather warmer than—the norm, most especially in summer. On the other hand, reconstructed mean values for air temperature for the January–April period as well as DB4°C reconstructions, as based on the width of tree rings and chrysophyte assembly, respectively, point to the occurrence of a period of cooling at this time (Figures 1 and 2). More in line with the results of the reconstruction based on historical sources are the newest reconstructions of mean temperatures in February and March obtained on the basis of dendrochronological data from northern Poland (Koprowski et al., 2012), as well as southern Poland (Szychowska-Krąpiec, 2010). The Baltic Sea region in the 15th century was, on average, colder than the preindustrial mean (950–1900), in both winter and summer (Schimanke et al., 2012). Similar conditions occurred in Germany and Switzerland (see Fig. 3.1 in Niedźwiedź et al., 2015), and in central Europe, although here the summer temperature was only slightly colder than in the 20th century. On the other hand, in Scandinavia the summers at this time were usually warmer than the 442–1970 mean (see Fig. 3.2 in Niedźwiedź et al., 2015). In conclusion, it can be stated that the majority of reconstructions (including those made for Poland) estimate the 15th century as a cold period.

As can be seen from this brief summary, the state of knowledge concerning changes in air temperature in Poland in the period 1001–1500 is very limited indeed, existing reconstructions being very uncertain. In some periods there are even cases in which opposing trends for the course of air temperature are presented.

The 1501–1800 Period

Knowledge concerning climate change in Poland is much fuller from the 16th century onward, hence the several successful attempts at the reconstruction of air temperatures in this period carried out on the basis of documentary evidence brought together (Maruszczak, 1991; Sadowski, 1991; Przybylak et al., 2004, 2005; Przybylak, 2011), dendrochronological data (e.g., Bednarz, 1996; Wójcik et al., 1999, 2000; Przybylak et al., 2001, 2005; Kaczka, 2004; Niedźwiedź, 2004; Szychowska-Krąpiec, 2010; Koprowski et al., 2012; Opała & Mendecki, 2014; Opała, 2015), as well as biological proxies (Hernández-Almeida et al., 2015).

Certain reconstructions of air temperature on the basis of documentary evidence that are characterized by a higher (daily) level of detail have also been generated, though obviously these deal with shorter overall periods of just a couple of decades (e.g., Bokwa et al., 2001; Limanówka, 2001; Nowosad et al., 2007; Niedźwiedź, 2010; Pospieszyńska & Przybylak, 2010; Przybylak & Marciniak, 2010; Przybylak et al., 2010b, 2014a, 2014b; Przybylak & Pospieszyńska, 2010).

Winter Season

In relation to the 18th, and additionally the 15th, century, the widening of the historical databases done recently has markedly increased the level of completeness and reliability of reconstructions of air temperature in Poland. This can be discerned when old reconstructions of air temperature (after Przybylak et al., 2004, 2005) are compared with the new ones presented in Figure 3. The analysis of these reveals that the coldest winters in the period 1501–1800 (as judged in terms of departures from the 1901–1960 mean in the range −3.5 to −3.4°C) occurred in the decades 1511–1520, 1541–1550, 1571–1580, 1701–1710 and 1741–1750. All decades (except 1521–1530) in the three analyzed centuries were colder than in the period 1901–1960 (black bars). On average, the coldest winters were those of the 16th century (an anomaly then of −2.4°C), while the warmest characterized the 17th century (anomaly −2.2°C). The differences between the secular means are thus very small.

Within the period under study, winters worthy of the description extremely cold (being 2 standard deviations below the long-term mean for the period 1901–1960) occurred most often in the 16th century (32 cases) and the 17th (21) (Figure 4). There were fewest of these (18) in the 18th century. This finding stands in a good agreement with the DB4°C reconstruction (Figure 2), which also shows that winters in the 16th and 17th centuries were colder than in the next two centuries. It was in the decade 1641–1650 that the greatest number of very severe winters (6) was recorded (these having indices of −2 and −3). There were also a number of examples of this category of winter (i.e., 5) in both the 1511–1520 and 1731–1740 periods.

