Late winter freeze-thaw cycles

March 20, 2025

Late winter freeze-thaw cycles

On March 5th, 2025, Northfield accumulated over 12 inches of snow overnight. Before the blizzard, temperatures were reaching above 40 ^oF and there was no snow on the ground for several weeks. After the blizzard, campus was warm during the day and below freezing at night, melting 12 inches of snow in only a few days. Have you ever wondered how these freezing and thawing conditions impact the environment?

In 2023, there were 17 more unfrozen days than the long-term average (1979 – 2023) in the contiguous 48 states (U.S Environmental Protection Agency 2025). The number of unfrozen days in a winter is increasing every year due to climate change. In Minnesota, days are 1 – 2 ^oF warmer and nights are 2 – 5 ^oF warmer (Mahedi et al. 2024).

(U.S. Environmental Protection Agency 2025)

This map of the upper portion of North America shows whether the annual number of unfrozen days increased or decreased from 1979 to 2023. Minnesota winters are getting warmer.

Winters and cold temperatures, as harsh as they may feel, are critical for many organisms that are adapted to the freezing conditions. Plants go through a process called acclimation in late summer that is triggered by longer nights. Acclimation involves physiological changes that prepare the plant for a winter dormancy. Dormancy is overcome by prolonged periods of above-freezing temperatures in the spring, which signals for a plant to begin growing (Weisenhorn et al. 2024). If these prolonged above-freezing temperatures occur too early, plants can exit dormancy before spring, and can be damaged by later periods of cold weather.

Not only do frequent thaws threaten trees and other plants, but they affect all levels of the ecosystem. Warmer temperatures lead to less snow. Snow over the soil or a layer of ice over a lake acts as a blanket throughout the winter . This layer helps insulate the roots of plants and hibernating animals. Without any insulation, when another freeze cycle comes through, many plants and animals are left vulnerable. Populations of insects may be affected by early thaws too. For example, tick populations are controlled by freezing temperatures, and more unfrozen days could lead to more ticks in the following summer (U.S. Environmental Protection Agency 2025). Freeze-thaw cycles have been found to cause substantial nitrogen and carbon release, leaving less of these vital materials in the soil when plants need to grow in the spring and summer (Ding et al. 2023). They also heavily influence the watersheds in Minnesota, regulating how water is moving throughout the soil and all of the bodies of water (Wang et al. 2020).

Freeze-thaw cycles are an important ecological process, and are gaining more attention as their frequency changes with climate change. When you are outside as winter transitions into spring, pay attention to what you observe. Can you find buds on the trees? Are you seeing any insects? Is this earlier than you may expect? How will this be different next year? Observations like these are important to be able to track and understand how our world is changing.

References

Ding, S., Zou, Y., and Yu, X. 2023. Freeze-thaw cycles alter the growth sprouting strategy of wetland plants by promoting denitrification. Communications Earth & Environment, 4(1), 57.

Mahedi, M., Rajewski, D., Ceylan, H., Kim, S., Takle, E. S., and Cho, I. H. 2024. Have climate change and warmer winters altered freeze-thaw patterns?. Transportation Geotechnics, 46, 101250.

U.S. Environmental Protection Agency. 2025. Climate Change Indicators: Freeze-Thaw Conditions. U.S. Environmental Protection Agency. https://www.epa.gov/climate-indicators/climate-change-indicators-freeze-thaw-conditions.

Wang, Q., Qi, J., Wu, H., Zeng, Y., Shui, W., Zeng, J., and Zhang, X. 2020. Freeze-Thaw cycle representation alters response of watershed hydrology to future climate change. Catena, 195, 104767.

Weisenhorn J., Miller B., Trappe J., Schuh M., and Wimmer M. 2024. How will this warm winter affect my plants?. University of Minnesota Extension.