Air temperature changes until now

Comparison of the temperature trends in 1961 – 1990 and 1991 – 2008 shows that the average annual temperature increased by 0.8⁰C between these two periods, with the greatest temperature increase in January and August (by 1.5⁰C) and the smallest in September, October and December (about 0.2⁰C). … The trend in increase in the average annual temperature since 1961 corresponds to 0.33⁰C/10 years; the winter and summer trends are higher, corresponding to 0.44⁰C/10 years and 0.43⁰C/10 years, while the lowest temperature increase is in the autumn (0.08⁰C/10 years) (1).

The trends have been increasing over the past 15 years. Since the beginning of the 20th century the trend in increasing temperature corresponds to 0.17⁰C/10 years, with comparable trends for the summer and winter seasons (1).

The numbers of tropical summer days and tropical nights have increased substantially in recent years, while the numbers of frost and ice days have decreased (1).

Winter, spring and summer are characterized by an increase in the number of hours of sunshine, decreasing cloud cover and decreases in the relative humidity. In contrast, in the autumn, when the temperature and daily amplitude decrease, we can observe a reduction in the number of hours of sunshine and an increase in cloud cover and relative humidity (1).

In the period 1961-2019, the diurnal temperature range has increased in spring and summer, and decreased in autumn and winter (22).

Urban heat island

In the period 1981-2016, the average urban heat island intensity was 2.3 °C for Prague and 1.8 °C for Bucharest, and it exceeded 4 °C in 6-10% of the cases. The highest values occurred in August in both cities, when built areas still accumulate radiant heat during daytime and release it slowly during night, while the rural neighbourhood starts cooling more rapidly during longer nights in August. City enlargement, traffic increase and conversion of green areas into built up zones have constantly augmented the urban heat island intensity in both cities during the last decades (23). Both Prague and Bucharest will be at least 1 °C warmer by the middle of the century (24).

Heat wave and cold wave changes until now

In the Carpathian Region (encompassing Croatia, Hungary, Slovakia, Czech Republic, Poland, Ukraine, Romania and Serbia), heat wave events have become more frequent, longer, more severe and intense over the period 1961 - 2010, in particular in summer in the Hungarian Plain and in Southern Romania (7). Cold wave frequency, average duration, severity, and intensity over this period, on the other hand, generally decreased in every season except autumn. In this study, a heat wave was defined as at least five consecutive days with daily maximum temperature above the long-term 90th percentile of daily maximum temperatures. Similarly, a cold wave was defined as at least five consecutive days with daily minimum temperatures below the long-term 10th percentile of daily minimum temperatures (7). The trend analysis shows a general tendency to more frequent, longer, more severe and more intense heat wave events in every season in the entire Carpathian Region. On the other hand, the cold waves show a general tendency to less frequent, shorter, less severe, and less intense events (7). 

The Carpathian Region and the Mediterranean area are the two European hotspots showing a drought frequency, duration, and severity increase in the past decades and in particular from 1990 onwards (8). When drought effects are exacerbated by heat waves or vice versa, such combination may cause devastating effects, as it happened in summer 2003 in Central Europe (9). 

In another study, a heat wave was defined as a continuous period during which daily maximum air temperature is higher than 30.0 °C in at least 3 days, mean daily maximum air temperature over the whole period is higher than 30.0 °C, and mean daily maximum air temperaturedoes not drop below 25.0 °C (6). For this definition of a heat wave, and using data from a network of meteorological stations covering the area of the Czech Republic over 1961–2006, it was shown that 1994 was the year with the most severe and longest heat waves, and the 1994 heat waves were overtaken by those of 2003 only in the southwest region. The other two seasons with enhanced heat wave characteristics were 1992 (mainly in the eastern part of the country) and 2006 (in the western part, including central lowland region surrounding Prague). The July 2006 heat wave, lasting 33 days, was the longest and most severe individual heat wave in Prague since 1775 (5).

Precipitation changes until now

The mean annual total precipitation varies from slightly more than 400 mm in the western part of the Czech Republic up to more than 1400 mm in the mountains to the north. Almost two thirds of the annual total falls in the warm half of the year (April–September) (11). 

