Annual, seasonal and monthly variations in precipitation during the period 1911-2011 were evaluated using spatially interpolated monthly precipitation records at 165 stations with more than 30 years of data (of which 32 stations had 101 years of data referring to full length of study period). Historical trends and cyclic patterns of precipitation and links to teleconnection indices were analysed. The results show that annual precipitation in Finland increased by 0.92 ± 0.50 mm year −1 (p < 0.05) during the study period , and showed negative relationships with the East Atlantic/West Russia (EA/WR) teleconnection pattern (ρ = −0.41, p < 0.05) through the years 1950-2011. However, there were spatial (north-south/east-west) differences between regions. The time cycle (λ) for annual precipitation cycles was estimated to 149 years (R 2 =0.15), and 1964 was indicated as the turning point from a negative (dry) to a positive (wet) phase. Analysis of seasonal and monthly cycles showed larger variations, depending mainly on teleconnection effects. Analysis of seasonal precipitation determined increasing trends for winter (by 0.46 ± 0.19 mm year −1 ) and summer (by 0.32 ± 0.29 mm year −1 ), while no clear trend was found for spring and autumn precipitation (p > 0.05). Winter precipitation was most strongly correlated with the North Atlantic Oscillation (NAO) index (ρ = 0.55, p < 0.05), while the EA/WR pattern was the most significant teleconnection index for precipitation variations in spring (ρ = −0.33, p < 0.05) and summer (ρ = −0.49, p < 0.05); the precipitation variability during autumn was negatively associated with the Scandinavia (SCA) pattern (ρ = −0.40, p < 0.05). The results clearly indicate that Finland has experienced wetter climate than normal conditions during recent decades, and precipitation in the Fenno-Scandinavian climate is controlled by a number of climate teleconnection indices, not just the NAO as previously assumed.

A before-after-control approach was used to analyze the impact of peatland restoration on hydrology, based on high temporal resolution water-table (WT) data from 43 boreal peatlands representative of a south-boreal to north-boreal climate gradient. During the study, 24 forestry drained sites were restored and 19 pristine peatlands used as control sites. Different approaches were developed and used to analyze WT changes (mean WT position, WT fluctuation, WT hydrograph, recession, and storage characteristics). Restoration increased WT in most cases but particularly in spruce mires, followed by pine mires and fens. Before restoration, the WT fluctuation (WTF) was large, indicating peat temporary storage gain (SG). After restoration, the WT hydrograph recession limb slopes and SG coefficients (Rc) declined significantly. Drainage or restoration did not significantly affect mean diurnal WT fluctuations, used here as a proxy for evapotranspiration. Overall, the changes in WT characteristics following restoration indicated creation of favorable hydrological conditions for recovery of functional peatland ecosystems in previously degraded peatland sites. This was supported by calculation of bryophyte species abundance thresholds for WT. These results can be used to optimize restoration efforts in different peatland systems and as a qualitative conceptual basis for future restoration operations.

Annual and seasonal variations in surface air temperature (SAT) during the period 1961-2011 were analysed using daily mean temperature data sets from regular grid points (10 × 10 km 2 ) throughout Finland. The Mann-Kendall nonparametric test was used to detect significant historical trends in SAT and Spearman's correlation coefficient ( ) to test the relationships between SAT patterns and various atmospheric circulation patterns over the northern hemisphere. The results showed that mean annual SAT in Finland increased (p < 0.05) by 0.4 ± 0.2 ∘ C per decade during the study period and that the SAT was significantly ( = 0.58, p < 0.05) positively correlated with the Arctic Oscillation (AO) index. However, there were spatial differences within Finland for both the trends and relationships with the atmospheric circulation. Analysis of seasonal mean SAT identified significant (p < 0.05) warming trends for both spring (by 0.4 ± 0.2 ∘ C per decade) and summer (by 0.3 ± 0.2 ∘ C per decade). Winter and spring mean SATs were most strongly (p < 0.05) correlated with the AO index ( = 0.72 and 0.42, respectively), while the most significant teleconnection pattern for mean SAT in summer was the East Atlantic (EA) pattern ( = 0.43, p < 0.05); and in autumn the EA/West Russia (WR) pattern ( = −0.59, p < 0.05). These results provide a detailed spatial picture of climate warming in Finland in recent decades and reveal that interannual variation of the SAT in Finland is closely linked with a number of atmospheric circulation patterns, not just the AO and North Atlantic Oscillation (NAO). Annual and cold-season SAT are mainly influenced by the AO and NAO, whereas the EA, EA/WR, Scandinavia (SCA) and West Pacific (WP) patterns play an important role for warm-season SAT.

