For millennia, ecosystems in Greenland and throughout the Arctic have been regulated by seasonal changes that govern the greening of vegetation and the migration and reproduction of animals. But a rapidly warming climate and disappearing sea ice are upending that finely tuned balance.

From their perch on a rocky ridge in southwestern Greenland, graduate students Rebecca Walker and Conor Higgins peer through binoculars, looking for caribou. It’s a cool, June day and the tundra is ablaze with tiny magenta, pink, and yellow wildflowers. Crystalline lakes dot the glacially carved valleys, and from the round-topped mountains you can catch the glint of the massive Greenland Ice Sheet to the east. Below, the Watson River tumbles toward Kangerlussuaq Fjord, 12 miles to the west. It’s quiet, save for bird song, the rush of the wind, and the frequent crash of ice shearing off nearby Russell Glacier. Two decades ago, Walker and Higgins would have seen hundreds of caribou from the top of this same hill, set amid an ancestral caribou calving grounds. But these days the herds are a fraction of their former size, and Walker and Higgins spot only a handful of females and two calves a mile away. The ecologist supervising the students — Eric Post of the University of California, Davis — says the decline is very likely linked to a rapidly warming climate that is driving the schedules of caribou and the tundra plants they eat seriously out of balance. Post first came to the area 25 years ago to study calving in large herds of caribou. But around the early 2000s, he began noticing a major change.

The growing season for plants now starts on average nearly three weeks earlier than a decade ago.

“As it got warmer and warmer and the growth season started earlier and earlier, the caribou calving season wasn’t starting earlier to the same extent,” Post says. The advancing plant growth was being triggered by warmer temperatures early in the season, while the caribou take their cues from the length of day. As a result, studies suggest, the plants may have already passed their nutritional peak by the time the herds arrived to eat them. Post determined that in years with the earliest plant growth, caribou parents produced the fewest calves, and those born were more likely to die young — possible victims of nutritional stress. “The caribou population at the site has declined pretty dramatically since this work began,” he says. This growing mismatch between peak vegetation growth and the birth of caribou calves shows how phenology — the timing of seasonal events such as plants flowering, birds migrating, insects hatching, and animals giving birth — is being knocked out of balance in our rapidly warming world. In his quarter-century of studying the impacts of global warming on Greenland’s ecosystems, Post, 51, has observed what he calls “dramatic and disturbing” changes in the seasonal clock. The growing season for plants in his study plots now starts on average nearly three weeks earlier than a decade ago. Some plants — the “racehorses” — are especially able to capitalize on the warmer temperatures and abundant water released by earlier spring thaws. The winners — a small, pinkish wintergreen called Pyrola grandiflora, and a Carex sedge — have advanced the start of their first spring growth by a stunning 26 days or more. That’s half of the Arctic’s brief growing season.

Researchers Rebecca Walker and Conor Higgins check plant growth stages in southwestern Greenland. Cheryl Katz / Yale e360

Not only are the plants greening up earlier and caribou herds declining, but the landscape has changed dramatically over the years that Post has been making annual trips to his study site, just north of the Arctic Circle. Shrubs like gray willow, normally dwarf, are now burgeoning — changing the very look of the tundra’s traditional landscape. “When we got out there for the first time 25 years ago, it was really low shrub tundra — you’re talking about knee-high… or even lower,” he recalls. “Now there are shrubs up to your shoulders and head in many places.” Earth’s springtime clock is advancing around the globe as the planet warms up. It’s being seen in phenomena such as butterflies emerging earlier, pollen seasons starting ahead of schedule, and the choreography of birds and bees going out of sync, which could adversely affect plant pollination. A new study finds the seasonal cycle of temperatures in the atmosphere itself is changing, with the air heating up faster and earlier in summers, especially at mid-latitudes. But nowhere is the clock speeding faster than in the Arctic. Post’s latest research finds that in the past decade, the onset of spring has moved up four days for every 10 degrees north of the equator. That builds to a 16-day advance on average north of the 59th parallel. And the rate of change appears to be accelerating.

Long-distance migrants, such as Arctic-breeding birds, face some of the greatest timing mismatches.

These rapid changes threaten to uncouple the tightly synchronized springtime routines of Arctic ecosystems. Plants here have just a few short weeks to bloom, set fruit, and produce seeds; and animals that depend on them must reproduce and rear their young before the snows return. There’s little room for schedule deviations, and Post is concerned that “there may be a tipping point somewhere in the near future.” Once reached, he says, the ecosystem might not return to normal, leading to a new state that’s “completely unpredictable.”

