The boreal forest wraps around the globe at the top of the Northern Hemisphere in North America and Eurasia. Also known as taiga or snow forest, this landscape is characterized by its long, cold and snowy winters. In North America it extends from the Arctic Circle of northern Canada and Alaska down into the very northern tip of the United States in Idaho, Washington, Montana, and Minnesota. It’s the planet’s single largest biome and makes up 30 percent of the globe’s forest cover.

Moose are the largest ungulate in the boreal, adapted with their long legs to wade in its abundant marshes, lakes and rivers eating willows, aspen and other plants. In the southern boreal forest of northern Minnesota, moose were once plentiful, but their population has plummeted. Thirty years ago, in the northwest part of the state, there were some 4,000; they now number about a hundred. In the northeast part, they have dropped from almost 9,000 to 4,300. They’ve fallen so far, so fast that some groups want them listed as endangered in the Midwest.

Moose carcasses deteriorate rapidly before they are found, and so forensics has not been able to determine why they are dying. Some experts surmise it could be that tens of thousands of ticks that mob an animal and weaken it. Others think it’s a parasite called liver flukes, or the fact that winters have gotten so warm the animals can’t regulate their body temperature and die from heat stress.

But Dennis Murray, a professor of ecology at Trent University in Peterborough, Ontario, thinks the dying moose of Minnesota and New Hampshire and elsewhere are one symptom of something far bigger – a giant forest ecosystem that is rapidly shrinking, dying, and otherwise changing. “The boreal forest is breaking apart,” he says. “The question is what will replace it?”

Many scientists, in fact, are deeply concerned about the state of the world’s largest forest. The Arctic and the boreal region are warming twice as fast as other parts of the world. Permafrost is thawing and even burning, fires are burning unprecedented acres of forest, and insect outbreaks have gobbled up increasing numbers of trees. Climate zones are moving north ten times faster than forests can migrate. And this comes on top of increased industrial development of the boreal, from logging to oil and gas. The same phenomena are seen in Russia, Scandanavia, and Finland.

These disturbing signals of a forest in steep decline are why NASA just launched a large-scale research project called ABoVE — Arctic Boreal Vulnerability Experiment, a “major field campaign” with 21 field projects over the next decade. But the studies will confirm in detail what many know is well underway.

“Boreal forests have a potential to hit a tipping point this century,” said Anatoly Shvidenko, of the International Institute for Applied Systems Analysis and a co-author of a survey of a recent research on boreal forests in the journal Science. “It is urgent we place more focus on climate mitigation and adaptation with respect to these forests.”

A tipping point would include the mother of all concerns: the unbridled melting of permafrost, one of the main thrusts of the ABoVE project. The permafrost in the boreal is more susceptible to thawing than in the Arctic because it’s closer to the freezing point. If large-scale melting occurs it would release more carbon dioxide and methane, which have been bound up in the frozen soil for thousands of years, and bring on more warming, and then more thawing, a dangerous loop. “Scientists call it a positive feedback, but most people call that a vicious cycle,” said Peter Griffith, chief support scientist for the ABoVE project.

Murray has been researching the boreal forest for 25 years, and he and his colleagues have seen many changes firsthand. In British Columbia, 80 percent of the province’s mature lodgepole, another boreal species, have recently died from the mountain pine beetle, whose range and season both expanded greatly because of a warmer world. White and black spruce, the main trees species in the boreal, are also dying in vast numbers “The southwest Yukon looks dramatically different than it did 25 years ago when I did my master’s degree,” he said. “Everywhere you go there is deadfall.”

NASA has also seen a lot of forest browning in its studies, large swaths of forests with reduced growth that turn brown in the hotter than normal summers, and which contribute to much bigger and hotter fires and the thawing of the permafrost. “Severe fires burn a foot or more of organic soil and that’s the insulating layer that keeps the permafrost frozen,” said Scott Goetz, lead scientist on the ABoVE project. “Once that is removed the thaw is much more rapid.”

The fires also change the vegetation. “When you remove that organic soil you get deciduous re-growth that lasts for decades,” said Goetz. “Maybe it will never come back as coniferous.”

Typically, when black spruce dies, it is replaced with new black spruce. “That’s been going on for a long, long time, [but] not anymore,” Murray says. “You lose spruce and you lose everything that lives in spruce and that is basically everything in the boreal forest. We’re seeing the same phenomena that we’ve seen with moose with lynx and snowshoe hares. And caribou are going belly up very, very fast. Their ranges are receding northward rapidly.”

The boreal is also home to some 5 billion birds. Many species have shifted their ranges north. “Climate change is having an impact much more quickly than we thought,” said Jeff Wells, a senior scientist with the International Boreal Campaign who focuses on birds. “Shifts that researchers thought would take place over 50 or a hundred years have taken place over a decade.”

