Whether you follow the astronomical or meteorological definition, we look to our calendar to tell us the season. By contrast, changes that we see in nature and associate with spring—daffodils emerging, birds chirping—come about through environmental cues such as warmer temperatures, more sunlight and snowmelt.
With many areas experiencing increasing average temperatures, more spring days above normal, and earlier dates of last freeze. Now we explore the impact of these changes on the natural world. We find that the timing of natural phenomena, also known as phenology, has shifted. As a result, spring is coming earlier across much of the United States.
‘Leaf out’ is a common marker that scientists use to keep track of spring phenology—it’s the time that leaves emerge on early spring plants (namely, honeysuckle and lilac). According to our analysis, 76% (181) of the 239 cities analyzed are seeing their average annual first leaf come earlier since 1981. Flagstaff, Ariz. registered the most dramatic change, with the first leaf coming 18 days earlier, followed by Colorado Springs, Colo. (17 days) and Reno, Nev. (17 days).
Earlier springs can cause a mismatch in the availability of food and other resources for animals during critical stages in their life cycle. In complex ecosystems where many species rely on each other, the impacts can be cascading. A recent study found that rising temperatures were associated with earlier spring migration in birds, potentially throwing them out of sync with the peak abundance of insects they eat or the blooming of flowers they pollinate. Early spring warming can also make plants more vulnerable to damage by subsequent frosts that are still normal for a given area. Known as a “false spring,” this phenomenon resulted in $1 billion of losses in fruit crops across the Southeast United States in 2017. Phenology is one of the oldest kinds of environmental science, and it could be more important than ever for monitoring how the natural world is responding to a changing climate.