Spring is Coming Early

Meteorological spring, which is based on calendar dates, started more than two weeks ago. Astronomical spring, based on Earth’s orbit around the sun, starts in two days. While these dates have not changed significantly since the development of our modern calendar, the date when spring weather arrives (as measured by first bud data) is earlier than in recent decades.

While many Americans may welcome that prospect, this shift has ecological consequences beyond milder temperatures and earlier flowers. Changing temperature cues can disrupt the temporal match between plant and animal processes, leaving insects without food and plants without pollinators. An earlier growing season can also increase water stress while leaving plants more vulnerable to frost damage after starting growth too soon.

For this analysis, we used a spring onset index developed by Mark D. Schwartz (University of Wisconsin-Milwaukee) and others at the USA National Phenology Network. This index uses temperature measurements from individual weather stations to estimate the first day of leaf emergence in each state. The change in spring arrival date was obtained by comparing the 1991-2010 average with the 1961-1980 average. Across the Lower 48, the average “first leaf” date has shifted from March 20, the beginning of astronomical spring, to St. Patrick’s Day three days earlier. 

The shift in spring arrivals is even greater across a number of states in the Southwest and Southeast, by five days or more. Several New England and Mid-Atlantic states have also experienced this magnitude of change, while only three states have seen a spring shift of less than than two days.

A few days change in our spring patterns may not sound like much, but with projected temperature changes this shift could become much more dramatic. A 2013 study by Su-Jong Jeong and other Princeton researchers predicts that red maple leaves will, on average, emerge 16 days earlier by 2100. In New York and several other northern states, the change may be as much as 38 days.