Poland’s Climate in the Last MillenniumClick to view larger

Figure 4. Decadal frequencies of occurrence of extreme cold and very cold winters (DJF, indices -3 and -2) and of extreme warm and very warm summers (JJA, indices 3 and 2) (after Przybylak, 2011). Key: indices -3 and +3 denote that temperature in winter and summer, respectively, is lower (higher) than 2σ‎ from reference period 1901–1960 mean. Indices -2 and +2 denote smaller deviations varying between 1.41 and 2.0σ‎.

Very warm winters were also noted most often (8 times) in the 16th century, while they were not noted at all in the 18th. The reliability of this assessment is much lower, however, because those keeping notes always paid greater attention to severe winters than to mild ones or very warm ones that were less burdensome.

Comparison of the reconstruction for temperatures in winter (Figure 3) with that involving temperature in the January–April period (Figure 1) indicates rather good concordance in the 18th century, as well as disparities in the 16th and 17th. According to Przybylak et al. (2004), the probable reason for the lack of accord in the results obtained may be: not fully comparable data (mean temperature for different periods of the year overlapping only partly), as well as imperfections in the reconstructed air temperature values. On the other hand, better relationships are seen between recently reconstructed DB4°C, based on biological proxies (Figure 2), and winter temperatures obtained from historical sources (Figure 3).

Figure 1.3 in Luterbacher et al. (2010) reveals the existence of a strong correlation for the period of the last 500 years between averaged temperature in winter from the area of Poland and the temperatures in almost all regions of Europe. At the same time, the coefficient for the correlation between reconstructed values for air temperature in winter in the Czech Republic (Przybylak et al., 2004, 2005) and in Poland (Figure 3) in the 1501–1800 study period has a value of zero. Particular sub-periods do in fact display weak or moderate correlations, but these change sign. In the 16th century, a strong positive correlation (r = 0.58) was obtained, while in the 17th and 18th centuries, the correlations were inverse (respective r values of −0.27 and −0.43). It is this radical change in the nature of the link (which most probably arose in the late 16th and early 17th centuries) that is responsible for the lack of correlation in the period as a whole.

For this reason, the correlations calculated for particular centuries would seem to be more reliable, even if they are not statistically significant. Certain disparities between the studied reconstructions of winter temperatures for the Czech Republic and Poland may also result from different methods applied in the reconstruction. These include:

  1. l. Availability of historical source materials, greater for the Czech Republic, and allowing for a thermal (indexing) assessment of different months, as opposed to seasons only in the case of Poland;

  2. 2. Different selections of years included within decades (e.g., Czech Republic 1500–1509, Poland 1501–1510).

The greater concordance and stability of the courses for the two air temperature series in the summer months probably results from the far lesser variability from day to day and year to year than is to be found for winter temperatures. This should favor the making of fewer errors in the course of the reconstruction of air temperatures for summer as opposed to winter, even where the number of indirect data is much smaller.

Comparisons between reconstructed thermal conditions for Poland in the winter period and analogous ones for Latvia (Jevrejeva, 2001) and Estonia (Tarand & Nordli, 2001) point to weaker connections than in the case of the Czech lands. However, it should be recalled that re-creations of air temperature in the Baltic States have made use of long-term observations of the ice cover at the ports of Riga and Tallinn, this giving good insight into the severity of winters, but obviously offering lesser comparability. On the other hand, results from Poland show good correspondence to the winter temperatures for the entire Baltic Sea region, in particular for the 17th century (see Schimanke et al., 2012).

Summer Season

The reconstruction of air temperature for the summer season on the basis of historical data is markedly less complete than that for winter (Figure 3). This is linked to the fact that less information is accessible, but it does not prevent general outlines from looking quite legible. The summers in the studied period of several centuries extending from 1461 to 1800 were primarily warmer than in the reference period (Figure 3). However, the positive departures are markedly smaller than the negative ones characterizing winter. The greatest mean decadal values for these, not exceeding 1°C, were mainly reported in the second half of the 16th century. The coolest summers (0.5°C down on the mean for 1901–1960, on average) were noted in the decade 1731–1740. Moreover, in periods of study involving five different decades (1521–1530, 1591–1600, 1671–1680, 1691–1700, and 1761–1770), they were slightly (on average 0.1°C) cooler than contemporary ones. The centuries’ mean departures noted for temperature were always somewhat positive, ranging from 0.1°C in the 18th century to 0.3°C in the 16th.

