Lake ice seasonality over the past 320-570 years

Lake and river ice seasonality (dates of ice freeze and breakup) responds sensitively
to climatic change and variability. We analyzed climate-related changes using direct human
observations of ice freeze dates (1443-2014) for Lake Suwa, Japan, and of ice breakup
dates (1693-2013) for Torne River, Finland. We found a rich array of changes in ice
seasonality of two inland waters from geographically distant regions: namely a shift towards
later ice formation for Suwa and earlier spring melt for Torne, increasing frequencies of
years with warm extremes, changing inter-annual variability, waning of dominant
inter-decadal quasi-periodic dynamics, and stronger correlations of ice seasonality with
atmospheric CO2 concentration and air temperature after the start of the Industrial
Revolution. Although local factors, including human population growth, land use change, and
water management influence Suwa and Torne, the general patterns of ice seasonality are
similar for both systems, suggesting that global processes including climate change and
variability are driving the long-term changes in ice seasonality.