Long‐term lake ice phenological records from around the Northern Hemisphere provide unique sensitive indicators of climatic variations, even prior to the existence of physical meteorological measurement stations. Here, we updated ice phenology records for 60 lakes with time‐series ranging from 107–204 years to provide the first re‐assessment of Northern Hemispheric ice trends since 2004 by adding 15 additional years of ice phenology records and 40 lakes to our study. We found that, on average, ice‐on was 11.0 days later, ice‐off was 6.8 days earlier, and ice duration was 17.0 days shorter per century over the entire record for each lake. Trends in ice‐on and ice duration were six times faster in the last 25‐year period (1992–2016) than previous quarter centuries. More extreme events in recent decades, including late ice‐on, early ice‐off, shorter periods of ice cover, or no ice cover at all, contribute to the increasing rate of lake ice loss. Reductions in greenhouse gas emissions could limit increases in air temperature and abate losses in lake ice cover that would subsequently limit ecological, cultural, and socioeconomic consequences, such as increased evaporation rates, warmer water temperatures, degraded water quality, and the formation of toxic algal blooms.

Lake ice phenology has been recorded for decades, providing us with long‐term records to investigate the impact of climate change in lakes since the Industrial Revolution. Here, we examine the trends and drivers of 18 lakes across the Northern Hemisphere, with 156–204 years of data, starting in the 1810s. We show that: (a) trends in ice phenology are faster than found by previous studies. Ice‐on is 11 days later per century, ice‐off is 9 days earlier per century, and ice cover duration is 19 days shorter per century; (b) there are significant breakpoints in the 1850s, 1870s, mid‐1890s, and mid‐1990s, after which trends in ice phenology are even faster, and associated with changing weather and climate; and (c) local air temperatures explain the most variation in ice phenology, on average 36.5%, followed by progressive climate change explaining around 17.5% on average, with teleconnection patterns explaining the least variation. Our findings support the assertion that broad‐scale climatic changes have led to more rapid lake ice loss in lakes distributed across the Northern Hemisphere, with potential widespread impacts on critical ecosystem services that lake ice provides.

Lake ice cover is essential to conserving the global freshwater supply for the 50 million lakes that freeze each winter. Here, we ask when lakes across the Northern Hemisphere may permanently lose ice cover. A K‐means cluster analysis from 31 lakes identified four clusters of lakes vulnerable to losing ice cover, including shallow and deep lakes in regions where winter air temperatures hover ∼0 °C and larger and deeper lakes in colder regions. By the end of this century, we estimate that up to 5,679 lakes of 1.35 million HydroLAKES may permanently lose ice cover if greenhouse gas emissions (GHG) continue to be emitted at current levels. In the Northern Hemisphere, lakes in southern and coastal regions, some of which are among the largest lakes in the world and in close proximity to large human populations, are the most vulnerable to permanently losing ice.

Aim: Many freshwater fishes are migrating poleward to more thermally suitable habitats in response to warming climates. In this study, we aimed to identify which freshwater fishes are most sensitive to climatic changes and asked: (i) how fast are lakes warming? (ii) how fast are fishes moving? and (iii) are freshwater fishes tracking climate?Location: Ontario, Canada.Methods: We assembled a database containing time series data on climate and species occurrence data from 10,732 lakes between 1986 and 2017. We calculated the rate of lake warming and climate velocity for these lakes. Climate velocities were compared with biotic velocities, specifically the rate at which the northernmost extent of each species shifted north.Results: Lakes in Ontario warmed by 0.2°C decade⁻¹ on average, at a climate velocity of 9.4 km decade⁻¹ between 1986 and 2017. In response, some freshwater fishes have shifted their northern range boundaries with considerable interspecific variation ranging from species moving southwards at a rate of −58.9 km decade⁻¹ to species ranges moving northwards at a rate of 83.6 km decade⁻¹ over the same time period. More freshwater fish species are moving into northern lakes in Ontario than those being lost. Generally, predators are moving their range edges northwards, whereas prey fishes are being lost from northern lakes.Main Conclusions: The concurrent loss of cooler refugia, combined with antagonistic competitive and predatory interactions with the range expanding species, has resulted in many commercially important predators moving their range edges northwards, whereas prey species have contracted their northern range edge boundaries. Trophic partitioning of range shifts highlights a previously undocumented observation of the loss of freshwater fishes from lower trophic levels in response to climate‐driven migrations.

Monitoring the timing of ice-on and ice-off has been instrumental in estimating the long-term effects of climate change on freshwater lakes and rivers. However, ice thickness has been studied less intensively, both spatially and temporally. Here, we quantified the trends and drivers of ice thickness from 27 lakes and rivers across North America. We found that ice thickness declined on average by 1.2 cm per decade, although ice thickness declined significantly in only four waterbodies. Local winter air temperature, cloud cover, and winter precipitation were the most important determinants of ice thickness, explaining over 81% of the variation in ice thickness. Ice thickness was lower in years and regions with higher air temperatures, high percentage of cloud cover, and high winter precipitation. Our results suggest that warming is contributing to thinning ice, particularly at high latitudes, with potential ramifications to the safety of humans and wildlife populations using freshwater ice for travel and recreation.