During the summers, in turn, it was far more usual for hot (often also very dry) summers to be noted, than cool and wet ones. Very hot or extremely hot summers were most often noted in the 17th century (15 cases) and the 16th (14 cases) (Figure 4). In turn, there were far fewer of them in the 18th century. Very cool or extremely cool summers were not noted more than three times in any century.

The summer air temperature changes described in Poland for 1461–1800 are generally in line with analogical reconstructions made for central Europe, Germany, Switzerland, and Scandinavia, as well as the Baltic Sea region in terms of the character of their changes. All show that the coldest summers were observed in the 17th century, and that summers were clearly warmer the rest of time. On the other hand, the differences between them are seen in temperatures, where in Poland the historical summers were slightly warmer than the reference summers (1901–1960), while in all other reconstructions they were colder than the different reference periods utilized (see Niedźwiedź et al., 2015). For this reason, reliable comparison of values is impossible.

The previous reconstruction of mean temperatures in the summer season is in essence concerned with lowland Poland. However, Niedźwiedź (2004) recently reconstructed mean summer-season temperatures for the Tatra Mountains in the post-1550 period (Figure 5), identifying two warm phases (1550–1575 and 1676–1792), two cool phases (1576–1675 and 1793–1895), as well as a (more) contemporary period extending from 1896 to 2004 and characterized pre-1990 by a decided prevalence of negative anomalies.

Under the latter reconstruction, the warmest summer seasons are seen to have arisen in the period 1676–1688, with a culmination in 1687 (2.1°C above the long-term norm from 1927–2004). It was only warmer than that in 1992, when the anomaly was of +2.4°C. In contrast, the coo1est summer seasons in the Tatra Mountains were those occurring in the periods 1580–1595, 1830–1850, 1910–1925, and 1970–1985. The coolest summer of all came in 1913, when the mean temperature was 2.5°C below the long-term norm.

When comparing the reconstruction for summer-season air temperatures in the Tatra Mountains (Figure 5) with those presented previously for lowland Poland (Figure 3) and the Czech Republic (Przybylak et al., 2004), it is necessary to note (perhaps not surprisingly) the greater concordance with the Czech series. The coefficient of correlation (r) for the reconstructed values for summer-season temperature in the Czech lands (1500–1799) as opposed to lowland Poland (1501–1800, new reconstruction) assumes a value of 0.29. The lowest value is that for 17th-century data (r = 0.27), and the highest for the 18th century (r = 0.66). Nevertheless, in no period are the links between the series analyzed statistically significant.

In summary, it needs to be noted how our knowledge of Poland’s climate over the 300-year period analyzed has increased greatly in the 21st century, albeit still being very limited, especially when it comes to summer. However, the quality is sufficient to support the contention that the period 1501–1800 saw Poland’s climate subject to greater thermal continentality than is noted these days. The result of that was far colder winters and slightly warmer summers than could be noted in the years 1901–1960.

Post-1800

Recently, Przybylak (2010) offered a synthetic presentation of existing knowledge on changes in Poland’s air temperature in the period over which observations using instruments have been possible, and carried out. Thus, only the key results concerning mean annual and seasonal values for air temperature are presented here. The mean annual temperatures in Warsaw and in Cracow rose from 1801 to almost the end of the 20th century—by more than l °C (Figure 6). The trends to changes in air temperature are upward at a rate in excess of 0.5°C per 100 years and are statistically significant (Trepińska and Kowanetz, 1997; Lorenc, 2000; Przybylak et al., 2013). Such significant increases also characterize the series for the Baltic coast involving the period 1836–1990 (Miętus, 1996), as well as that for Poland as a whole extending between 1901 and 2000 (Kożuchowski & Żmudzka, 2003) (Figure 6).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 5. Mean summer (June–August) temperature variations in the Tatra Mountains (Hala Gąsienicowa, 1520 m a.s.l.) from 1550 to 2004 (after Niedźwiedź, 2004). Key: z—standardized values, average = 0.0, standard deviation = 1.0; solid line—year-on-year course; heavy line—11-year running means.

It has emerged that the areally averaged temperature for Poland as a whole is exceptionally strongly correlated with temperatures noted for Cracow and Warsaw, with the effect that the thermal series of the two stations may indeed be used to characterize this element of the country’s climate in relation to the whole of Poland, of course leaving aside the distinctive mountain areas of the far south.

