Last updated December 4, 2018

Season Creep

As climate change advances, spring is arriving much sooner, while winters are becoming shorter and milder. Changes in the timing of the seasons has been documented around the world — through studies on plant and animal development and life cycles, temperature and snow cover — and informally dubbed “season creep.” 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.

US trends and projections

Earlier/ warmer springs

Global warming drives season creep.[1] In the United States, studies have documented an advance in the timing of springtime lifecycle events across plant and animal species in response to increased temperatures.[2]

The continental US experienced widespread earlier green-up (when plants go from winter dormancy to photosynthesis) and last spring freeze dates over the period from 1920 to 2013.[3] From 1950 to 2013, green up and last spring freeze advanced by 4.2 and 7.9 days respectively.[3] 

In the Northeast, the last spring frost is about a week earlier now than it was 30 years ago on average. This leads to increased chances of frost damage since the start of growth for many plants has shifted even earlier than the last frost date.[4]

Longer/ warmer falls

Hardwood forests in the Northern Hemisphere are holding their green leaves for over a week longer than normal.[5]

The date of the first autumn freeze in the Pacific Northwest has been delayed 9 days since 1950.[6]

Drier, warmer autumn weather may be extending summer smog well into the fall in the Southeastern US.[7]

Shorter/ warmer winters

According to the 2014 US National Climate Assessment, “Observed long-term trends towards shorter, milder winters and earlier spring thaws are altering the timing of critical spring events such as bud burst and emergence from overwintering.”[1]

Global trends and projections

Earlier/ warmer springs

The IPCC states, “It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales, as global mean surface temperature increases.”[8] Recent warming of the Northern Hemisphere is well documented and typically greater in winter and spring than other seasons.[9]

In Eurasia and North America, the length of time in a calendar year when temperatures are consistently warm enough for agricultural activity lengthened by 10 days between 1982 and 2011.[10] In Eurasia, the growing season increased by 13 days, and in North America, it increased 6 days.[10] The increase closely tracks the pace of warming in the spring.[10]

Research indicates that natural variability can, at best, explain only one-third of the rate of “creep” in the arrival of spring in North America.[11]

Spring snow cover extent has decreased in the Northern Hemisphere. During the months of March and April from 1922 to 2012, snow cover decreased by 7 percent and the decline shows a strong negative correlation with land temperature.[8]

Longer/ warmer falls

In North America, the end of the growing season was delayed by 8.1 days from 1982 to 1999 and delayed by another 1.3 days from 2000 to 2008.[5]