The physical characteristics of ice formation in water courses are ideal for studying climatic variation in space and time. We used a large set of observations for 9-116 years (1890-2019) of the timing of freeze-up and break-up, and the length of ice-free season for 99 Norwegian lakes to elucidate variation in ice phenology across space and time. The dataset of Norwegian lakes is unusual, covering considerable variation in altitude (4 – 1401 m above sea level) and climate (varying from oceanic to continental) within a large latitudinal and longitudinal gradient (58.2 – 69.9 °N; 4.9 – 30.2 °E).

The average date of ice break-up occurred later in spring with increasing altitude and latitude, but earlier in spring with increasing longitude. The average date of freeze-up and the length of the ice-free period decreased significantly with altitude and longitude. No correlation with distance from the ocean was detected. The geographical gradients are related to regional climate due to adiabatic processes (altitude), solar radian (latitude) and the degree of continentality (longitude).  There was a significant lake area effect as small lakes froze-up earlier due to less volume. There was also a significant trend that lakes were completely frozen over later in the autumn in recent years. After accounting for the effect of long-term trends in the large-scale NAO index, a significant but weak trend over time for earlier ice break-up was detected. An understanding of the relationship between ice phenology and geographical parameters is a prerequisite for predicting the potential consequences of climate change on ice phenology. Changes in ice phenology will have consequences for the behaviour and life cycle dynamics of the aquatic biota.

We collated observations from 101 Norwegian lakes, covering a wide range in latitude (58.2 – 69.9 °N), longitude (4.9 – 30.2 °E) and altitude (4 – 1401 m a.s.l.). The lakes are situated in three major climatic zones (boreal, subalpine, alpine) and with varying distances from the ocean. Thus, they differ in several geographic characteristics (Figure 1, Appendix 1). Most of the lakes are relatively small (median area 6.9 km²), although the dataset also includes Norway’s largest lake, Mjøsa (369.3 km²). The catchment area varies between 7.1 and 18101.9 km² (median 235 km²) and mean annual inflow to the lakes varies between 5.6 10⁶ and 9935.7 10⁶ m³ year^-1 (median 256 10⁶ m³ year^-1). About 50 % of the lakes (N = 53) were developed for hydropower production with an annual water level variation varying from 1 to 30.3 m. The lake and catchment information were extracted from www.nve.no.  

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