In line with expectations, the greatest increases in air temperature are those reported for winter, which exceed 2°C in relation to Cracow, Warsaw, and the Baltic coast (Fig. 5.6 in Przybylak, 2010). Indeed, all four studied seasons of the year feature a distinct increase in air temperature over the last 200 years. Furthermore, most of the trends for air temperature are statistically significant—other than those for the summer in Cracow, Warsaw, and the Baltic coast, as well as winter temperatures for Poland as a whole. Thus, recent air temperature changes in Poland fit very well with the changes of areally averaged temperature for Europe and the entire globe (see IPCC, 2007, 2013).

Ground-Surface Temperature

Climatic studies first turned to the reconstruction of mean annual temperatures of the ground surface in the work of Čermak (1971), and later Lachenbruch and Marshall (1986). Elsewhere in the world, such data were not used widely until the 1990s, when a number of reconstructions of temperature of this kind appeared around the world (e.g., Shen & Beck, 1991; Shen et al., 1995; Majorowicz, 1996; Šafanda et al., 1997; Majorowicz & Šafanda, 1998; Huang et al., 2000; and Pollack & Huang, 2000).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 6. Long-term courses of 11-year running mean annual air temperatures in Poland during the period of instrument-based observation (after Przybylak, 2010, modified). Key: Data for Cracow after Trepińska (1971) and Matuszko (2007), for Warsaw after Lorenc (2000), for the Baltic coast after Miętus (1996), and for Poland as a whole after Żmudzka (2008).

The first reconstruction of temperatures at the land surface over the last 500 years in southwest Poland was presented by Wójcik et al. (1999). In subsequent years, further such reconstructions arose, albeit using varying methodologies, among others for northern Poland (Wójcik et al., 2000) and finally for the country as a whole beyond the mountains (Majorowicz et al., 2001, 2004; Przybylak et al., 2005).

The works in question dealt with the geothermal reconstruction methods in detail, as well as application of the research results. Thus only selected, most important results from the earlier research are presented here.

Figure 7 presents four reconstructions (curves 1–3 and 5) for changes over time in mean annual ground-surface temperatures in Poland, as obtained using the functional space inversion methodology proposed by Shen and Beck (1991). These are shown against the background of the course obtained for Warsaw mean temperatures (curve 4); and the courses of all curves make clear the existence of a period of cooling from 1700, with a minimum being reached in the early 19th century. Warming commenced from around the 1800–1850 period, this lasting up to the beginning of the second half of the 20th century, with only a slight cooling in that second half, prior to a return to an upward trend. The increase in temperature over the last 500 years in Poland is in the range 0.5–0.95°C, depending on the assumed level of conductivity, the error for the reconstruction adopted, and the groups of boreholes chosen for analysis. Most reconstructions from deep-drilled sites with measurements maintained at constant depths also point to a local maximum around 1700–1720, preceding a 19th-century minimum. A similar course to ground-surface temperatures was reported from inter alia central Czech Republic (Šafanda et al., 1997), western Canada and the United States (Skinner & Majorowicz, 1999), as well as around the globe (Pollack & Smerdon, 2004). The amplitude to the changes in Poland is nevertheless seen to be smaller than in the North American case.

Poland’s Climate in the Last MillenniumClick to view larger

Figure 7. Reconstructions of the ground-surface temperature history in Poland (curves 1–3) and their comparison with the analogical reconstruction (curve 5) based on the deepest point temperature profile with depth in the Grodziec borehole, as well as with air temperature (curve 4) (after Majorowicz et al., 2001). Key: curve l—reconstruction from the continuous temperature logs that indicate minor ground-surface temperature warming, curve 2—reconstruction derived from continuous temperature 1ogs indicative of major warming, curve 3—average based on the above two groups, curve 4—homogeneous air temperature series from Warsaw (11-year running mean) (Lorenc, 2000).

Majorowicz et al. (2001) showed how reconstructed ground-surface temperature values for Poland are highly correlated with the mean annual values for air temperature that have been obtained for Warsaw. The coefficient for the correlation between the series obtained from boreholes (all holes; curve 3—Fig. 7) and the aforementioned Warsaw series (curve 4—Fig. 7) is of r = 0.79. A still stronger correlation occurred between the series obtained from inversion of thermal profiling with holes showing marked near-surface warming on measuring curves (Fig. 7—curve 2) and the instrument-based Warsaw series (r = 0.89). It is worth adding that such values for correlation coefficients obtained are markedly higher than those calculated between series for annual tree growth and air temperature.

