Triin Saue, a researcher at the Center of Estonian Rural Research and Knowledge, studied how the arrival of spring in Estonia has changed over time. She based her analysis on daily air temperature data from 21 Estonian weather stations covering the years 1965–2020. She defined the start of spring as the date on which the average daily air temperature rose permanently above 5 degrees Celsius.

"When looking at the entire period covered by the study, the start of spring has shifted earlier by approximately one to three weeks at most observation stations," Saue said.

The study confirmed that spring is arriving earlier across Estonia, though the extent of the changes varies considerably by region. In general, spring begins earlier in southern parts of the country and later along the coast and on the islands.

The largest changes were observed primarily at inland stations in southern and central Estonia.

"At several inland stations in southern and central Estonia, including Tartu-Tõravere, Võru, Valga and Viljandi, the shift toward an earlier start of spring exceeded two weeks over the entire observation period. At the same time, substantial changes were also recorded at some coastal stations," Saue said.

Although changes were on average more pronounced at inland stations than at coastal ones, the differences could not be explained solely by distance from the sea. According to the researcher, the course of spring is shaped by multiple factors simultaneously, including the influence of the sea, local conditions and atmospheric processes. As a result, the study did not identify a simple geographic pattern that could be used to describe changes in the timing of spring's arrival across Estonia as a whole.

Same data, different results

However, the study focused not only on whether spring is arriving earlier, but also on how such a change should be measured in the first place. Climate research often relies on so-called climate normals, or reference periods against which changes are assessed. These are typically 30-year periods. Saue compared four partially overlapping climate normal periods: 1965–1990, 1971–2000, 1981–2010 and 1991–2020. She found that the average onset of spring depends significantly on which reference period is chosen.

"Using the same dataset, an analysis of the entire 1965–2020 period may show a very strong shift toward an earlier start of spring, while an analysis based on the most recent climate normal, 1991–2020, may indicate much weaker changes or, at some stations, almost no change at all," Triin Saue explained.

At some observation stations, estimates of how much earlier spring had arrived differed by more than 10 days depending on the reference period used. This means that the same dataset can produce different pictures of the pace of climate change depending on the methodology applied. The reason is that climate does not change at a constant rate over time.

The study showed that the strongest shift toward an earlier spring at many Estonian stations occurred during the 1971–2000 period. In later decades, the trend became considerably more moderate in many locations.

"If the reference period coincides with a phase of rapid change, the resulting trend will be stronger. If it falls within a period when changes are slower or more variable, the trend will be weaker," Saue said.

A comparison of the two most recent climate normals, 1971–2000 and 1991–2020, showed that the trend toward earlier springs has slowed in many parts of Estonia in recent decades. At some weather stations, changes were very small or absent altogether. However, this does not mean that climate change has stopped. Rather, it suggests that changes do not occur at the same pace everywhere or at all times.

The researcher also found that changes in the timing of spring's arrival are neither temporally nor geographically uniform across Estonia. At some stations, the rapid changes observed in the late 20th century were followed by a later slowdown. In other regions, strong changes continued in recent decades or the results were more variable.

No clear nationwide pattern emerged. As a result, changes in the onset of spring cannot be described using a single simple linear trend.

"This shows that a single trend may not tell the whole story and that it is important to examine results across multiple reference periods when assessing the impacts of climate change," Saue said.

Mirror of climate change

The timing of spring's arrival is a phenological indicator, meaning it describes how climate conditions influence the annual development of nature. Saue emphasized that the onset of spring does not replace air temperature measurements and is not necessarily a more reliable indicator. However, it provides different information.

"Temperature directly reflects changes in the climate, while the onset of spring shows how those changes are expressed through the progression of the seasons and the functioning of natural systems. For that reason, the two are complementary indicators rather than competing ones," she explained.

That is precisely why scientists consider the onset of spring to be one of the most sensitive indicators of climate change. It helps researchers understand how changes in temperature are transmitted into the natural environment and affect the annual cycles of plants, animals and entire ecosystems.

The study's main message extends beyond the timing of spring's arrival and speaks more broadly to climate change research. According to the author, studies of phenology and climate change should always clearly specify which climate normal is being used. Whenever possible, results should be compared across multiple reference periods.

Doing so would make assessments of climate change more transparent and make it easier to compare findings across different studies.

"When interpreting a trend, it is important to know the time period on which it is based. That is why it is not enough to look only at the final figure — it is also important to understand how it was derived," Triin Saue said.

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