In the first half of the 20th century, annual total precipitation at the Prague-Klementinum station exhibited a very slight increase, followed by a slight decrease in the second half (5.6 mm/10 years), caused by the decrease in total precipitation in the warm half of the year – especially in summer and autumn; winter totals increased slightly. The trend in decrease in annual total precipitation corresponds to a decrease in total precipitation by approx. one percent per decade (1).

The last 40 years

The overall trend in the precipitation regime is an increase of average annual total precipitation in the 1991 – 2008 period by 2.9% compared to 1961 – 1990. A clear increase of more than 5% is apparent in February and March and in the August to October period; the monthly total precipitation in the April to June period was lower by 2 to 6% than in the 1961 – 1990 normal period (1).

Extreme total precipitation values occur more frequently, reflected in the repeated occurrence of floods, especially after 1995 (e.g. more extensive floods in 1997, 1998, 2002, 2004, 2005 and 2007). Greater occurrences of extreme weather events have been recorded especially in the last ten years (1). An analysis has been carried for rainfall data in the Czech Republic over the period 1961 to 2011 (10). The contribution of heavy, short-term precipitation to the precipitation total significantly increased at about half of 17 weather stations spread over the country. However, whether these changes are due to climate change is yet unclear. The observed changes are not exceptional on a longer time-scale, and may just as well point to climate variability on a time scale of decades (10).

Snow cover

The average number of days with snow cover at positions up to 600 m above average sea level has decreased over the past twenty years by an average of approx. 15% compared to the usual number of days in the middle of last century (shortening of the season by 12 days); this decrease was approximately half as great at higher altitudes. The maximum snow cover has decreased by 25% at lower altitudes and by up to 30% at higher altitudes. Total amounts of new snow in the winter exhibit similar trends (1).

Regional differences

While the temperature trends are more or less homogeneous throughout the country, trends in precipitation exhibit certain differences. In the eastern part of the country, reductions in summer precipitation are more marked and are also the cause of the overall decrease in annual totals; in the western part, the increase in winter precipitation is more marked, leading to a slight increase in annual total values (1).

Wind climate changes until now

In recent years, it seems that the systematically positive trend in the North Atlantic oscillation over the past decades and consequent increase in wind speeds that have been apparent in western Europe in the past few years are already becoming manifested in this country (1).

Air temperature changes in the 21st century

Between now and 2030, an increase in average annual temperatures of 0.24⁰C/10 years is projected. This corresponds well with global values and values given for Europe (0.2⁰C/10 years (2)). With respect to 1961 – 1990, the median estimate of average annual temperature in 2030 is projected to be 1.2⁰C higher (1).

Lower trends in increases in the temperature in the warm seasons and higher trends in the cold half of the year indicate that the temperature differences between the seasons will become less distinct. Maximum and minimum temperatures will change similar to changes in the average temperatures. Maximum temperatures will exhibit a trend towards a clear increase in the winter and summer; the minimum temperatures will tend to increase especially in the summer and partly also in the autumn and winter (1).

If no action is taken to reduce greenhouse gas emissions, annual mean temperature in central Europe may increase up to 4-4.5°C in continental regions by the end of the 21st century (4).

Heat wave changes in the 21st century

A stochastic model was used to reproduce basic characteristics of heat waves in the Czech Republic in the present climate (1961–2006), and subsequently estimate characteristics of future heat waves under several assumptions of summer warming towards the future (2007–2100). These assumptions have been estimated from regional climate models and climate change scenarios. These assumptions are warming during the 21st century of 0.2 °C/decade (lower bound), 0.5 °C/decade (mid-estimate, current trend) and 0.9 °C/decade (upper bound) (5).

Under the mid-estimate, probabilities of long and severe heat waves sharply increase in the Czech Republic. Heat waves with a severity and duration comparable to the record-breaking ones in 2006 and 1994 may be expected to occur once in around 4–8 years in the mid-21st century, while at the end of the 21st century most summer seasons will have heat waves comparable to those of 2006 and 1994. The estimated future frequencies of severe heat waves differ by an order of magnitude between the upper bound and lower bound scenarios, which reflects large uncertainties in future summer warming projections over central Europe (5).

