Climate Change Indicators: Freeze-Thaw Conditions
This indicator tracks the number of days the U.S. and North American landscape is not frozen.
This figure shows the number of days the ground is considered unfrozen in the contiguous 48 states compared with the 1979–2021 average. For each year, the bar represents the number of days shorter or longer than average. Positive numbers represent years with more unfrozen days than average. Choosing a different long-term average for comparison would not change the shape of the data over time.
Data source: Kim and Kimball, 20224
Web update: September 2023
This figure shows the number of days the ground is considered unfrozen in Alaska compared with the 1979–2021 average. For each year, the bar represents the number of days shorter or longer than average. Positive numbers represent years with more unfrozen days than average. Choosing a different long-term average for comparison would not change the shape of the data over time.
Data source: Kim and Kimball, 20225
Web update: September 2023
This map of North America shows where the number of annual unfrozen days either increased or decreased from 1979 to 2021. Colors show total change over this 42-year period, based on the long-term average rate of change. Dark gray areas were not included in this analysis because they are either open water, barren land, permanently frozen (glaciers, for example), or land that almost never freezes.
Data source: Kim and Kimball, 20226
Web update: September 2023
Key Points
- Within the contiguous 48 states, 2021 had 11 more unfrozen days than the long-term average (1979–2021). The six least frozen years on record have all occurred within the last 10 years of data. The number of unfrozen days in the contiguous 48 states has increased at an average rate of about four days per decade (see Figure 1).
- Alaska’s unfrozen days have increased at an average rate of about four days per decade, although the most recent year of data (2021) was close to the long-term average (1979–2021) (see Figure 2).
- Across North America, the number of unfrozen days in a year has increased since 1979, but trends vary widely with broad-scale regional patterns (see Figure 3). Within the United States, the number of unfrozen days per year has increased the most in parts of the West, Mid-South, and North Central states. Parts of Alaska and northern Canada have also seen a sizable increase in unfrozen conditions. Conversely, some areas in the U.S. and Canadian interior have experienced a long-term increase in frozen conditions.
Background
The freezing point (32°F) is a critical temperature threshold for many aspects of life. The number and timing of days with temperatures below freezing affect ecosystems and many sectors of the economy. For example, many crops only grow during periods when they are not limited by frost damage.1
The number of days below freezing during a year determines which plants can thrive, what food sources are available for animals, and when and how animals migrate or hibernate. Freezing temperatures also help reduce populations of certain insects and other pests that can damage crops and forests or spread diseases to humans (see the Lyme Disease indicator, for example); conversely, a mild winter with fewer days of frost can lead to more of these pests during the following spring and summer.2 Temperatures below freezing also allow precipitation to fall as snow rather than rain (see the Snowfall indicator), which is important for regional water supplies and for places where winter activities, like skiing and snowmobiling, are an important part of the economy. Regional temperature conditions also drive the freezing and thawing of lakes and rivers (see Lake Ice and Ice Breakup in Three Alaskan Rivers, for example).3
As the Earth’s climate warms overall (see the U.S. and Global Temperature indicator), the number of days below freezing has decreased in most parts of the United States. Continued reduction in frozen days could lead to a variety of effects on ecosystems, drought, wildfire risk, agriculture, natural resources, and the economy.
About the Indicator
This indicator tracks the total number of unfrozen days per year in the contiguous 48 states and Alaska. Here, a “frozen day” is any day in which frozen conditions are present at ground level in both morning and evening. An “unfrozen day” is the opposite.
The indicator is based on satellite measurements that detect whether the land surface is frozen. For a broader perspective, it also shows results in a map for North America. It is complementary to EPA’s Length of Growing Season indicator, but rather than relying on temperature thresholds, it offers a unique perspective based on ecologically relevant physical changes from different data sources and calculations. This indicator focuses on whether the landscape itself is frozen (i.e., frozen conditions are present at ground level), and it counts all frozen or unfrozen days throughout the year. In contrast, the growing season indicator is based on air temperatures and it focuses on one continuous stretch of days between the last frost of spring and the first frost of fall.