Harris and Chapman (1998) modified the method of functional space inversion, bearing in mind the most realistic possibility of it being applied to establish the mean ground-surface temperature within the time interval preceding instrumental data. The result is the POM (pre-observational mean temperature) approach. The model the authors adopted proceeds on the assumption that—in the period of instrumental measurements—the reconstructed ground-surface temperature is the same as the air temperature measured at a standard meteorological station.

On the other hand, in the period prior to the first measurement in a measured air temperature series, what is sought is the mean temperature value that bests approximates the course to anomalies of temperature of rocks, together with the depth denotes for thermal profiles obtained in the case of holes. Using this method, Przybylak et al. (2001) calculated that the increase in the ground-surface temperature between the pre-instrumental and current periods was of ca. 1.5°C, and hence 0.5–0.6°C more than was calculated with the aid of the first method. In their view, one of the reasons for disparities in the results obtained may lie with differences in the assumptions underpinning the two methods.

Precipitation

The 1001–1800 Period

The reconstruction of atmospheric precipitation has met with far greater problems than in the case of air temperature, this mainly reflecting the more limited influence of the former on natural phenomena (e.g., tree rings, Zielski, 1997), as well as on the life and activity of people in the temperate zone (hence the far fewer references in historical sources to precipitation conditions as opposed to thermal conditions). As a result, the number of works describing precipitation in Poland in the pre-instrumental period is very limited. Generally, there are two studies (Maruszczak, 1991; Przybylak et al., 2004) in which this issue is addressed in the case of periods lasting several hundred years, along with several (Bokwa et al., 2001; Limanówka, 2001; Nowosad et al., 2007; Filipiak, 2007; Przybylak et al., 2008; Przybylak & Marciniak, 2010; Przybylak et al., 2014a) that address only short, isolated periods.

Another significant hindrance met with in the reconstruction of precipitation is that this is the most temporally and spatially variable of all the climatic elements. Maruszczak (1991) offers only very general information on (what the author judges to be) average annual humidity conditions—in a period the work does not define precisely—these being understood to be precipitation conditions. These were average in the 11th century, while the 12th century was the wettest at any point in the whole millennium. This century is also very rich in precipitation in the Baltic Sea catchment (Schimanke et al., 2012). The beginning of the 13th century was marked by a decline in precipitation, to the point where this was below the norm in that century’s second half. In the 14th century, quantities of precipitation increased, to the point where they were variable but close to the norm from the middle part of the 14th century through to the mid-15th century. In the subsequent 100 years there were considerable changes in humidity relations, though the author does not indicate in what direction. However, it can be judged from the context that the climate became wetter, in the years 1480–1520 at least. A hypothesis of this kind is supported in data published by Przybylak et al. (2004) for Poland (see Figure 8), and by the modeling work of Schimanke et al. (2012) for the Baltic Sea catchment. There is little information about changes in the annual cycle of precipitation from historical to present times. From two periods (1656–1685 and 1739–1770) for which such information is available for the NE part of Poland and for Gdańsk, it can be, respectively, stated that the observed changes are quite small (Filipiak, 2007; Przybylak & Marciniak, 2010).

In contrast, on the basis of an analysis of numbers of days with precipitation, Limanówka (2001) declared that the first half of the 16th century had far less precipitation than do contemporary times. However, it would seem that the notes compiled by the professors of the Cracow University did not take account of the lightest precipitation, which they may well have missed altogether. A similar problem for the first two years of Chrapowicki ’s diaries (1656 and 1657) was reported by Nowosad et al. (2007). It therefore seems likely that the number of days with precipitation given by Limanówka (2001) is underestimated and should be corrected.

The years from the mid-16th to mid-17th centuries brought a preponderance of average conditions where humidity was concerned (Maruszczak, 1991). Similar results were also obtained by Przybylak et al. (2004) (see, i.a., Figure 8).