But the changes in such a large system are complex. The forest isn’t merely receding north, it’s dying in some places in its interior and surviving quite well in others. A 2015 study found that while boreal spruce forest in interior Alaska is dying out because of warm temperatures, the same forest on the state’s west coast is thriving.

The dying spruce and moose and lynx and other changes are the dots that scientists are trying to connect to get a sense of what the boreal forest in North America will look like in the next half century.

The caveat is that in a system this massive and complex there are so many variables it’s difficult to forecast with much certainty. The projected changes, for example, are based on the Intergovernmental Panel on Climate Change’s temperature forecast of a rise of some 10 degrees by 2100. Should things warm faster or go higher, the changes could come faster and be deeper or different than modeled. And many changes could simply be unpredictable.

Murray’s modeling shows that a large gap is opening in the boreal in Ontario and Quebec, a gap some 500 by 350 kilometers. It will essentially split the contiguous forest into two separate forests — one to the east, the other to the west — as the black spruce and white spruce disappear. That gap could then become grassland or Carolinian, the forest ecosystem that dominates south of the boreal in Ontario, characterized by oak, maple, black walnut and other deciduous species.

As the spruce goes, so goes its inhabitants. “We’re predicting there’s going to be no lynx, moose, or hare in central Ontario in the years to come,” Murray says, “so we’re going to have disjunct populations on either side.” It could cause a break between populations of species and a loss of genetic diversity.

Murray has coined a term for the patchy quality of the effects of warming on the boreal – climate fragmentation. “Because it’s a gradual transition, it won’t be real boreal and it won’t be real deciduous,” says Murray. “Ecosystems will likely be less stable and more open to invasives.”

Changes in snow cover are already driving changes. In some places snow covers the ground for shorter periods, and the conditions of the snow are also changing. Instead of three feet of fluffy snow all winter, temperatures rise and melt the snow and then get cold and freeze it and compact it into ice, again and again. In such conditions, lynx, which have thick, padded paws that serve as de facto snowshoes and allow them to hunt atop deep snow, loses it evolutionary advantage. “It affects their ability to survive,” says Murray. “Coyotes, on the other hand, are extremely plastic and flexible. They weren’t in the boreal a hundred years ago, but they are there now. They aren’t everywhere, because they don’t do well in deep snow, but that’s changing.” They are competing with lynx by foraging for their kills and preying on hares and squirrels.

The die-off of much of the boreal forest could have serious and unpredictable repercussions on the global climate system. When British Columbia’s lodgepole forests died, the province went from being a carbon sink to a carbon source because the dead forests released massive amounts of stored CO2. If the spruce forests of the boreal disappear they could also alter climate systems by releasing CO2. Because they are dark, they also absorb heat, but a treeless, snow-covered landscape would reflect far more solar energy back into the atmosphere.

Knowing what could happen in the next 50 or 60 years, and where, is important for conservation and other strategies. The boreal, especially in Canada, isn’t just a forest. It’s woven into the fabric of the region’s life and culture. Hundreds of First Nations call the boreal home, and they depend on it for sustenance, hunting moose and caribou, picking berries, and fishing. Paper and lumber industries also depend on a healthy boreal.

The research NASA is doing will be used to help determine how communities and land managers in the boreal might adapt to changes. Dall sheep in the Yukon, for example, are in decline and a subject of one of NASA’s studies. The results will be given to provincial and federal officials in Canada to make decisions on issues ranging from hunting to access to land management.

Meanwhile, in the hope of slowing the thawing of boreal permafrost, the International Boreal Conservation Campaign has launched a “cold carbon storage campaign.” This initiative is aimed at managing the boreal in ways that keep as much as possible frozen — no road building atop it, for example, or the creation of new large scale carbon reserves.

But such measures can only do so much. The real decisions on the fate of the boreal are up to the world community. It’s not a good sign that when I contacted Canadian Forest Service scientist Sylvie Gauthier, the lead author of the Science paper that surveyed threats to the boreal, I was told by a public affairs officer that the interview would be on “deep background only” with no attribution to her or her employer. The federal government of Canada, facing some the world’s most serious climate change threats, has gone to great lengths to squelch any discussion of this issue.

Yet even as they redouble their research efforts, scientists know the ultimate answer is not about adaptation or more research, but a rapid reduction in global CO2 emissions, which so far has shown little hope of being achieved. NASA’s Griffith says the situation reminds him of his father, a small-town doctor. “Even when the patient ignored his advice over and over again, he would continue to treat the patient,” he said. “A lot of us in the climate and ecosystem world are finding ourselves dealing with a similar kind of problem.”