Climate Change
Climate Change Indicators in the United States
This figure depicts plant hardiness zones in the lower 48 states in 1990 and 2006.
Data source: Arbor Day Foundation, 2006 1
This figure depicts changes in plant hardiness zones in the lower 48 states between 1990 and 2006.
Data source: Arbor Day Foundation, 2006 2
Key Points
- Between 1990 and 2006, hardiness zones have shifted noticeably northward, reflecting warmer winter temperatures (see Figures 1 and 2).
- Large portions of several states have warmed by at least one hardiness zone; for example, large parts of Illinois, Indiana, Ohio, and Missouri have shifted from Zone 5 to Zone 6, reflecting a sizable increase in average low temperatures (see Figures 1 and 2).
- A few scattered areas, mostly in the West, have cooled by one hardiness zone, while a few smaller areas have cooled by two hardiness zones (see Figure 2).
Background
Plant hardiness zones are regional designations that help farmers and gardeners determine which plant species are expected to survive a typical winter. Locations are assigned a numbered plant hardiness zone based on an average of the lowest temperatures recorded each winter.
Average annual minimum temperature is used to determine hardiness zones because a single low temperature event such as a freeze is far more likely to harm plants than a single high-temperature event, such as an unusually warm day. Minimum temperature is considered a critical factor in a plant's ability to survive in a particular location.
As temperatures increase, plants are able to survive winters in areas that were previously too cold for them to thrive. These changes in growing patterns can influence agricultural production, and changes in wild plant distribution can have wide-ranging effects on ecosystems. For instance, the animal species present in a location could change as the animals move to seek out their preferred food source, or an invasive plant could harm native plant species.
About the Indicator
The U.S. Department of Agriculture first published a plant hardiness zone map of the United States in 1960, and revised the map in 1990. This map is divided into numbered zones based on average annual low temperatures in 10-degree increments. For example, areas in Zone 7 have an average annual minimum temperature of 0 to 10°F, while areas in Zone 8 have an average annual minimum temperature of 10 to 20°F.
In 2006, the Arbor Day Foundation revised the map based on 15 years of temperature data collected by 5,000 National Oceanic and Atmospheric Administration weather stations across the United States. To determine how plant hardiness zones have shifted over time, this indicator compares the 1990 U.S. Department of Agriculture hardiness zone map with the 2006 Arbor Day Foundation hardiness zone map.
Indicator Confidence
Changes in plant hardiness zones do not address maximum temperatures or the amount of precipitation present in a location, which can also affect plants' ability to thrive. Plant hardiness zones also do not take into account the regularity and amount of snow cover, elevation, soil drainage, and the regularity of freeze and thaw cycles. As a result, plant hardiness zone maps are less useful in the western United States, where elevation and precipitation vary widely. For example, both Tucson, Arizona, and Seattle, Washington, are in Zone 9 according to the 2006 map; however, the native vegetation in the two cities is very different.
Data Sources
The maps used in this indicator are available online at: www.arborday.org/media/ map_change.cfm. The data used to create the map were provided by the National Oceanic and Atmospheric Administration's National Climatic Data Center, which provides temperature data and maps through its website at: www.ncdc.noaa.gov/oa/ncdc.html.
Indicator Documentation
- Download related technical information PDF (5 pp, 35K)