Air temperature changes until now

Estonia lies in the transition zone between maritime and continental climate. Local climatic differences are due, above all, to the neighbouring Baltic Sea, which warms up the coastal zone in winter and has a cooling effect, especially in spring. The topography, particularly the heights in the southeastern part of Estonia, plays an important role in the distribution and duration of snow cover. As a result of these factors, the summers are moderately warm (the mean air temperature in July is 15-17°C) and winters are moderately cold (the mean air temperature in February is between –3.5 to –7.5°C) (1).

Mean air temperature

Within the limits of the continuous time series (1737-1995), the linear trend is 1.5ºC. Since the beginning of the 1920s, there has been a positive linear trend. The most intensive warming to date has taken place in March, April, and May, with a gradient of 1.5ºC per 100 years. From October to February the trend has been moderate (0.9ºC per 100 years on average). No significant trend was evident in summer (June - September) (2).

During the second half of the 20th century the annual mean air temperature in Estonia increased by 1.0-1.7°C (3, 14). For the period of 1965– 2005 a statistically significant trend of mean annual temperature increase of ca 0.3 ± 0.1°C per decade) was detected for both the Baltic Sea region and Estonia (17). Seasonality plays an important part in climate warming in Estonia. A statistically significant increase in the monthly mean temperature is present only during the period from January to May, with the greatest increase in March (up to 4°C). For the rest of the year, practically no change in the annual mean air temperature has been identified. During the period 1961–2004 the winter seasonal air temperature increased by 3.2°C on average (1). From another study a significant mean monthly temperature increase over the period of 1965– 2005 was detected for January, April and July for both the Baltic Sea region and Estonia (17).

Maximum and minimum temperature

Statistically significant increasing trends in maximum and minimum temperatures were detected for March, April, July, August and annual values over the period 1951–2010 in Lithuania, Latvia and Estonia. At the majority of stations in these countries, the increase was detected also in February and May in case of maximum temperature and in January and May in case of minimum temperature (15). For Estonia, the increase in annual mean maximum and minimum temperature during 1951–2010 was about 0.3°C/decade (15).

Changes in atmospheric circulation

Part of the major climate changes observed in the Baltic Sea region during the late 20th century can be related to changes in atmospheric circulation (4). It has been suggested that the increased frequency of anti-cyclonic circulation and westerly wind types have resulted in a slightly warmer climate with reduced seasonal amplitude and reduced ice cover (4). Recent research using tree-ring based chronologies indicates that the variability of recent decades may lie within the natural range (5).

Length summer and winter

Observations show that between 1891 and 2003 winter has shortened by 29 days in southern Estonia and summer has become 11 days longer (20).

Precipitation changes until now

Since annual precipitation exceeds evaporation approximately twofold, the climate is excessively damp. The mean annual precipitation is about 550-650 mm, ranging from 520 mm on some islands to almost 730 mm in the uplands. The seasonal variation in precipitation is similar throughout the country, the driest months being February and March. From then on, precipitation gradually increases until July and August, after which it decreases towards winter and spring. The lowest annual precipitation may be less than 350 mm on the coast, but inland regions sometimes have more than 1,000 mm. The highest daily rainfall recorded is 148 mm (1).

Since 1966 precipitation series in Estonia have been homogeneous. They indicate an increase during the cold half-year and also in June. A significant increase in precipitation has occurred in winter period (29%). During the period 1961 - 2004 the winter seasonal precipitation increased over 45 mm (1), or approximately 10 mm per decade during 1966 - 2015 (18). A slight positive trend in heavy precipitation events over the last half century has been reported; this increase is expected to continue and accelerate in the future by up to 22 % (16). A significant increase of annual total precipitation was detected for the Baltic Sea region (13 ± 6 mm per decade) and for Estonia (28 ± 12 mm per decade) for the period of 1965 - 2005 (17), although this trend is mostly insignificant for the period 1966 - 2015 (18).

The duration of snow cover decreased significantly during the second half of the 20th century (6), with the greater decrease occurring in the western and central parts, exceeding one day per year in some regions. A decrease in mean snow cover depth and in water equivalent from 1961 to 2001 has also been observed, with the largest decrease occurring during February (7). Decreases in the duration of snow cover in Latvia and Lithuania have been found during the past five to seven decades. A slightly decreasing trend in the duration of the snow cover (up to −4 days/decade) and depth of snow (up to −13 cm/decade) has also occurred in most of Poland during the second half of the 20th century.

