Freeze and breakup dates of ice on lakes and rivers provide consistent evidence of later freezing and earlier breakup around the Northern Hemisphere from 1846 to 1995. Over these 150 years, changes in freeze dates averaged 5.8 days per 100 years later, and changes in breakup dates averaged 6.5 days per 100 years earlier; these translate to increasing air temperatures of about 1.2 degrees C per 100 years. Interannual variability in both freeze and breakup dates has increased since 1950. A few longer time series reveal reduced ice cover (a warming trend) beginning as early as the 16th century, with increasing rates of change after about 1850.

Often extreme events, more than changes in mean conditions, have the greatest impact on the environment and human well-being. Here we examine changes in the occurrence of extremes in the timing of the annual formation and disappearance of lake ice in the Northern Hemisphere. Both changes in the mean condition and in variability around the mean condition can alter the probability of extreme events. Using long-term ice phenology data covering two periods 1855-6 to 2004-5 and 1905-6 to 2004-5 for a total of 75 lakes, we examined patterns in long-term trends and variability in the context of understanding the occurrence of extreme events. We also examined patterns in trends for a 30-year subset (1975-6 to 2004-5) of the 100-year data set. Trends for ice variables in the recent 30-year period were steeper than those in the 100-and 150-year periods, and trends in the 150-year period were steeper than in the 100-year period. Ranges of rates of change (days per decade) among time periods based on linear regression were 0.3−1.6 later for freeze, 0.5−1.9 earlier for breakup, and 0.7−4.3 shorter for duration. Mostly, standard deviation did not change, or it decreased in the 150-year and 100-year periods. During the recent 50-year period, standard deviation calculated in 10-year windows increased for all ice measures. For the 150-year and 100-year periods changes in the mean ice dates rather than changes in variability most strongly influenced the significant increases in the frequency of extreme lake ice events associated with warmer conditions and decreases in the frequency of extreme events associated with cooler conditions.

We examined spatial patterns of trends in ice phenology and duration for 65 waterbodies across the Great Lakes region (Minnesota, Wisconsin, Michigan, Ontario, and New York) during a recent period of rapid climate warming . Average rates of change in freeze (3.3 d decade 21 ) and breakup (22.1 d decade 21 ) dates were 5.8 and 3.3 times more rapid, respectively, than historical rates for Northern Hemisphere waterbodies. Average ice duration decreased by 5.3 d decade 21 . Over the same time period, average fall through spring temperatures in this region increased by 0.7uC decade 21 , while the average number of days with snow decreased by 5.0 d decade 21 , and the average snow depth on those days decreased by 1.7 cm decade 21 . Breakup date and ice duration trends varied over the study area, with faster changes occurring in the southwest. Trends for each site were compared to static waterbody characteristics and meteorological variables and their trends. The trend toward later freeze date was stronger in large, low-elevation waterbodies; however, freeze date trends had no geographic patterns or relationships to meteorological variables. Variability in the strength of trends toward earlier breakup was partially explained by spatial differences in the rate of change in the number of days with snow cover, mean snow depth, air temperature (warmer locations showed stronger trends), and rate of change in air temperature. Differences in ice duration trends were explained best by a combination of elevation and the local rate of change in either temperature or the number of days with snow cover.

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The meromictic Sophia and Garrow lakes are probably saline relicts of cutoff fjords on the uplifted Cornwallis and Little Cornwallis islands in the High Arctic of the Canadian Archipelago. Sophia and Garrow have brackish (S = 2–4) upper and hypersaline (S = 55–90) lower waters with ion ratios (especially the lower waters) similar to that of the sea. Substantial oxygen extends well below the chemocline in Sophia. The stability of these lakes is among the highest known. Because of their size, depth, and unusually warm lower waters, it seems likely that an unfrozen "thermal chimney" extends beneath the lakes through the surrounding permafrost. We feel that the hypersaline gradients evolved primarily by descent of "salt fingers" during freeze out from above, and not by solute rejection from the ground during uplift and permafrost growth, or through taliks connected to the sea, as proposed by Page et al. (1984. Limnol. Oceanogr. 29: 564–573).