According to the stochastic model, the return period of a heat wave reaching or exceeding the length of the 2006 heat wave in Prague is estimated to be around 120 years in 2006. Owing to an increase in mean summer temperatures, probabilities of the very long heat waves have already risen by an order of magnitude over the recent 25 years, and they are likely to increase by another order of magnitude by around 2040 under the summer warming rate assumed by the mid-scenario. Even the lower bound scenario yields a considerable decline of return periods associated with the long and most severe heat waves (5).

If a moderate increase in the variance of summer temperature, which has been observed over western and central Europe in the recent decades and appears to be a likely future scenario according to climate model simulations, is taken into account in addition to the mid-scenario of the warming over the 21st century, the return periods of the record-breaking heat waves in the Czech Republic shorten to around 3–5 years in the middle of the 21st century (5).

Heat waves in Central Europe 

In the last three decades of the previous century Central Europe has experienced 22 heat waves. Future changes in the frequency of occurrence of these heat waves were studied for this part of Europe, that includes most of Germany, the Czech Republic, Slovakia, the Southwest of Poland, Northern Austria and Hungary (18). In this study a heat wave is defined by at least three consecutive hot days. A hot day occurs when on average maximum daily temperature over Central Europe exceeds the 90th percentile of the distribution of daily maximum summer temperatures for the period 1970 - 1999. This value of the 90th percentile for this historical period is also kept as the reference for quantifying future hot days and heat waves.

Compared to this historic period, the frequency of heat waves is projected to increase by a factor 2 in the near future (2020 - 2049). For the late twenty-first century (2070 - 2099), the projected frequency increase of heat waves depends on the rate of climate change: under a high-end scenario of climate change (RCP 8.5), 3-4 heat waves per summer are projected, compared to about two heat waves under a moderate scenario of climate change (RCP 4.5). These projections are based on a large number of combinations of global and regional climate models (18).

The 1994 heat wave is found to be the most distinctive during the 1970 - 1999 period. It lasted 16 days and was associated with large excess mortality in the Czech Republic (19), Poland (20) and other Central European countries. This heat wave has been ranked as the most severe in Central Europe over the whole 1950 - 2012 period (21). Such extraordinary heat waves will probably still be rather rare in the near future. At the end of this century, however, at least one event per decade similar to the 1994 heat wave is projected for Central Europe (18). 

Precipitation changes in the 21st century

Projections for the end of the century

In plain words, rain falls in three distinct ways: by large-scale depressions, by local intensive showers, and when moist air is forced upwards over rising terrain, such as a mountain. In scientific terms these forms of precipitation are called stratiform, convective, and orographic, respectively. In Europe convective precipitation typically occurs in summer, for instance during thunderstorms. The precipitation is generally intense but of short duration. Stratiform precipitation occurs throughout the year.

The general future trend for Europe is an increase of mean precipitation in northern Europe, and a decrease in the south (12). The overall character of precipitation in Europe will also change. The contribution of convective precipitation to total yearly precipitation amounts will probably increase, especially in summer (and to a lesser degree in spring and autumn). Again in plain words: the number of intense showers will increase (13). The difference in future projections for northern and southern Europe is quite clear, but what about Central Europe, the transition zone between increasing and decreasing precipitation?

Future changes in precipitation in this transition zone were studied by looking at the Czech Republic. Projections of future precipitation patterns were made with regional climate models (driven by GCMs) under an intermediate (RCP4.5) and high-end scenario (RCP8.5) of climate change. This was done for the period 2071-2100 against 1971-2000 (as a reference) (14).

According to these simulations, mean seasonal amounts of total precipitation tend to increase in all seasons except summer (14). The part of this precipitation related to large-scale depressions (stratiform) showed a similar pattern (highest increase in winter and spring, by about 15%, slight decrease in summer by a few per cent) while local intense (convective) precipitation tends to increase throughout the year (especially in spring and summer, by about 10-20%). Previous studies showed that annual mean precipitation may increase by up to 10% in most central European regions by the end of the 21st century if no action is taken to reduce greenhouse gas emissions,(4).