Snow cover changes until now

During the period 1950-2016 the median number of days in a year with snow cover at 22 stations across Estonia was 112. There was a negative trend in snow cover duration due to the earlier snow melting in spring at the majority of these stations. The end date of the permanent snow cover has shifted earlier by 10-30 days in 66 years and its duration has decreased accordingly (19).

Wind climate changes until now

Over the last century the mean wind speed increased by 0.5-0.8 m/s. The increase of mean wind speed is characteristic mainly of the cold season (November to February). No significant change in wind speed is observed during the warm period (May–July). During 1966–2005, generally, south-westerly and westerly winds have increased, whereas north-easterly, easterly and south-easterly winds have decreased. The winds of maximum frequency have changed from south-east to south-west (1).

Sea water temperature changes until now

The duration of sea ice decreased significantly during the second half of the 20th century (2). Over this period, the date by which sea ice appears has been very consistent, but the date by which it disappears at the end of winter has become earlier. The end of winter and the start of spring occur much earlier than before (by 19–39 days) (1).

In the 20th century, temperatures in the Baltic Sea basin showed a marked increase during the early part of the century (termed the early 20th century warming) until the 1930s; this was followed by a smaller cooling that ended in the 1960s, and then another warming thereafter. For the entire period (1871–2004) of a time series of Baltic land-based data, the largest trends are present in the spring; the trends in all seasons are positive and most of them are statistically significant (7).

There is great variation in the extent of the sea ice cover on the Baltic Sea from year to year. In extremely mild winters, the Bothnian Bay, parts of the Gulf of Finland and the Bothnian Sea, and shallow coastal regions in the Gulf of Riga are covered by ice, and the maximum ice coverage is only about 12% of the total area of the Baltic Sea. In average winters, the ice-covered region in March consists of the Gulf of Bothnia, the Gulf of Finland, the Gulf of Riga, northern parts of the Baltic Proper, and shallow coastal areas further south. In extremely severe ice winters, almost all of the Baltic Sea freezes over. The latest extremely severe winter occurred in 1986/1987, and the latest winters with the Baltic Sea totally frozen were in 1941/1942 and probably also in 1946/1947 (7).

A climate warming is reflected in time series data on the maximum annual extent of sea ice and the length of the ice season in the Baltic Sea. On the basis of the ice extent, the shift towards a warmer climate took place in the latter half of the 19th century. The length of the ice season showed a decreasing trend by 14–44 days during the 20th century, the exact number depending on the location around the Baltic Sea. The ice extent, the date of ice break-up, and the length of the ice season show a correlation with the NAO index (7).

In the Baltic Sea the region of the highest warming in the 1980s and 1990s was in the eastern Baltic region south and east from Tallinn and St Petersburg. Atmospheric circulation has been shown to have had a large (~70%) impact on temperature variations: the westerly wind flow from the North Atlantic in winter was particularly strong during the 1990s; strong westerly winds generally bring milder air towards Fennoscandia, thus contributing to the mild Fennoscandian winters of this decade (7).

Air temperature changes in the 21st century

Climate change scenarios for the year 2100 indicate a significant increase in air temperature (by 2.3–4.5°C) and precipitation (by 5–30%) in Estonia. The highest increase is expected to take place during winter and the lowest increase in summer (2,8).The continentality of the climate will be reduced and the influence of the sea will become more pronounced. Winters will become milder and hibernating conditions will improve (2).

Comparing the predicted air temperature changes with Estonian local observational time series, we see that they stay within the limits of air temperature fluctuations of earlier years. From this it can be concluded that an increase in average temperatures will not bring about any catastrophic change in Estonia (2).

According to calculations based on a regional climate model and two different emission scenarios, wintertime average daily temperatures in the period 2071–2100 are simulated to increase with respect to the period 1961–1990 from 3° to more than 7ºC in east Europe and Russia depending on which emission scenario and which driving global model is used (10). The warming in the cold end of the temperature distribution is even larger. The strongest warming occurs on cold days.

The strong increase in wintertime temperature in east Europe and Russia is probably connected to the reduction of the snow cover in the scenario runs. The mechanisms involved are feedback processes involving temperature, snow cover and albedo. With decreasing snow cover the albedo becomes lower. The lower albedo implies that more shortwave radiation is absorbed in the ground which in turn leads to higher surface temperatures. The largest reduction of the length of the snow season is calculated to be in a zone reaching from central Scandinavia through southern Finland and the Baltic countries and further towards the southeast into Russia (11).

