January 7, 2022
On Dec. 21, 2021, the EPA issued a new report, Seasonality and Climate Change: A review of observed evidence in the United States, showing how climate change is affecting seasonal events and processes across the United States. Longer growing seasons, more heat waves, earlier snowmelt, and changes in leaf and bloom dates: These are just a few of the ways in which climate change is altering the nature of seasonal events. The report uses long-term historical data tracking dozens of climate indicators to describe these changes and how they affect physical, ecological and human systems, as well as our everyday lives.
“With climate change, we are seeing longer and more frequent summer heat waves, more intense and prolonged wildfire and pollen seasons, and many more seasonal impacts,” said EPA Administrator Michael S. Regan. “This report collects the long-term data on seasonal events, and provides a new resource for understanding climate change impacts on our environment and our health.”
Many of the changes underway can lead to harmful impacts on the environment and human health. More frequent heat waves can increase incidence of heat stroke, respiratory problems, and other adverse health conditions. Prolonged wildfire and pollen seasons can lead to increased exposure to unhealthy air quality and extra risks for people with asthma and allergies. Mountain snowpack plays a key role in the water cycle in the western U.S., and changes in mountain snowpack can affect agriculture, winter recreation, and tourism in some areas, as well as plants and wildlife. While a few changes can be beneficial– such as longer growing seasons for crops or reductions in winter heating fuel costs – the vast majority of effects on the climate are detrimental to human health and society.
The report considers three aspects of seasonality: 1) shifts in the timing of seasonal events such as the timing of first and last frost; 2) changes in the duration of seasonal events such as the length of the wildfire season; and 3) changes in the variability of events or processes that occur during certain times of the year such as the change in the number of major hurricanes or intensity of heat waves.
These are a few of the documented changes in seasonality across the United States:
- Seasonal temperatures: All seasons have warmed in the U.S., with winter temperatures increasing by nearly 3°F since 1896.
- Spring snowpack: Since the 1950s, there has been widespread declines in spring snowpack across the West. In addition, the timing of peak snowpack shifted earlier by an average of 9 days between 1982 to 2018.
- Timing of spring runoff: In parts of the country where streamflow is strongly influenced by snowmelt, the timing of winter-spring flow carried by rivers and streams is happening at least eight days earlier since 1940.
- Growing Season: The average length of the growing season in the contiguous 48 states has increased by nearly two weeks since the beginning of the 20th century.
- Pollen Season: The season for ragweed pollen grew longer at eight of nine study locations in the Midwest since 1995.
- Heat Wave Season: The average heat wave season across 50 major U.S. cities is 47 days longer than it was in the 1960s.
The report can be found here: https://www.epa.gov/climate-indicators/seasonality-and-climate-change
In addition to support from the scientific literature, this report draws on data and findings from EPA’s Climate Change Indicators resource. EPA works in partnership with more than 50 data contributors from various government agencies, academic institutions, and other organizations to compile a key set of indicators related to the causes and effects of climate change.
Information about Climate Change Indicators: https://www.epa.gov/climate-indicators
Information about climate change: https://www.epa.gov/climate-change