season creep
n. Earlier spring weather and other gradual seasonal shifts, particularly those caused by global climate change.
Examples
2006
This year, for the first time in more than two decades of serious gardening, the 66-year-old Wynnefield grandfather couldn't resist. It was so warm, he planted in mid-March and never gave it a second thought.

"Is it global warming?" Ford wonders, "or is this a quirk?"

His perception of earlier springs — an idea known as "season creep" — may or may not be related to long-term warming trends. Yet the reality of year-to-year weather weirdness recently, coupled with the ever-present impulse to outsmart Mother Nature, has prompted more than a few gardeners to shun conventional horticultural wisdom.
—Virgina A. Smith, “Out on a limb,” The Philadelphia Inquirer, April 07, 2006
2006
Ice records on Grand Traverse Bay show a disturbing new pattern. After 13 decades in which the bay froze at least seven winters out of every 10, the rate slipped in the 1980s. In the 1990s, the bay froze only three times. So far this decade, once.

Observers see that as one more sign of what some call "season creep," or evidence of global warming.
—Barbars Arrigo, “Why less winter ice is the pits for state,” Detroit Free Press, April 03, 2006
2006 (earliest)
While to some, an early arrival of spring may sound good, an imbalance in the ecosystem can wreak havoc. Natural processes like flowers blooming, birds nesting, insects emerging, and ice melting are triggered in large part by temperature. As temperatures increase globally, the delicately balanced system begins to fall into ecological disarray. We call this season creep.
—Jonathan Banks, “Season Creep: How Global Warming Is Already Affecting The World Around Us” (PDF), National Environment Trust, March 21, 2006
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