Hilppa Gregow

Plant phenological records in northern Finland since the 18th century as retrieved from databases, archives and diaries for biometeorological research

Plant phenological data from northern Finland, compiled from several sources, were examined as potential biometeorological indicators of climate change since the 18th century. A common feature of individual series was their sporadic nature. In addition to waning enthusiasm, wartime hardships and crop failures had caused gaps in recording observations during the 18th and 19th centuries. The present study’s challenge was to combine separate records, as retrieved from several historical archives and personal diaries, into a single continuous series. To avoid possible biases due to the variability of data availability each year, each phenomenon-specific mean series was transformed into normalized site-specific index series. These series were compared to each other and to a regional instrumental temperature series (years 1802–2011). The inter-phenomena correlations were high. Moreover, a strong biometeorological response of the phenological series, most especially to monthly mean temperature in May, and seasonally to the April through June temperatures, was identified. This response focused on slightly later spring months compared to the responses in an earlier study conducted for southern Finland. The findings encouraged us to compute a total phenological index series as an average of all available phenomenon-specific index series for northern Finland. The earliest phenological springs were found as a cluster in the recent end of the record, whereas the anomalously-late phenological spring could be found through the centuries. This finding could indicate that potential future warming could result in an earlier onset of phenological springs (i.e. as experienced by the plants), with a remaining possibility of late phenological springs. To conclude, it was shown that the indices are reliable biometeorological indicators of the April through June temperature variations and thus of the climate variability in the region.

Skilful seasonal predictions of Baltic Sea ice cover

The interannual variability in the Baltic Sea ice cover is strongly influenced by large scale atmospheric circulation. Recent progress in forecasting of the winter North Atlantic Oscillation (NAO) provides the possibility of skilful seasonal predictions of Baltic Sea ice conditions. In this paper we use a state-of-the-art forecast system to assess the predictability of the Baltic Sea annual maximum ice extent (MIE). We find a useful level of skill in retrospective forecasts initialized as early as the beginning of November. The forecast system can explain as much as 30% of the observed variability in MIE over the period 1993–2012. This skill is derived from the predictability of the NAO by using statistical relationships between the NAO and MIE in observations, while explicit simulations of sea ice have a less predictive skill. This result supports the idea that the NAO represents the main source of seasonal predictability for Northern Europe.

Multimodel estimates of the changes in the Baltic Sea ice cover during the present century

We project changes in the annual maximum ice extent and the maximum coastal fast ice thickness in the Baltic Sea during the ongoing century. The influence of future warming on the ice conditions was assessed using the November–March Baltic coastal mean temperature as a predictor for the annual maximum ice extent (MIB), and the local freezing degree-day sum as a predictor for the fast ice thickness. Future winter temperatures were derived by adjusting observational baseline-period temperatures in accordance with temperature projections based on 28 global climate models (GCMs) participating in the Coupled Model Intercomparison Project Phase 5. Under the Representative Concentration Pathway (RCP) 4.5 scenario, the ensemble-mean trend of MIB is −6400 km2/10 yr, and from the 2060s onwards in a typical winter MIB remains below 80×103 km2. If the RCP8.5 scenario is realised, the corresponding estimates are −10 900 km2/10 yr for the trend and 60×103 km2 for a typical MIB. For cold rather than typical winters, the projected rate of decrease in MIB is even faster. During the late century under RCP8.5, in 9 out of 10 yr the ice would only cover 5–20% of the total sea area. The projected trends in the mean annual maximum ice thickness are −7.6 … −3.3 cm/10 yr, depending on location and applied scenario. In the 2040s under both scenarios, and in the 2080s under RCP4.5, the ice thickness may still exceed 60 cm in the northernmost Bay of Bothnia, while elsewhere in the Gulf of Bothnia and in the Gulf of Finland, it will vary between about 10 and 40 cm. In the 2080s under RCP8.5, virtually no ice occurs outside the Bay of Bothnia. For both the ice extent and thickness, the spread among the responses based on the temperature projections of individual GCMs is considerable. Nonetheless, a robust finding is that the Baltic Sea is unlikely to become totally ice-free during this century.

Phenological patterns of flowering across biogeographical regions of Europe

Long-term changes of plant phenological phases determined by complex interactions of environmental factors are in the focus of recent climate impact research. There is a lack of studies on the comparison of biogeographical regions in Europe in terms of plant responses to climate. We examined the flowering phenology of plant species to identify the spatio-temporal patterns in their responses to environmental variables over the period 1970–2010. Data were collected from 12 countries along a 3000-km-long, North–South transect from northern to eastern Central Europe.Biogeographical regions of Europe were covered from Finland to Macedonia. Robust statistical methods were used to determine the most influential factors driving the changes of the beginning of flowering dates. Significant species-specific advancements in plant flowering onsets within the Continental (3 to 8.3 days), Alpine (2 to 3.8 days) and by highest magnitude in the Boreal biogeographical regions (2.2 to 9.6 days per decades) were found, while less pronounced responses were detected in the Pannonian and Mediterranean regions. While most of the other studies only use mean temperature in the models, we show that also the distribution of minimum and maximum temperatures are reasonable to consider as explanatory variable. Not just local (e.g. temperature) but large scale (e.g. North Atlantic Oscillation) climate factors, as well as altitude and latitude play significant role in the timing of flowering across biogeographical regions of Europe. Our analysis gave evidences that species show a delay in the timing of flowering with an increase in latitude (between the geographical coordinates of 40.9 and 67.9), and an advance with changing climate. The woody species (black locust and small-leaved lime) showed stronger advancements in their timing of flowering than the herbaceous species (dandelion, lily of the valley). In later decades (1991–2010), more pronounced phenological change was detected than during the earlier years (1970–1990), which indicates the increased influence of human induced higher spring temperatures in the late twentieth century.