Changes in winter

Key takeaways

• The Northeast U.S. (NEUS) winters have warmed faster than summers.
• There have been increases in days above freezing, decreases in days with snow cover, and fewer nights below freezing.
• Deep snowpacks are becoming increasingly ephemeral, decreasing from a historical baseline.
• Winter has been getting shorter, as the timing of fall has shifted later, and spring earlier.
• Climate projections indicate that winters will continue to warm and lose snow in the future, but uncertainty remains regarding the magnitude of warming.

Recent research has shown that climate changes in winter, such as soil freezing and snow cover, are having strong and often surprising impacts on species and ecosystems in seasonally snow-covered areas such as high-elevation and alpine habitats. Changes in winter are impacting ecosystem structure and function with important consequences for the carbon cycle (carbon sequestration, decomposition, and export), which influences production in agricultural and forest habitats.

Winter Temperatures

The cold season in NEUS is warming at a faster rate than the warm season. Over the past 100 years, minimum winter temperatures have increased 0.16 °C/decade, with commensurate reductions in snowpack duration and frequency of extreme cold conditions, as well as an increased proportion of winter precipitation falling as rain instead of snow. Winter heat waves have become more common. Average air temperatures in New England have shown the greatest increases during the winter season; over the last half-century, winter temperatures have risen by more than 3 °F (1.5°C). This trend is projected to continue with winter temperatures rising. The winter season has also been getting shorter over past decades, as the timing of fall has shifted later, and spring earlier - each by about a week or more.
Extreme cold winter temperatures in the NEUS have been observed in recent decades and are thought to be the result of rapid warming in the Arctic, which influences the strength and meandering of the jet stream. These unusual cold temperatures were the exception as the rest of the world experienced some of the highest temperatures on record. Studies of these extreme temperature events are an emerging area of climate science. Recent research suggests it is likely that North America will experience additional extreme winter temperatures, though they are expected to vary in intensity and frequency over time. Increases in the amplitude of the jet stream in winter may also explain the observed increases in winter storms affecting the Northeast United States.

Winter Precipitation and Snowpack

Annual winter precipitation has been increasing; however, future projections of precipitation are generally less certain than temperature. Projections for the Greater New England region consistently predict wetter winters, with more precipitation falling as rain than snow. Warmer temperatures, fewer annual snowfall events, and more precipitation falling as rain have already led to a fewer number of days with snow on the ground, and are expected to lead to less annual snowfall totals and snowpack depths; however, projections of more intense snowfall events suggest local increases in snow totals may occur, including in high elevation habitats. Snowpacks are thinning in the NEUS, melting earlier, and have shown shorter durations. Snowlines on mountains also show signs of retreating upslope. Climate projections for the 21st century suggest a continuation of observed trends of decreased snow depth and duration of snow cover due to warming and the advancement of spring. Snowpack quality and characteristics, such as texture (e.g., more ice-like, harder and crustier snow), may change in the Greater New England due to increased freezing and thawing over the winter season as well as more rain and freezing rain events.
Snow acts as an important soil insulator and decreased snowpacks and ground cover during winter can result in colder soils and increase soil freezing depths. This phenomenon has been shown to lead to increased root mortality, decreases in soil decomposition, increased nitrogen, as well as other changes in soil chemistry. There is still high uncertainty concerning the interactive effects of changes in snow insulation and air temperatures and how this will impact fish and wildlife species and habitats in Massachusetts as well as the Greater New England region.

Winter storms

Winter storms are projected to have an increase in winter precipitation and in the future will result in more rainfall and less snowfall due to atmospheric warming. This leads to model projections indicating a decrease in snowfall frequency in the NEUS in the future (Zarzycki, 2018). This, however, does not discount the likelihood of individual high-impact snowfall events. In fact, during sufficiently cold conditions, snowstorms in the future can drop more snow because of the ability of the warmer atmosphere to hold more moisture (Zarzycki, 2018). In fact, the frequency of heavy snowfall in the NEUS has increased over the past three decades (Whitehead et al., 2023), which is likely caused by interactions between warming in the western Atlantic Ocean and frequent Arctic air outbreaks (Cohen et al., 2018). Higher winter warming will result in an overall increase in days and nights with temperatures above freezing, leading to decreases in snow cover as well as snow depth (Burkowski et al., 2022). These changes are likely to impact coastal and southern regions of the NEUS first with high elevation regions remaining more resilient (Burkowski et al., 2022). A projected increase in winter precipitation in the form of rainfall is also projected to increase surface runoff and peak river flows in winter in the future (Siddique and Palmer, 2021; Siddique et al., 2020).

Snowmelt and Hydrology

Earlier winter-spring peak stream and river flows (in the range of 7-10 days) have been observed in the Northeast and are thought to be linked to earlier snowmelt and increased rain-on-snow episodes. This trend is projected to continue during the 21st century. A shift toward higher winter flows and lower spring and summer flows has been documented in the Connecticut River Watershed using climate-driven streamflow simulations. Changes in the timing and the magnitude of spring snowmelt in the eastern U.S. are crucial to maintain ecosystem functions since some aquatic species rely on seasonal streamflow for critical life cycle events. Larger peak flows can contribute to increases in river scour magnitude and frequency and negatively affect survival of some species and life stages. In addition, changes in winter climatic conditions due to decreased snowpack, increased temperatures, and precipitation may increase nutrient and pollution inputs into regional watersheds, negatively impacting water quality and downstream habitats.

References

Adapted from

Staudinger, M.D., A.V. Karmalkar, K. Terwilliger, K. Burgio, A. Lubeck, H. Higgins, T. Rice, T.L. Morelli, A. D'Amato. 2024. A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S. DOI Northeast Climate Adaptation Science Center Cooperator Report. 406 p. https://doi.org/10.21429/t352-9q86

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