The projected summer drying in Central Europe (15) is associated mainly with a decrease of stratiform precipitation; this decrease could be caused by a decrease in the frequency of frontal systems reaching Central Europe in summer (and autumn) due to a northward shift of the Atlantic-European storm track (16). Local intense (convective) precipitation, therefore, will contribute more to summer rainfall in Central Europe. Currently, convective precipitation In the Czech Republic contributes 50% to summer rainfall, and 35% and 15% in spring and autumn, respectively. These results agree with previous studies where in most scenario runs, the projected change in extreme precipitation in summer is of the opposite sign than a change in mean seasonal totals, the latter pointing towards generally drier conditions in summer (3).

Overall, it will rain harder in Europe in the future, both during heavy showers and during large-scale depressions (17). This increase will be more pronounced under the high-end end than the intermediate scenario of climate change: higher temperatures lead to more intense rainfall.

The steady rainfall of large-scale depressions (stratiform) is by its nature more important for agriculture; a decrease in combination with higher temperatures can lead to larger soil moisture deficits. More frequent and intense precipitation (convective), projected for the future climate, is not able to fully compensate the deficit of light and moderate precipitation because dry soil cannot absorb all water from intense precipitation events. Furthermore, extreme convective precipitation may cause flash floods and landslides, thus representing additional hazards and damages (3,14). 

Projections for 2030

Simulated changes in total precipitation indicate the possibility of a slight increase in annual totals (on an average by approx. 4% in 2030 compared to 1961 – 1990), higher in the winter and spring (maximum from February to April), lower in the summer and autumn (minima from July to November). The possible reduction in total precipitation by 2 to 8% in the period from May to October, together with increased evaporation in these months, indicates the risk of an increase in water deficit in soils (1).

Uncertainty in these projections

For Europe, and especially for central Europe, model projections of the precipitation regime are burdened by a much higher level of uncertainty than similar temperature projections. This is demonstrated in the effect of the relatively complicated orography of the continent, which currently does not permit sufficiently detailed modelling of precipitation processes, as well as of the predominant impacts of climate change in Europe – increased temporal and spatial variability in precipitation (1). The inter-model and intra-model variability and related uncertainties in the pattern and magnitude of the change is large, but the scenarios tend to agree with precipitation trends recently observed in the area, which may strengthen their credibility (3).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for the Czech Republic.

1. Ministry of the Environment of the Czech Republic (2009)
2. Solomon et al. (ed.) (2007), in: Ministry of the Environment of the Czech Republic (2009)
3. Kyselý and Beranová (2008)
4. Commission of the European Communities (2007)
5. Kysely (2010)
6. Huth et al. (2000), in: Kysely (2010)
7. Spinoni et al. (2015)
8. Spinoni et al. (2013), in: Spinoni et al. (2015)
9. Fink et al. (2004); Ciais et al. (2005), both in: Spinoni et al. (2015)
10. Hanel et al. (2016)
11. Tolasz et al. (2007), in: Hanel et al. (2016)
12. Frei et al. (2006); Boberg et al. (2010); Heinrich and Gobiet (2012); Rajczak et al. (2013), all in: Rulfová et al. (2017)
13. Berg et al. (2013); Fischer et al. (2015), both in: Rulfová et al. (2017)
14. Rulfová et al. (2017)
15. Kyselý and Beranová (2009); Kyselý et al. (2011); Hanel and Buishand (2012), all in: Rulfová et al. (2017)
16. Lehmann et al. (2014), in: Rulfová et al. (2017)
17. Alexander et al. (2006); Giorgi and Coppola (2009); Rajczak et al. (2013); Wagner et al. (2013); Jacob et al. (2014); Lehtonen et al. (2014); Fischer et al. (2015), all in: Rulfová et al. (2017)
18. Lhotka et al. (2018)
19. Kyselý and Huth (2004), in: Lhotka et al. (2018)
20. Kuchcik (2001), in: Lhotka et al. (2018)
21. Lhotka and Kyselý (2015a), in: Lhotka et al. (2018)
22. Zahradníček et al. (2021)
23. Zak et al. (2020)
24. Cheval et al. (2017), in: Zak et al. (2020)