This indicator was developed by dividing the country into a grid with thousands of cells, using satellite measurements to calculate the number of unfrozen days in each cell for every year, and then averaging all the grid cells together. Figures 1 and 2 show annual averages for the contiguous 48 states and Alaska, respectively. These graphs compare the number of unfrozen days per year against the 1979–2021 long-term average. Figure 3 shows how the annual average change for this period looks geographically. The calculations for this indicator include all land areas that experience freeze-thaw cycles that affect plant life—which means it does not cover areas that are permanently frozen, areas that never freeze, land with no plant life, or large bodies of water.
About the Data
Indicator Notes
At any given location, the number of days experiencing frozen conditions varies from year to year as a result of normal variation in weather patterns. With more than four decades of data, though, variability should not affect the conclusions that can be inferred from the long-term trends shown in this indicator.
A decline in frozen conditions does not necessarily lead to a corresponding increase in agricultural productivity. This is because plant productivity also depends on plant-specific temperature thresholds (some yields go down if temperatures get too warm), pollinators, and seasonal limitations in water and nutrient availability.7 Agricultural production also depends on management activities (e.g., choice of crop varieties suited to changing climate conditions, timing of planting, irrigation, pesticide and fertilizer applications) and natural weather events, such as storms.
This indicator differs from EPA’s Length of Growing Season indicator because it is based on satellite data rather than on-the-ground weather stations, and because it counts the total number of unfrozen days throughout the year—not just the stretch of consecutive days between the last spring frost and the first fall frost. Both indicators are useful for different reasons: growing season is especially meaningful for agricultural crops, while a broader look at frozen conditions is important for understanding ecological impacts, including biological and hydrological processes. In addition, the satellite method used here can pick up local variations in areas that might not have many long-term weather stations that measure air temperature. The two indicators show similar patterns, which is consistent with multiple lines of evidence pointing to similar conclusions about the nation’s changing climate.
Data Sources
This indicator is based on data collected by National Aeronautics and Space Administration (NASA) satellites and analyzed by the University of Montana. EPA’s indicator is an updated version of an analysis originally published in the scientific literature in 2017.8
Technical Documentation
References
1 IPCC (Intergovernmental Panel on Climate Change). 2022. Climate change 2022: Impacts, adaptation, and vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Pörtner, H.-O., D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, and B. Rama (eds.). Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar6/wg2. doi:10.1017/9781009325844.
2 IPCC (Intergovernmental Panel on Climate Change). 2022. Climate change 2022: Impacts, adaptation, and vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Pörtner, H.-O., D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, and B. Rama (eds.). Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar6/wg2. doi:10.1017/9781009325844.
3 Kim, Y., J.S. Kimball, J. Glassy, and J. Du. 2017. An extended global Earth system data record on daily landscape freeze-thaw status determined from satellite passive microwave remote sensing. Earth Syst. Sci. Data 9:133–147.
4 Kim, Y., and J. Kimball. 2022 update to data originally published in: Kim, Y., J.S. Kimball, J. Glassy, and J. Du. 2017. An extended global Earth system data record on daily landscape freeze-thaw status determined from satellite passive microwave remote sensing. Earth Syst. Sci. Data 9:133–147.
5 Kim, Y., and J. Kimball. 2022 update to data originally published in: Kim, Y., J.S. Kimball, J. Glassy, and J. Du. 2017. An extended global Earth system data record on daily landscape freeze-thaw status determined from satellite passive microwave remote sensing. Earth Syst. Sci. Data 9:133–147.
6 Kim, Y., and J. Kimball. 2022 update to data originally published in: Kim, Y., J.S. Kimball, J. Glassy, and J. Du. 2017. An extended global Earth system data record on daily landscape freeze-thaw status determined from satellite passive microwave remote sensing. Earth Syst. Sci. Data 9:133–147.
7 USGCRP (U.S. Global Change Research Program). 2017. Climate science special report: Fourth National Climate Assessment, volume I. Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.). https://science2017.globalchange.gov. doi: 10.7930/J0J964J6.
8 Kim, Y., J.S. Kimball, J. Glassy, and J. Du. 2017. An extended global Earth system data record on daily landscape freeze-thaw status determined from satellite passive microwave remote sensing. Earth Syst. Sci. Data 9:133–147.
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