Precipitation changes in the 21st century

Climate change scenarios for the year 2100 indicate a significant increase in precipitation (by 5–30%) in Estonia. The highest increase is expected to take place during winter and the lowest increase in summer (8).The increasing trend in autumn and winter precipitation indicates a seasonal shift and demonstrates a general tendency of climate change in Estonia from a continental climate towards a more maritime climate (2). Estimates of precipitation changes, however, are more dispersed than those of temperature changes: no annual trend of precipitation changes can be observed (2).

Projections of climate change show a future decrease in mean annual maximum snow depth everywhere over Northern Europe. This decrease is smaller in the northern parts of the Baltic Sea basin than in the southern areas. The simulations also show a decrease in the duration of the snow season. In areas such as Denmark, Germany, Poland, and most parts of the Baltic countries, where the present-climate snow depth is small, the scenario simulations show a complete lack of snow cover (7).

Wind climate changes in the 21st century

During the winter period, cyclonic activity will become more intensive and the wind speed will increase (2).

Mean and extreme geostrophic wind speeds in Northern Europe have been projected for the future periods 2046–2065 and 2081–2100, and compared with the baseline 1971–2000 (based on nine global climate models and the SRES A1B, A2 and B1 scenarios) (12). The geostrophic wind speed is a theoretical, calculated wind speed indicative of true surface wind speed. The results show:

• Mean wind geostrophic speeds: During the windiest time of the year, the monthly mean wind speeds will start to increase in the Baltic Sea already in 2046–2065. In Finland, increases are largest (5–7%) in November and January by 2081–2100. In November–February 2081–2100, a positive shift of 5–10% is projected to materialize in the Baltic Sea.
• Extreme geostrophic wind speeds: The extreme wind speeds (10-year return level estimates) will increase on average by 2–4% in the southern and eastern parts of Northern Europe, whereas a decrease of 2–6% dominates over the Norwegian Sea. These results agree with results on the future projections of 20-year return level estimates of gust winds that showed that the increase in winds is dominant in a zone stretching from northern parts of France over the Baltic Sea towards northeast (13).

Sea water temperature changes in the 21st century

The annual mean sea surface temperature of the Baltic Sea is projected to increase by between 2°C and 4°C between 1961–1990 and 2071–2100 (7).

The projected decrease of ice cover of the Baltic Sea over the next 100 years is dramatic. Towards the end of the 21st century, the Bothnian Sea, large areas of the Gulf of Finland and the Gulf of Riga, and the outer parts of the southwestern archipelago of Finland would, on average, become ice free. The length of the ice season would decrease by 1–2 months in the northern parts and 2–3 months in the central parts of the Baltic Sea. Some of the model simulations predicted that totally ice-free winters could occur (7).

On average, Tallinn will be out of the freezing zone of the Baltic Sea in the second half of the 21st century (2).

Uncertainties in climate projections

The projected changes in precipitation are far more uncertain than those for temperature. Hence, quantitative projections of changes in river flow remain largely uncertain. For the Baltic Region even the sign of precipitation and runoff changes is inconsistent across the current generation of models. The uncertainty in findings about future climate change impacts refers particularly to extreme events (9).

Uncertainties of climate change projections increase with the length of the future time horizon. In the near-term, e.g. 2020s, climate model uncertainties play the dominant role, while over longer time horizons, uncertainties due to the selection of emission scenarios become increasingly significant (9).

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 Estonia.

1. Ministry of the Environment of Estonia (2009)
2. O’Brien (ed.) (2000)
3. Jaagus (2006b), in: Kont et al. (2007)
4. Omstedt et al. (2004)
5. Esper et al. (2002), in: Omstedt et al. (2004)
6. Jaagus (1997); Tooming and Kadaja (1999), both in: Kont et al. (2007)
7. HELCOM (2007)
8. Kont et al. (2003)
9. Kundzewicz (2009)
10. Räisänen et al. (2003), in: Kjellström (2004)
11. Kjellström (2004)
12. Gregow et al. (2011)
13. Nikulin et al. (2011), in: Gregow et al. (2011)
14. Jaagus (2006), in: Jaagus et al. (2014)
15. Jaagus et al. (2014)
16. Reckermann et al. (2011), in: Norwegian Meteorological Institute (2013)
17. Männik et al. (2015)
18. Jaagus et al. (2018)
19. Viru and Jaagus (2019)
20. Kull et al. (2008), in: Ruosteenoja et al. (2020)