The forces behind the heightened advance at the highest latitudes are complex. A key factor is that temperatures in the Arctic have risen twice as fast as the rest of the planet, and the increase is accelerating. That rapid warming trend has caused sea ice to thaw and snow cover to recede, exposing more dark, heat-absorbing surfaces that further warm the region — an effect known as “Arctic Amplification.” The earlier warming is triggering head starts in spring activity at the lower end of the food web — such as plants and insects — while the animals that consume them can’t change their schedules fast enough to keep up. Long-distance migrants, such as Arctic-breeding birds, face some of the greatest timing mismatches because conditions in their wintering grounds give them little sign of what’s going on up north.

In years with early plant growth, caribou produce the fewest calves. Those that are born are more likely to die young due to nutritional stress. Christian John / UC Davis

Post’s nearly 15-square-mile study site used to be a paradise for caribou, which are, essentially undomesticated reindeer. Some years, he saw as many as 600 of them at a time. “When you see several hundred animals moving together across the landscape, it’s a beautiful sight,” he says. “Now it’s down to around 90, and it’s really hard to find big groups anymore.” A key driver of changes across Greenland’s ecosystem is the rapid decline of Arctic sea ice, says Post. “We’ve found that as sea ice diminishes there are more days in the calendar amenable to plant growth, and those fall earlier in the year,” he says. Declining sea ice not only boosts local temperatures by opening up heat-absorbing ocean, but it may also draw heat from the warming ocean into the atmosphere, or alter circulation. The upshot, says Post, is that “sea ice is not only responding to temperature increases, it’s also having a sort of cascading effect on local air temperature as it retreats.” As for plants, the response to earlier warming varies markedly by species. While some, like wintergreen, are sprinting out the gate, others, like birch, have barely budged. And still others, like shrub willows now exploding across much of the Arctic tundra, are not starting their growth earlier, Post says, but may be taking advantage of the growing season’s later end.

The annual growing cycle for harebell flowers now begins 10 days earlier than it did a decade ago. Eric Post

“You can’t generalize about the Arctic tundra,” Post says. “These individual species have very distinct strategies for what they can do and will do when the environment is changing.” In an effort to forecast how potential future warming, coupled with the loss of grazing animals, could affect tundra vegetation, Post fitted some of his test plots with greenhouses and animal-excluding fences. The results were striking. “In the first five years, we saw a shift in the tundra plant community away from grasses and sedge dominance to shrub dominance,” he says. “So as you warmed the tundra and excluded large herbivores (caribou and muskoxen), the shrub willows and birches began to take over the community.” Over a decade-and-a-half, shrub heights actually quadrupled. Shrubs are similarly expanding in tundra across the Arctic. This summer, Post reversed his experiment. Plots that were warmed and protected from grazing will now face natural conditions. “The question is, has the plant community or the tundra system been pushed to such a different state of plant composition and species richness that it can’t be returned to the state it was in originally?” he asks.

Can species with intertwined lifecycles somehow adapt to these schedule mismatches?

To calibrate the shifts in the botanical clock, graduate students Walker and Higgins crawl across Post’s study plots every day from May to the end of June, examining each tiny plant to determine its growth stage and noting the dates the various plant species hit landmarks, such as putting out first shoots, budding, and starting to fruit. Each day they also walk a route covering more than 10 miles as they count caribou and muskoxen. Similar research finds that shifting seasons are straining ecological relationships throughout the Arctic. On Bylot Island in Nunavut, Canada, earlier springs are opening a gap of up to 20 days between the peak nutrition of plants and the hatching of greater snow geese. “The goslings need to feed well and they need to feed on plants when they are at that early stage of growth,” explains Gilles Gauthier, biology professor and scientific director of the Center for Northern Studies at the University of Laval in Quebec, who has been studying this High Arctic site for nearly 30 years. “Because as plants get older and mature, their protein content goes down, and it’s not so good for the growing goslings.” Although the geese have sped up their schedule to some degree, they can’t keep up with the plants because, like other vertebrates, their timing is largely tied to the length of day. “If, for instance, plants start growing 10 days early, geese will only start nesting five days early,” says Gauthier. “Which means in the long term if springs keep advancing, this mismatch will be increasing more and more.”

Migratory birds such as snow buntings, seen here near Kangerlussuaq, increasingly miss the peak nutrition of plants. Eric Post

In northeast Greenland, earlier snowmelt is shortening the time that blooming plants and the insects that pollinate them overlap. Changes to the Arctic’s smallest creatures’ timetables can reverberate throughout the ecosystem, says Amanda Koltz, a global change ecologist at Washington University in St. Louis. Ecologists stress, however, that climate change impacts on Arctic phenology are far from uniform. “We talk about the Arctic and we talk about climate change, but it’s affecting different parts of the Arctic very differently,” says Laura Gough, a plant ecologist at Towson University in Maryland, who has studied the region around the University of Alaska’s Toolik Field Station for more than two decades and seen little evidence of phenological shifts there so far. “It’s hard to come up with a general statement that makes sense and that actually applies across all these regions.”