In situ observation data for 25 lakes (obtained from the Finish Environmental institute [SYKE] and the Swedish Hydrological and Meteorological Institute [SHMI]) were used to validate the satellite-derived phenological dates. There was strong agreement between in situ data and satellite estimates of BUE (R2 = 0.65, RMSE = 6.16 days, MBE = −1.38 days, n = 287). Freeze-up dates (FUE) were also estimated, but the low sun angle during the freeze-up period affected the ability of the satellite sensor to collect adequate data. This resulted in less reliable estimates of freeze-up dates and their exclusion from further analysis (R2 = 0.32, RMSE = 32.8 days, MBE = 16.2 days, n = 274). The earliest BUS occurred at the end of March in the southern part of the northern European study area, and the latest was in mid-June in the northern portion of the northeast Canadian study area (Fig. 1). BUE followed a similar spatial pattern three to four weeks after BUS.

Statistical analysis of the 14-year mean BUS and BUE time series showed a temporal progression to an earlier start of these events across all five sampling areas (Fig. 1). Greater variation in BUE compared to BUS is evident in the Alaskan, northeast Canadian, and central Siberian study areas (as indicated by the error bounds in Fig. 1). The central Siberian area had the strongest rate of change for BUS and BUE (slope −1.05 days/year and −0. 72 days/year respectively), whereas the northern European area had the weakest rate of change for BUS and BUE (slope −0.10 days/year and −0.14 days/year respectively). Except in the northern European study area (51% BUS, 54% BUE), the great majority (76–97% BUS, 69–95% BUE) of lakes showed a negative trend (i.e. earlier dates; Table 1). Since the statistical significance level of individual lake trends varied within study areas, presumably due to variation among lakes in such factors as water volume (see above), the overall trends and significant negative trend (p < 0.05) are discussed separately.

Table 1 Summary of observed changes. Full size table

With the exception of the central Siberian study area, a relatively small proportion of lakes in each area showed a significant trend for the break-up period (e.g., central Siberia BUS, 33%; northeast Siberia BUS, 5%; see Table 1). Whether a trend was recognized as significant or not was strongly dependent on the completeness and length of underlying time series. For over 97% of the lakes, full 14-year BUS and BUE time series were extracted (see Methods). However, the length of record is relatively short, and this largely explains the high number of non-significant trends. In all but the Alaskan and Central Siberian study areas, more lakes showed a significant trend for BUE than for BUS (Table 1). The great majority of identified trends, whether significant or not, are negative, indicating a general pattern of earlier break-up of lake-ice. The number of positive trends (i.e. a later ice-out) corresponds to <0.1% of significant trends for all study areas except Northern Europe, which shows 17% and 29% positive trends for BUS and BUE, respectively, although absolute numbers are low (Table 1). Below, we discuss rates and spatial patterns of change by study area.

Northern European study area

The trend signal is mixed for the Northern Europe study area (Fig. 2). The average overall rate of change observed for all 1,802 lakes included in the study was −0.10 and −0.14 days/year with only very few lakes (BUS 14, BUE 18) showing a significant trend. Lakes with significant negative trends (<1% of lakes) were located predominantly on the northern Kola Peninsula; here, BUS and BUE changed by on average −0.98 days and −0.89 days per year, respectively (Table 1).

Figure 2 The spatial distribution and magnitude of all significant trends of breakup start and end for all study areas. Each dot represents a lake for which statistically significant trend was observed. The colour value represents the magnitude of the trend. This figure was drawn using ESRI ArcMap 10.3 (http://www.esri.com). Full size image

Northeast Canada study area

The overall observed rate of change for 2,994 lakes in Northeast Canada study area was stronger with mean change of −0.31 and −0.34 days per year for BUS and BUE, respectively, than observed in Northern Europe (Table 1). Significant negative trends similar in magnitude to the North European study area were observed for lakes in the glaciated terrain of the Precambrian shield of northeast Canada (Fig. 2). Lakes with significant negative trends (<3% of lakes) show a shift towards an earlier BUS, with a mean rate of −0.93 days/year. The magnitude of change in BUE was even higher with an average change of −1.05 days/year (Table 1, Fig. 2).

Alaskan Arctic Coastal Plain study area

The 1,303 studied lakes on the Alaskan Arctic Coastal Plain show a stronger response then observed in Northeast Canada and Northern Europe, with the shift towards earlier BUS showing a mean rate of −0.60 days/year. The BUE was on average −0.34 days/year earlier (Fig. 2). Over 18% of the lakes show a significant negative trend for BUS with a mean rate −0.94 days/year; ~9% show a mean rate of −0.89 days/year for BUE (Table 1).

Central Siberia study area

The most pronounced change in spring break-up was observed in the central Siberia study area (Fig. 2). The mean observed rate of change for 1234 studied lakes was −1.05 days/year for BUS and −0.72 days/year for BUE. Over 33% of lakes, a much higher portion than in any other study area, showed significant negative trend for BUS with mean rate of −1.40 days/year. Eleven percent showed a significant negative trend for BUE with mean rate of −1.10 days/year (Table 1).

Northeast Siberia study area

For 6,028 studied lakes in Northeast Siberia, the largest study area, the mean observed change for BUS and BUE was −0.34 and −0.37 days/year, respectively. Many lakes (BUS 5%, BUE 8%) show significant negative trends (Fig. 2), with BUS occurring on average −0.86 days/year earlier and BUE −0.95 days/year earlier. A few significant positive trends were observed, distributed randomly, which could indicate error in the extraction process but may alternatively reflect individualistic lake behaviour, as this area has by far the highest lake density and hence number of lakes observed. If extreme and positive values are disregarded, a weak southeast to northwest gradient in the magnitude of the change can be detected, with greatest change in the northwest.

Temperature as a controlling factor for lake ice phenology

We examined a range of temperature variables derived from the ERA-Interim Reanalysis 2-m air temperature dataset at a 0.7-degree spatial resolution20. Of these, the date of the 0 °C isotherm is most strongly related to BUS and BUE. For BUS the strongest relationship was found in the northeast Siberia study area (r = 0.81, p < 0.001) and the weakest for the northern European area (r = 0.63, p < 0.001; Fig. 3). The relationships are slightly weaker for BUE (r = 0.56–0.76, p < 0.001), as the end of break-up is expected to be affected by factors other than temperature alone, such as wind speed, snow cover and rainfall. These factors were excluded from further analyses due to coarse spatial resolution of the available climate data, which required downscaling of BUS/BUE for further analysis and generated noise in the regressions. Further analysis of the temperature-related variables is provided in the supplementary information.