Lake ice phenology (timing of ice breakup and freeze up) is a sensitive indicator of climate. We acquired time series of lake ice breakup and freeze up, local weather conditions, and large-scale climate oscillations from 1981-2015 for seven lakes in northern Wisconsin, USA, and two lakes in Ontario, Canada. Multiple linear regression models were developed to understand the drivers of lake ice phenology. We used projected air temperature and precipitation from 126 climate change scenarios to forecast the day of year of ice breakup and freeze up in 2050 and 2070. Lake ice melted 5 days earlier and froze 8 days later over the past 35 years. Warmer spring and winter air temperatures contributed to earlier ice breakup; whereas warmer November temperatures delayed lake freeze. Lake ice breakup is projected to be 13 days earlier on average by 2070, but could vary by 3 days later to 43 days earlier depending upon the degree of climatic warming by late century. Similarly, the timing of lake freeze up is projected to be delayed by 11 days on average by 2070, but could be 1 to 28 days later. Shortened seasonality of ice cover by 24 days could increase risk of algal blooms, reduce habitat for coldwater fisheries, and jeopardize survival of northern communities reliant on ice roads.

The duration of seasonal winter ice cover has declined in many mid‐ and high‐latitude regions around the world as climate continues to warm. We obtained data on lake ice breakup dates, air temperature, precipitation, and large‐scale climate oscillations for 152 lakes across the northern hemisphere from 1951 to 2014. Ninety‐seven percent of study lakes exhibited earlier ice breakup trends. Forty‐six percent of the variation in ice breakup trends was driven by spring air temperatures and elevation across the northern hemisphere. However, changes in ice breakup have not always been in a gradual or linear pattern. Using the sequential T‐test analysis of regime shifts, we found evidence of abrupt changes in mean ice breakup for 53% of lakes with shift years identified between 1970 and 2002. Concurrently, we found abrupt changes in mean spring and winter air temperatures, winter precipitation, and large‐scale climate oscillations that occurred either the same year or 1 yr prior. Earlier ice breakup and the shortening of the ice season will have consequences for winter heritage, local economies, and lake ecosystems around the world.

No abstract