By Climate Central
For most of the country spring has sprung earlier this year, but is this anything more than a single warm year? It seems that it is. During the past several decades, with the exception of a few states, spring weather has, indeed, been arriving earlier.
In the interactive above, you can see how much earlier spring has arrived state-by-state, measured by the date of “first leaf.” As you hover over any state, it'll display two boxes: a gray box that represents the day spring used to arrive (based on the 1961-1980 average) and a colored box that represents how much earlier spring has arrived in recent years (based on the 1991-2010 average).
Nationwide, the date of “first leaf” has clearly shifted — arriving roughly three days earlier now on March 17 (1991-2010 average) from March 20 (1961-1980 average). This shift affects all sorts of biological processes that are triggered by warmer temperatures — not just flowering, but animal migration and giving birth and the shedding of winter coats and the emergence from cocoons. How much will an earlier spring disrupt the intricate natural balance between the tens of thousands of species that depend on each other for food, reproduction and ultimately, survival? No one really knows.
The data behind the map comes from an index for the onset of spring developed by Mark D. Schwartz (University of Wisconsin-Milwaukee) and USA National Phenology Network colleagues. The index, based on daily minimum and maximum air temperatures measured at individual weather stations, estimates the first day that leaves appear on plants in a given state. Developed from cloned lilac and honeysuckle observations, the models are most applicable to temperature-responsive forest trees, shrubs and agricultural crops planted in temperate regions with adequate rainfall, or in temperate dry regions where irrigation is used.
NOTE: An earlier version of this map was published on March 21. The story has been updated based on consultations with Schwartz and colleagues. They have examined trends in spring onset since 1900 across the lower 48 states, and find complex patterns of differential warming across regions, especially in the first half of the 20th century. Our updated analysis focuses after 1960 when trends show a more cohesive spatial pattern across the U.S.