Weak trends in ice phenology of Estonian large lakes despite significant warming trends

We studied changes in air temperature (AT) in Tartu, Estonia, since 1866; ice phenology in two Estonian large lakes since the 1920s; and daily surface water temperatures (SWT) in these lakes since the 1940s. The Mann-Kendall test showed increasing AT trends in all seasons with biggest changes in spring. The strongest increase in SWT occurred in April and August. The temperature increase has accelerated since 1961. Despite significant trends in the seasonal AT and SWT of Estonian large lakes, trends in ice phenology were weak or absent, implying that the processes governing ice phenology are more complex than those governing lake SWT. Greater snowfall was associated with later ice breakup, longer duration of ice cover, and greater ice thickness, while the relationship between winter rainfall and these ice parameters was the opposite. In the deeper Lake Peipsi, ice-on occurred later and ice-off earlier than in the shallower Vrtsjarv. The dates of both ice-on and ice-off responded more sensitively to AT in the case of Peipsi than in the case of Vrtsjarv. An increase of the average November-March AT by 2A degrees C would presumably halve the ice cover duration in Peipsi but shorten it only by about 20% in Vrtsjarv.