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.
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