After a mild winter across much of the United States, February brought abnormally high temperatures, especially east of the Rockies. Spring weather arrived more than three weeks earlier than usual in some places, and new research released Wednesday shows a strong link to climate change.

By the 2017 calendar, the first day of spring is March 20. But spring leaves arrived in mid-January in some parts of the South, and spread northward like a wave. The map above plots the date of “first leaf,” a temperature-based calculation of when vegetation that has been dormant starts to show signs of life. This year, with the exception of a few small areas, the wave has arrived much earlier than the 30-year average.

An early spring means more than just earlier blooms of fruit trees and decorative shrubs like azaleas. It can wreak havoc on schedules that farmers follow for planting and that tourism officials follow for events that are tied to a natural activity like trees blooming. Some plant species that bud early may be susceptible to a snap frost later, and early growth of grasses and other vegetation can disrupt some animals’ usual cycles of spring feeding and growth.

First leaf can vary greatly from year to year and location to location, but the general long-term trend is toward earlier springs.