Climate Matters•October 16, 2024
Warmer Fall, Later Freeze
KEY CONCEPTS
Fall warming can delay the first freeze of the season and disrupt important ecological cues in forests, farms, and gardens.
Climate Central analyzed how the first fall freeze date has changed since 1970 in 200 U.S. cities.
The first freeze is arriving later in 85% (170) of cities — 11 days later, on average, than in the early 1970s.
The first freeze is now arriving at least two weeks later in 57 cities — especially cities in the Northeast and Upper Midwest.
A later freeze can lead to worse fall allergies, lingering pests, and reduced summer fruit yields.
Download local data
Warmer fall…
The fall season has warmed by 2.4°F on average across the contiguous U.S. since 1970.
A warmer fall season means that risky heat, high cooling demand, wildfires, and allergies linger later into the year — affecting health, ecosystems, and the economy.
Fall warming can also delay the first freeze of the season and disrupt important ecological cues in forests, farms, and gardens — from fall foliage to hibernation, bird migration, and fruit ripening.
…Later freeze
Climate Central analyzed how the date of the first fall freeze (the first day with minimum temperatures at or below 32°F) has changed since 1970 in 200 U.S. cities. See Methodology below for details.
The first freeze is arriving later in 170 cities, or 85% of the 200 analyzed.
Across these 170 cities, the first freeze is arriving 11 days later, on average, than in the early 1970s.
The first freeze is now arriving at least two weeks later in 57 cities — especially cities in the Northeast and Upper Midwest.
The five cities that have seen the biggest shift in their first freeze since 1970 are: Reno, Nev. (41 days later); Bend, Ore. (37 days later); Toledo, Ohio (28 days later); Boise, Idaho (27 days later); Manchester, N.H. (27 days later); and Helena, Mont. (26 days later).
Later freeze, worse allergies, lingering pests, less fruit
Warming driven by carbon pollution results in more freeze-free days each year and therefore longer annual growing seasons. According to the EPA, the growing season is now more than two weeks longer on average across the contiguous U.S. compared to the beginning of the 20th century.
Although some plants and animals may benefit from longer growing seasons, they can also lead to more water and heat stress, and longer pollen and pest seasons.
Recent Climate Central analysis found that the annual freeze-free season has grown longer in 164 U.S. cities since 1970. This trend is due to both:
earlier spring thaw (last spring freeze happens earlier) and
later fall freeze (first fall freeze happens later).
Later fall freeze and longer growing seasons can:
Lengthen and worsen fall allergy season. In most U.S. areas, ragweed pollen typically peaks in September and lasts through October. But warmer fall temperatures and a later fall freeze extend the ragweed growing season.
Learn more: Later Fall Frost Extends Allergy Season.
Disrupt conditions that trigger fall color and leaf drop. Fall plant cycles are complex, but our understanding of their response to climate change is growing.
Learn more: Fall Foliage and Climate Change.
Allow mosquitoes to linger later. With warming, mosquitoes and other disease-carrying insects can emerge earlier and survive later into the year — meaning more opportunities to bite people and potentially transmit disease.
Learn more: Mosquito Days.
Impact fruit and nut crops — a nearly $27 billion industry in the U.S. A certain amount of sustained chilling (starting September 1) is key for high quality non-citrus fruit and nut yields. But 181 U.S. locations have seen this chill period decrease since 1980.
Learn more: Winter Chill Brings Summer Fruit.
Affect planting, pest management, and harvest schedules on farms and in gardens. Growing degree days (a measure of growing season heat accumulation required for both crop and pest development) have increased since 1970 across the entire U.S.
Learn more: Growing Degree Days.
LOCAL STORY ANGLES
When does the last spring freeze and first fall freeze typically occur in your area?
Check out NOAA’s national maps showing the historical date of first freeze and last freeze to understand when to generally expect freezing. Monitor NOAA’s National Weather Service for frost and freeze watches and warnings in your area.
The Freeze Date Tool, from the Midwestern Regional Climate Center and the USDA Midwest Climate Hub, provides information about the climatology of freezing temperature dates across the north-central and northeastern U.S.
The North Carolina State Climate Office has a Freeze Date Explorer tool to view both the long-term average first and last freeze dates, along with the observed annual occurrence of freeze dates since 1950. Other State Climate Offices may have similar tools.
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Daily temperature data since 1970 were obtained for 203 U.S. weather stations from the Applied Climate Information System. The annual first freeze date for each location was determined based on the first day of each year (starting August 1st) with a minimum temperature of 32°F or below.
Locations without a consistent freezing season (defined as fewer than three months of minimum temperatures at 32°F or below) were excluded from the analysis.
This analysis included 203 weather stations. For reported data summaries based on linear trends, however, only 200 stations are included due to data completeness measures that were not met by three stations: Dothan, Ala.; Hazard, Ky; and Wheeling, W.Va.