Last week, the United States was convulsed by some extremely unusual weather.

Temperatures across the eastern half of continent shot up. Boston and Buffalo both climbed above 70 degrees Fahrenheit, their hottest February temperatures ever measured. Washington, D.C., posted a string of summery days and will likely tally its warmest February ever. Cincinnati came close to breaking 80.

This capped off a weird winter. For the first time since modern record-keeping began in 1871, Chicago made it through January and February without any snow on the ground. Hot, dry air from the Mexican plateau handed a 94-degree day to Oklahoma City and an 80-degree day to Denver. And while the West experienced more typical coolness, historic amounts of rain and snow ensnarled California. Ski resorts in the Sierra Nevadas measured more than 500 inches of snow, breaking half-century-old records.

For most of the country, those unseasonably warm temperatures mean that the seasons themselves will arrive somewhat unseasonably. Trees and flowers will probably bloom much earlier this year: Some forecasters believe that the famous cherry blossoms in Washington, D.C., will flower before March 15, two and a half weeks earlier than the average.

Data from the U.S. Geological Survey shows that spring has already overtaken most of the American south. In this map, deep maroon represents areas where the spring bloom has started 20 days earlier than normal:  

USGS

This can prompt excitement (if you’re sick of winter) or apprehension (if you suffer from allergies). But it can also prompt wonder and worry at what it all means. Doesn’t it feel like plants bloom earlier and earlier every year? When spring arrives a month early, does it have a cost?

The answer, say climate scientists, is yes and yes. Thanks to human-caused climate change, spring is happening about 2.5 days earlier every decade. But to understand the deeper meaning and damage of that change, it’s worth first asking: What makes spring spring?

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For people who live between the tropics and the Arctic Circle, spring is two different phenomena that happen at the same time. The first is that the weather gets warmer: Daily high temperatures rise, nights stop getting so cold, and eventually frost no longer forms on the ground. The second is a natural renewal: plants bloom and grow, migratory birds and insects return, and many animals give birth or hatch. The study of this second event—and the other cycles of nature’s calendar—is called phenology.

Usually, these two events happen in concert: The warmer days of meteorological spring arrive in March or April, and phenological spring follows. But when the weather goes awry, it can confound the fine-tuned mechanisms of biology. So when scientists talk about spring getting earlier, they’re really asking two different questions: Has meteorological spring been happening differently or earlier in the year? And has that changed phenological spring?

“Spring is coming earlier,” says Jake Weltzin, executive director of the U.S. National Phenological Network. “But it depends on where you are.”

No matter how you measure it, meteorological spring is arriving earlier and earlier. Across the Northern Hemisphere, the first warmth of early spring arrives about a day earlier every decade. The last cold days get earlier and earlier as well. A recent study found that more than three-quarters of U.S. national parks experience an earlier spring than they did a century ago, and more than half of them are on the “extreme” side of that shift.

This is, in turn, triggering an earlier phenological spring. A 2006 analysis found that, on average, trees now leaf out about a day earlier than they did the decade before. And in most parts of the country, the agricultural growing season begins at least a week earlier now than it did in the 1960s.

Seasonal shift may sound methodical: Can’t farmers just assume that the growing season gets steadily earlier every year? But they can’t. Seasonal changes—which unfold predictably on decadal scales—are difficult to forecast from year to year. One year, cherry blossoms might bloom three weeks earlier than the average; the next, they might bloom a week later. The blooming season creeps earlier and earlier, but it does so erratically.

Weltzin says that this has high costs for tourism, for agriculture, and for ecology. Any town or region trying to organize an outdoor festival must now reckon with the new seasonal calendar.

“The Great Smoky Mountains has a firefly festival, where they have synchronous fireflies,” he told me. “Only a few places in the world have synchronous fireflies, so they have to plan ahead for weeks to think about how to manage the traffic on narrow backroads. The Park Service stretches to get out ahead of the issue,” but it struggles to do so when spring is so unpredictable, he said.

What’s destructive is that not all facets of meteorological spring are uniformly moving earlier. Some are staying the same. In many places in the United States, the date of the last frost of the year isn’t moving much at all. David Inouye, a biologist at the University of Maryland, has found that the last frost date in some parts of the Rockies has stayed put at June 10, even as warmth arrives earlier in the season.

Many wildflowers begin their growing season in late May, as the snow melts, he told me in an email. By the time these flowers have sprouted frost-sensitive buds a month later, the chilling season is over.

“But now sometimes snow melts in mid-April and by June there are a lot of plants with flowers and buds, and some of those are frost-sensitive. Most or all of the frost-sensitive flowers don’t bloom in years like that. And if the flowers are killed by frost, there is then no nectar or pollen for pollinators, and no seeds for animals that eat them,” he added.

“In some parts of the country, the timing of the last frost isn’t getting earlier as fast as the growing season,” said Mark D. Schwartz, a professor of geography at the University of Wisconsin Milwaukee. This is dependent on a number of local factors, but it proved especially damaging in 2012, when North America experienced the earliest “false spring” in the weather record.

These kinds of mismatches can upset complex relationships between animals and their environment. In the Arctic, some grasses bloom a month before normal, depriving hibernating animals of a crucial early-spring food source. Snowshoe hares turn white during the winter, and then brown during the summer, so that they can be better camouflaged against the ground. But now that snow is melting earlier in the year, many are still wearing their white coats in the spring mud—making them especially easy for predators to pick off.

These kind of seasonal mismatches are best documented in the Arctic, but researchers are increasingly finding them elsewhere. “When we look, we find evidence. But it’s patchy,” says Weltzin. Scientists are hampered by the lack of good long-term databases capturing multiple seasonal variables. The birding database Ebird is an extremely useful phenological database, for instance. It’s high-quality despite being managed by amateur birders. But it doesn’t tell scientists about the local availability of insects, seeds, or nectar.

Not all plants are subject to the whims of an early meteorological spring. While many grasses, flowers, and shrubs—like honeysuckle and lilac—bloom only in response to temperature, other plants wait for more certain signals.

The hardwood trees that dominate Eastern forests—oak, beech, and hackberry—all need to experience the longer days of astronomical spring before they begin to flower. (Astronomical spring begins this year on March 20, the vernal equinox.) These hardwoods are also the species most protected from a late frost: They can’t be fooled just by a false spring’s warmth; they also need the light of axial tilt. Researchers write that even when these sturdy trees are planted well south of their natural range, in subtropical climates where they could flourish in January or February, they still seek the light signal and never bloom before early March.

Often when people talk about climate change, they talk about how the world will change in the future. But an early spring is happening now. The same study that revealed how national parks are facing seasonal shift included a special warning for park rangers: “Managers who have worked in these parks for the past one to three decades are already working under anomalous conditions.”

But that warning applies many of us: The springs of the past 30 years have been “anomalous.” The national parks are not the only thing that have already changed. The natural calendar that guides all of our lives has already changed, too.