When Alaska's Lakes Freeze: Predicting Lake Ice From Satellites and Weather

Large lakes freeze less reliably in winter; modeling the weather helps predict the chances of freezing on lakes of different sizes. Image from the NPS's page.

Why lake ice matters

The seasonal freezing of lakes matters to the ecosystems that support fish, wildlife and people. In Southwest Alaska, lakes are critical habitat and support communities through subsistence fishing, transportation and recreation — but a warming climate is likely to change when, and how reliably, they freeze.

The detailed physical measurements that sophisticated lake-ice models need are rarely available for Alaska's lakes. So scientists Peter Kirchner and Mike Hannam of the Park Service's Southwest Alaska Network took another route.

The method

• Satellites: optical imagery from MODIS, 2002–2016, gave a record of when key lakes in the region froze and thawed.
• Survival analysis: a statistical technique, fed with a few readily available temperature and solar radiation predictors, modeled freeze-over and breakup — allowing for years when a lake didn't freeze at all.
• Back in time: with historical weather data, the model reconstructed ice seasons for 1982–2022.
• Checks: results were validated against observations at two lakes and water-temperature layering measured in three others. Predictions were good — off by 11 days on average for freeze-over and 16 days for breakup.

A graphic summary of the study. NPS figure.

What they found

• Volume matters: a lake's volume shapes its ice. Small lakes freeze regularly and for long periods; large lakes have shorter, more variable ice seasons, and some winters without a complete freeze.
• The best predictors: cumulative freezing degree-days best predicted freeze-over, and cumulative thaw degree-days best predicted breakup.
• Shrinking seasons: over roughly 40 years, the ice season has shortened on most, but not all, lakes. Most lakes under 10 km³ show a trend of 1 to 6 fewer days of ice per decade; most larger lakes show no clear trend or a slight gain of up to 2 days per decade.
• Missed freezes: lakes over 20 km³ more often failed to freeze — 37 times in 200 modeled lake-years, and 19 times in 60 in the satellite record. Three lakes seldom froze in any year, and four more large lakes began having years without freezing in the late 1990s.

What comes next

The approach opens doors. With climate model projections, the future of these lakes' ice seasons can be projected; machine learning might produce ice-season records for many more Alaska lakes, to be analyzed the same way; and other spatial analyses could sharpen predictions for each lake — all helping to prepare for a changing winter.

Not a safety guide: the study covers only whether a lake's surface is ice-covered. It says nothing about ice thickness, so it gives no guidance on safe travel over ice.

Sources

Based on "Lake Ice Phenology: A New Analysis Approach Yields New Insights," Southwest Alaska Inventory and Monitoring Network, National Park Service, summarizing P. B. Kirchner and M. P. Hannam, "Volume-mediated lake-ice phenology in Southwest Alaska revealed through remote sensing and survival analysis," Water 16(16): 2309 (2024); a work of the United States government in the public domain.