Look to the lilacs, not the heavens, if you want to know when spring really starts for plants and wildlife, say scientists like Alyssa Rosemartin of Newtown Square.

She and those who study phenology — nature’s calendar — use the emergence of lilacs as a sign spring has sprung. With climate change, that date can be much different than the official start of spring, or the vernal equinox, which occurs at 11:49 p.m. Thursday, when the length of day and night will be roughly equal.

Rosemartin, with the USA National Phenology Network at the University of Arizona, says lilacs are a good barometer because their initial leaf growth has been tracked for decades and tends to coincide with early blooming leaf-dropping trees.

“Spring has arrived in Philadelphia 16 days earlier this year than the long-term average,” said Rosemartin. “This year is not normal.”

Rosemartin studies when trees bloom, birds migrate, and leaves turn in the fall. That timing has implications for plants and wildlife. For example, trees that certain migratory birds depend on might bloom early, which then throws off the timing of the appearance of insects, a key source of food for the birds.

“In Europe, they have documented population declines of birds,” Rosemartin said.

Spring is coming earlier at the rate of about a day per decade, she said, "an indicator of changing climate.”

And that portends more pests, a prolonged allergy season, and disruption of agriculture. What it won’t do: Reduce the spread of the novel coronavirus, as scientists have warned.

Warming winters

Indeed, December, January, and February, also known as the meteorological winter, all were warmer than normal, and so far March is the same, as a chart shows.

As The Inquirer reported last week, the winter of 2019-20 (December, January, and February) had an average temperature of 39.4 degrees, the 10th-warmest on record for Philly.

Spring is arriving earlier

Bernadette Woods Placky, chief meteorologist at Climate Central, an organization of scientists that researches climate change, said that on average, spring had been arriving eight days earlier than it used to in Philadelphia through 2019. It’s more like 16 days this year.

Philly is just one of many cities experiencing what’s known as an early leaf-out, when trees start producing leaves. Leaf-out arrived earlier in 76% of 239 cities analyzed over a nearly 40-year long-term average, according to Climate Central. The data are from the National Phenology Network’s database, which goes back to 1981.

In some cases, animals are thrown off by the timing. There have been reports that this year’s warm winter has roused brown bears out of hibernation earlier than usual in the western United States and countries such as Russia, which had its warmest winter ever.

Larry Hajna, a spokesperson for the New Jersey Department of Environmental Protection, said the state’s native black bears don’t technically hibernate but go into torpor, when their heart rates and metabolism drop to conserve energy.

As a result, Hajna said, they might rouse during a warm spell, but go back into torpor when it gets cold again.

Spring is warmer

On average, a typical spring in Philadelphia is about 2.8 degrees warmer now than it was in 1970, according to data from the government’s Applied Climate Information System (ACIS), analyzed by Climate Central.

Jason Parker, a district manager for the Davey Tree Expert Co., said dandelions are already out, “which is crazy early.”

Parker said the mild winter and warm spring likely will mean insect issues on trees and plants, as it never got cold enough to kill as many insects as usual.

That also applies to Philly’s most invasive pest: the spotted lanternfly. “We anticipate very few spotted lanternfly eggs would have died off over the winter because it was so mild,” Parker noted.

More days above normal

According to NASA, global temperatures will continue rising for decades because of greenhouse gases, such as carbon dioxide and methane, that are produced through human activity.

Effects vary, but the Philadelphia region has more spring days with above-normal temperatures.

‘Last Freeze’ changing

Data also show the last freeze of the spring is coming earlier, meaning some vegetables can be planted earlier, though it’s a bit of a guessing game. An unexpected freeze, when air temperatures to 32 degrees or lower, can damage plants.

. A sudden freeze could also cause severe damage to fruit orchards that are blooming earlier due to unseasonably warm temperatures.

“If the trees start to bud early and if you have a real big cold spell, that can kill the buds and significantly reduce yields later in the year,” said Mark O’Neill, a spokesperson for the Pennsylvania Farm Bureau.

Around the nation

Philly is far from alone in experiencing a shifting spring. Much of the United States, other than some areas of the upper Midwest, is seeing warmer springs.

This was the second warmest winter on record globally stretching back to 1880, according to federal scientists. Only 2015-16 was warmer, and that was fed by an El Niño, a warming of Pacific Ocean water.

This winter, the average temperature in central Tokyo was the third highest on record, according to the Tokyo Times. It’s been warm enough there that cherry blossoms flowered on Saturday, the earliest recorded date ever and 12 days earlier than normal.

Rosemartin, of the phenology network, knows many view a warm winter and early spring combo as a good thing, but cautions: “You also have to consider the downside.”