The second half of the 17th century was, according to the reconstruction by Maruszczak (1991) a time of below-average precipitation. However, it is probable that the first two decades of the period had annual precipitation within the norm, albeit with more summer rain than now, but also more limited winter precipitation (Figure 9). High precipitation totals for June and July in this period are also documented by the reconstruction performed on the basis of dendrochronological data (see Fig. 2 in Przybylak et al., 2001). This is also confirmed by an analysis of the number of days with precipitation in the years 1656–1685 (Przybylak & Marciniak, 2010). From Figure 21.10 presented in this work, it can be concluded that the period 1677–1685 was also humid.

From the 18th century onward there was a prevalence of within-the-norm precipitation, except in the late-18th- and early-19th-century periods, when there was less (Maruszczak, 1991). In principle, the results obtained by Przybylak et al. (2004) confirm this, with the possible exception of the 1731–1750 period, which was most likely wetter than the norm (Figure 8).

Reconstructions of summer and winter precipitation totals from the area of the Czech Republic (see for example Fig. 2 in Brázdil, 1994) are similar to those obtained for Poland (see Table l and Fig. 4 in Przybylak et al., 2004). The most reliable results on precipitation amount over a major part of that century (years 1739–1770) come from direct measurement made in Gdańsk (Filipiak, 2007). This reveals that the first years of the period (up to 1750) were wetter, as is confirmed by results presented by Przybylak et al. (2004) for Poland as a whole. Between 1751 and 1766 there was in turn a dry period with annual precipitation in 13 years below the long-term norm calculated for the study period as a whole. However, it is noteworthy that it was at this time that there occurred the year (1755) that is considered to have been the wettest in the entire analyzed period, with annual precipitation at 140% of the norm. Interestingly, the driest year of all came in 1762, reporting just 63% of the norm.

Poland’s Climate in the Last MillenniumClick to view larger

Figure 8. Decadal frequencies of occurrence of summers (JJA) that were: (a) extremely wet and very wet (indices 3 and 2) and (b) extremely dry and very dry (indices -3 and -2) in Poland between 1501 and 1840 (after Przybylak et al., 2004).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 9. Differences in numbers of days with precipitation (a) and with snowfall (b) in Poland between periods that are historical (1657–1667, J. A. Chrapowicki’s diary) and modern (1961–1990, selected meteorological stations) (after Nowosad et al., 2007).

Post-1800

Przybylak (2010) summarized the state of knowledge of courses for precipitation in Poland during the instrumental-observation period. Thus, all that are presented in this section are selected most important results from that earlier work. The majority of publications concerning variability of precipitation in Poland relates to small areas only, or even to a single station (e.g., Gorczyński, 1912; Trepińska, 1969; Hohendorf, 1970; Kożuchowski, 1985; Twardosz, 1999, 2007; Twardosz & Niedźwiedź, 2001; Pospieszyńska & Przybylak, 2013; Przybylak et al., 2013). A large number of studies also discuss variability using data from across Poland, or certain regions thereof (e.g., Kaczorowska, 1962; Kożuchowski, 1985; Miętus, 1996; Kożuchowski & Żmudzka, 2003; Twardosz & Cebulska, 2010).

Thus far, the longest series of measured precipitation totals is that available for Wrocław (1791–present) (Przybylak et al., 2013). The driest period was noted in the first half of the 19th century, and the wettest from 1910 to 1950. The second-longest precipitation series is from Cracow (post 1812), and the third from Warsaw, which begun in 1813 (Figure 10). Przybylak (2010) in turn stated that the precipitation totals for the years to the end of the 1820s in Cracow are of 1imited reliability, reconstructed as they were on the basis of numbers of days with precipitation. If these are excluded from the analyses then the annual precipitation totals for Cracow do not show any more major changes over the two-century study period. It is nevertheless possible to distinguish wetter times like 1830–1860, 1900–1920 and 1960–1972, as well as the years since 1996 (Figure 10; Twardosz, 2007). The trend for annual totals in Cracow in the years 1812–2007 has been up by 23 mm per 100 years, but this does not attain statistical significance. Similarly nonsignificant changes over the years 1867–2007 characterize seasonal totals, other than for winters, in which the increase in the amount of precipitation has been significant (Przybylak, 2010; Twardosz & Cebulska, 2010).

