Estonia recently experienced what is likely its longest and most intense May heatwave, lasting a total of 18 days in Southern Estonia from May 14 to May 31. Heatwaves have become more frequent and intense worldwide. Unusually high temperatures over several consecutive days can severely impact health and increase mortality.

Below, I present the findings of a recently published study on heatwaves covering the three Baltic states. Various definitions of heatwaves are used in scientific discourse. In our study, we followed one of the most common definitions, which states that a heatwave occurs when the maximum or minimum daily temperature exceeds the 95th or 90th percentile for at least three consecutive days.

This means that the temperature was higher than on 95 or 90 percent of days during the same period in previous years. The measurements used in the study come from 40 weather stations covering the years 1951-2021.

The assessment of changes was based on the five most common heatwave indices: the number of heatwaves per year (HWN), the duration of the longest heatwave each year (HWD), the total number of heatwave days per year (HWF), the amplitude of the heatwave, i.e., the deviation of the hottest day of the year's hottest heatwave from the average temperature of the five summer months (May to September) (HWA), and the magnitude of the heatwaves, i.e., the deviation of the average temperature of all heatwave days from the average temperature of the five summer months (HWM).

The units for the first three indices are the number of days, while for the last two, they are degrees Celsius. As an additional simple indicator, we used the number of hot days per year, defining a hot day as one where the maximum daily temperature exceeds 27 degrees Celsius.

Figure 1 shows the seasonal distribution of hot days, indicating that the highest number of hot days occurred between July 6 and August 8, peaking on July 25 and 26. The number of hot days begins to increase gradually from May and remains relatively stable from June 7 to early July. After mid-August, the number of hot days decreases rapidly.

The average number of hot days varies significantly across the Baltic states. The left map in Figure 2 shows that the coastal areas and highlands of Estonia and Latvia experience only a few hot days. In contrast, in other inland regions, there are on average about ten hot days per year, and in southern Lithuania, up to 18 days. The cooler sea surface keeps the air above it cooler, resulting in very high temperatures being extremely rare on Estonia's coast.

The average number of heatwave days per year (HWF) is about 15 days for the 95th percentile and approximately 22 days for the 90th percentile. According to the study, the average number of heatwaves (HWN) per year is three to four and four to five, respectively. The average duration of the longest heatwave each year (HWD) ranges from five to six days and seven to eight days.

The right hand side map in Figure 2 indicates that the occurrence of heatwaves has increased significantly. The number of hot days has statistically increased across all weather stations.

In most stations, there has been an increase of 8-12 hot days over the 71-year period, representing a doubling in frequency. This trend is also evident in Figure 3. On a temporal scale, the sharp change can be pinpointed to the last three decades, starting from the 1990s.

The trend of increase is also evident in the time series of heatwave indices. The trends are stronger and statistically more significant in the indices representing the frequency and duration of heatwaves (HWF, HWN, HWD) compared to those reflecting their intensity (HWA, HWM). Additionally, changes are more pronounced and reliable when heatwaves are defined based on the 90th percentile compared to the 95th percentile.

Moreover, the changes are more significant when heatwaves are defined based on minimum temperatures rather than maximum temperatures. This result aligns with the observation that the rise in minimum temperatures has been faster than the rise in maximum temperatures. Trend analysis also reveals that changes are more pronounced in Estonia compared to Latvia and Lithuania.

Figure 4 shows the trend values (in days per decade) for the number of heatwave days (HWF) based on maximum temperatures (left) and minimum temperatures (right) for the period 1951–2021. Black dots indicate stations where the trend was statistically significant.

The frequency of heatwaves (HWF), which is the number of heatwave days, is one of the main indicators that has shown a particularly large increase in recent decades. Over the 71-year observation period, its value has more than doubled, as seen in Figures 4 and 5. A similar change has occurred in the number of heatwaves (HWN). During the observation period, two to three additional heatwaves have been added annually.

The dynamics of the third heatwave index, the duration of the longest heatwave (HWD), have been similar to the previous indices. However, the amplitude of heatwaves (HWA) and especially their magnitude (HWM) have increased much less over the observation period, making their trends unreliable.

The above findings indicate that with the overall warming of the climate, Estonia has experienced a sharp increase in the occurrence of heatwaves. The main indicators, such as the number of heatwave days, their frequency, and duration, have at least doubled over the long observation period. Assuming continued climate warming, it is clear that problems associated with excessive summer heat will only intensify in the long term.

--

Follow ERR News on Facebook and Twitter and never miss an update!