Environmental Protection Agency: Climate Change Indicators - Leaf and Bloom Dates

This indicator examines the timing of leaf growth and flower blooms for two widely distributed plants in the United States.

This indicator shows trends in the timing of first leaf dates and first bloom dates in lilacs and honeysuckles across the contiguous 48 states. Because many of the phenological observation records in the United States are less than 40 years long, and because these records may have gaps in time or space, computer models have been used to provide a more complete understanding of long-term trends nationwide.

The models for this indicator were developed using data from the USA National Phenology Network, which collects ground observations from a network of federal agencies, field stations, educational institutions, and citizens who have been trained to log observations of leaf and bloom dates. For consistency, observations were limited to a few specific types of lilacs and honeysuckles. Next, models were created to relate actual leaf and bloom observations with records from nearby weather stations. Once scientists were able to determine the relationship between climate factors (particularly temperatures) and leaf and bloom dates, they used this knowledge to estimate leaf and bloom dates for earlier years based on historical weather records. They also used the models to estimate how leaf and bloom dates would have changed in a few areas (mostly in the far South) where lilacs and honeysuckles are not widespread.

This indicator uses data from nearly 3,000 weather stations throughout the contiguous 48 states. The exact number of stations varies from year to year. For each year, the timing of first leaf and first bloom at each station was compared with the 1981 to 2010 average to determine the number of days’ “deviation from normal.” This indicator presents the average deviation across all stations, along with maps that compare the most recent 10-year period (2006–2015) with a mid-20th-century baseline (1951–1960) at individual stations. These time periods were chosen to match published studies.

Science Topics
Biology, Botany, Climate Change, Ecology
Middle School
6th Grade, 7th Grade, 8th Grade

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