Courses for total precipitation similar to those recorded for Cracow are present in other precipitation series shown in Figure 10. Particular good concordance is present between the Cracow series and the averaged series obtained for the Baltic coast, or for Poland as a whole. A good spatia1 correlation for precipitation in Poland was noted by Kożuchowski and Żmudzka (2003), on the basis of data for 1951–2000 (coefficients for the correlation between the averaged precipitation series for Poland and the series from particular stations assume values ranging from 0.6 to 0.8). The research done by many Polish climatologists also reveals that precipitation in Poland displays short-period oscillations over intervals of 2–4 years (Kożuchowski, 1985; Miętus, 1996; Kożuchowski & Żmudzka, 2003).

Poland’s Climate in the Last MillenniumClick to view larger

Figure 10. Long-term courses of 11-year running mean annual precipitation in Poland during the period of instrumental observations (after Przybylak, 2010, modified). Key: Data for Warsaw after Kożuchowski (1990), for Cracow after Twardosz (1999, updated), for the Baltic coast after Miętus (1996), and for Poland after Kożuchowski and Żmudzka (2003).

In conclusion, it should be noted that, over the last 200 years, Poland has not witnessed any more significant change in amounts of precipitation. Analysis of precipitation for Europe (Casty et al., 2007; Büntgen et al., 2011), as well as for the entire globe also show similar results (see, e.g., IPCC, 2007, 2013; Sheffield et al., 2012; Greve et al., 2014).

Conclusions and Final Remarks

In the scientific literature, such an extensive synthesis and review of the knowledge about climate change in Poland in the last millennium as presented here has not been available to date. Although some reviews were written in the 20th century, they are fragmentary and usually rather general, because quantitative information was very limited at the time. A marked progress in the acquisition of more precise information about the history of climate in Poland has been noted quite recently, in the 21st century. For this reason, the reliability of climate reconstructions for Poland in the last millennium presented here is higher than in those presented in the past. Nevertheless, the general scheme of climate change in the study period is similar in all available reviews and can be summarized as follows, including the results from the most recent quantitative climate reconstructions:

  1. 1. The existing incomplete reconstructions of climate in Poland over the last millennium indicate that the first 500-year period was warmer than the second (especially as regards the first 300 years). It was mainly the warmer winters that gave this effect, because summers could even be cooler at times, if the index adopted here is the frequency of occurrence of hot summers (see Fig. l in Sadowski, 1991). This was thus a period of major (and according to Sadowski [1991] even the greatest) oceanicity of the Polish climate. Thus, the history of Poland’s climate needs to recognize the so-called Medieval Warm Period, most probably persisting through to the early 14th century (according to data from Maruszczak, 1991) or the beginning of the 15th century (according to data from Sadowski, 1991). The mean air temperature then was probably higher by 0.5–1.0°C than what we experience now (1961–1990).

  2. 2. Beginning with the 15th century, the continentality of climate was maintained at a high level, and this continued through to the early 19th century. As a result, this period brought winters that were markedly cooler (by ca. 1.5–3.0°C in comparison with the 1901–1960 period), but also summers that were warmer (by ca. 0.5°C on average). Mean annual values for air and ground-surface temperatures were most probably lower than current ones by some 0.9–1.5°C. At this point it is possible to distinguish the so-called Little Ice Age, which began more distinctly around the middle of the 16th century and most probably ended in the second half of the 19th century.

  3. 3. The reconstruction of precipitation (the most temporally and spatially variable of the meteorological elements) is burdened with much greater uncertainty than the process involving air temperature. More precipitation than average was probably present in the 12th century (especially its second half), in the first half of the 16th century, and in the first half of the 18th century. In turn, precipitation below the norm probably characterized the second half of the 13th century and the first half of the 19th century, while remaining periods probably had conditions close to average. It is interesting to note that the periods with the lowest precipitation in Poland occurred in times of the strongest volcanic forcing in the last millennium.

  4. 4. The presented synthesis of knowledge on Poland ’s climate over the last millennium reveals that—despite the considerable progress made with research since the late 20th century—there is still much work to be done in the future.

Acknowledgments

Work on this review was supported by funds received from two research projects: PSPB-086/2010 (“Climate of Northern Poland during the Last 1000 Years: Constraining the Future with the Past” [CLIMPOL]) and the DEC-2013/11B/HS3/01458 (National Science Center funds).

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