Spring arrives earlier in Estonia over past half-century, study shows
A study based on data from 21 Estonian weather stations (1965–2020) finds that spring onset has shifted by one to three weeks depending on region, but the magnitude of change depends heavily on the chosen climate normal reference period.
Spring in Estonia has been arriving earlier over the last 55 years, but the extent of the shift varies significantly depending on the reference period used, according to a study by Triin Saue, a researcher at the Center of Estonian Rural Research and Knowledge. Saue analyzed daily mean air temperature data from 21 weather stations covering 1965–2020. She defined the start of spring as the date when the daily average temperature permanently exceeds 5°C.
At most stations, spring now begins one to three weeks earlier than in the past. The largest changes were observed at inland stations in southern and central Estonia, including Tartu-Tõravere, Võru, Valga, and Viljandi, where the shift exceeded two weeks. Coastal stations also showed changes, but no simple geographic pattern explains the variation across the country.
Climate normals matter
The study highlights how the choice of climate normal—a 30-year reference period—affects results. Saue compared four overlapping normals: 1965–1990, 1971–2000, 1981–2010, and 1991–2020. Using the same dataset, the entire 1965–2020 period shows a strong shift, while the most recent normal (1991–2020) indicates much weaker or no change. Differences between normals at some stations exceeded 10 days. The strongest shift occurred during 1971–2000; later decades saw a slowdown in many locations.
No single trend fits all
Saue concluded that a single linear trend cannot describe changes in spring onset across Estonia. Spring onset is a phenological indicator complementary to temperature measurements. The study's key message is that climate research should always specify the reference period and compare results across multiple normals to increase transparency and comparability.
