Season Creep

The Science and Climate Change Connection

As climate change continues to advance, spring is arriving much sooner, while winters are becoming shorter and milder. This phenomenon has been documented around the world and informally dubbed “season creep.”

In the United States, the growing season has lengthened by 10 days in the past 30 years (Barichivich et al. 2013). Many migratory bird species show up earlier. For example, northeastern birds that spend winter in the southern United States now return to the Northeast an average of 13 days earlier (NCA Chpt 16, p.561). Spring snowmelts have shifted so that peak melt flow now arrives 1-4 weeks earlier. Flowers are blooming earlier, including a week earlier on average for Washington D.C’s famous cherry blossoms. Hardwood forests in the Northern Hemisphere are holding their green leaves for over a week longer than normal (Jeong et al. 2011).

Global warming drives season creep (NCA Chpt 2, p.39). Natural variability can, at best, explain only one-third of the rate of “creep” in the arrival of spring (Ault et al. 2011).

Season creep is an example of how small changes can have a big impact. Climate change disrupts the critically important timing of events, such as snow melt and spring bloom, upon which ecosystems and agricultural industries depend (Cleland et al. 2006). For example, warmer winters can lead to early bud-burst or bloom of some perennial plants, resulting in frost damage when cold conditions occur in late spring. This was the case with Michigan cherries in 2012 (NCA Chpt 6, p.235). Maple syrup production requires cold temperatures for strong sap flow and good flavor, and the brevity of recent winters has cost producers.

Finally, season creep impacts biodiversity, with cascading effects on agriculture, tourism, hunting, and fishing. All species do not respond to the change of seasonal cues in the same way. This can lead to mismatches between the availability of flowers and their pollinators or predators and their prey (Kudo et al. 2013). For example, the pied flycatcher now migrates at the wrong time relative to its prey and has experienced a 90 percent population decline (Both et al. 2006). In some cases, these disruptions can enable takeover by invasive species, as witnessed at Thoreau’s Walden Pond.