Urban pluvial flooding now occurs more frequently than it has in past decades, mainly due to an increasing number of extreme precipitation events occurring in the context of a changing climate. To limit the evolving risks of urban pluvial flooding in a more environmentally friendly manner, the research community has recently paid increasing attention to Nature-Based Solutions (NBS), which are based on new green technologies. To meet the urgent demand for a comprehensive review of the most recent literature, this review conducts a systematic survey of the literature to characterize various NBS adopted in different regions of the world and to elaborate on the benefits and limitations of such NBS. The review highlights the role of NBS in urban flood risk management under ongoing climate change and rapid urbanization. It shows that NBS could effectively mitigate urban flooding caused by high-frequency precipitation events, with additional economic, ecological, and social benefits. However, NBS are less effective at helping cope with pluvial flooding caused by extreme precipitation events over a short period of time, while gray infrastructures also have limitations as a mitigation measure against extreme pluvial flooding. We thus recommend identifying flood risk management strategies by evaluating the performance of alternative combinations of NBS with gray infrastructures in preventing pluvial flooding in the cities. Finally, recent advances made in the applications of NBS are presented with suggestions (e.g., long-term monitoring) to improve urban flood adaptive management. This article is categorized under: Engineering Water > Planning Water Engineering Water > Sustainable Engineering of Water Science of Water > Water Extremes K E Y W O R D S extreme precipitation, nature-based solution, risk management, urban pluvial flood Yijing Huang and Zhan Tian equally contributed to this study.[Correction added on 27 April 2020, after first online publication: the unit of second affiliation has been updated and funding information has been reordered.]

Using daily mean temperature datasets applied at regular grid points (10 × 10 km 2 ) throughout Finland for the period 1961-2011, interannual variations and trends in thermal growing season (GS) parameters, in terms of start (GSS), end (GSE) and length (GSL) and their relationships with various atmospheric teleconnection patterns (ATPs) were analysed. The GSL in Finland naturally increases from north to south, in association with earlier GSS and later GSE. However, the results showed that during the last 50 years, GSL on national scale in Finland significantly (p < 0.05) increased, at a rate of 0.30 ± 0.18 days year -1 . This change was mainly attributable to earlier GSS (0.16 ± 0.13 days year -1 , p < 0.05). The East Atlantic/West Russia (EA/WR) pattern was the most significant ATP influencing variations in GSS ( = 0.33), GSE ( = −0.42) and GSL ( = −0.33) on national scale in Finland. Statistically significant (p < 0.05) trends in GSL were all positive (increasing) and mainly observed in the centre (Oulu), upper east (Kuopio) and west (Jyväskylä), north (Rovaniemi-Sodankylä) and south-west coastal areas (Helsinki-Turku-Vaasa) of Finland. Increasing GSL in central, upper eastern and western Finland was mainly associated with significantly earlier GSS; that in the north with significantly later GSE; and that in the most south-westerly areas with both earlier GSS and later GSE. These increases in GSL and corresponding shifts in GSS and/or GSE were significantly correlated with the East Atlantic (EA) pattern in northern and central Finland; with the EA/WR pattern in the south, south-east and south-west; and with the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) in south-west coastal areas. The results provide a detailed national and spatial picture of changes in GS parameters in Finland during the last 50 years and reveal that the interannual variability in GS parameters in Finland is strongly linked with a number of ATPs, such as the EA, EA/WR, NAO and AO.