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"findings suggest that methane released from low-latitude permafrost clathrates therefore acted as a trigger and/or strong positive feedback for deglaciation and warming. Methane hydrate destabilization is increasingly suspected as an important positive feedback to climate change"

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Snowball Earth termination by destabilization of equatorial permafrost methane clathrate

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This paper reports the broadest range of oxygen isotope values yet measured in marine sediments in methane seeps in Marinoan deglacial sediments underlying a thin interval of carbonate; these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise.

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"But in a region where temperatures can dip to 40 degrees below zero Fahrenheit, the Zimovs say unusually high snowfall this year worked like a blanket, trapping excess heat in the ground. They found sections 30 inches deep—soils that typically freeze before Christmas—that had stayed damp and mushy all winter. For the first time in memory, ground that insulates deep Arctic permafrost simply did not freeze in winter."

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On January 16, 2019, a new global study published inNature Communications confirmed that permafrost is thawing quickly across much of the world. Between 2007 and 2016, permafrost temperature increased by 0.29 ± 0.12 °C globally. The greatest warming was seen in parts of Siberia, up to 0.93 °C. Significant warming was also seen in Antarctica, and less in mountain regions. In much of the Arctic ground temperature increased because of rising average air temperatures, while increased snow thickness in some areas also contributed to warming the ground underneath.

Nikita Zimov was teaching students to do ecological fieldwork in northern Siberia when he stumbled on a disturbing clue that the frozen land might be thawing far faster than expected.

Zimov, like his father, Sergey Zimov, has spent years running a research station that tracks climate change in the rapidly warming Russian Far East. So when students probed the ground and took soil samples amid the mossy hummocks and larch forests near his home, 200 miles north of the Arctic Circle, Nikita Zimov suspected something wasn't right.

In April he sent a team of workers out with heavy drills to be sure. They bored into the soil a few feet down and found thick, slushy mud. Zimov said that was impossible. Cherskiy, his community of 3,000 along the Kolyma River, is one of the coldest spots on Earth. Even in late spring, ground below the surface should be frozen solid.

Except this year, it wasn't.

Every winter across the Arctic, the top few inches or feet of soil and rich plant matter freezes up before thawing again in summer. Beneath this active layer of ground extending hundreds of feet deeper sits continuously frozen earth called permafrost, which, in places, has stayed frozen for millennia.

But in a region where temperatures can dip to 40 degrees below zero Fahrenheit, the Zimovs say unusually high snowfall this year worked like a blanket, trapping excess heat in the ground. They found sections 30 inches deep-soils that typically freeze before Christmas-that had stayed damp and mushy all winter. For the first time in memory, ground that insulates deep Arctic permafrost simply did not freeze in winter.

"This really is astounding," says Max Holmes, an Arctic scientist with Woods Hole Research Center in Massachusetts.

The discovery has not been peer-reviewed or published and represents limited data from one spot in one year. But with measurements from another scientist nearby and one an ocean away appearing to support the Zimovs' findings, some Arctic experts are weighing a troubling question: Could a thaw of permafrost begin decades sooner than many people expect in some of the Arctic's coldest, most carbon-rich regions, releasing trapped greenhouse gases that could accelerate human-caused climate change?

Already, three of the last four years have been earth's hottest on record, with 2018 on schedule to be number four. And the poles are actually warming far faster, with areas 300 miles north of the Arctic Circle in Norway reaching 90 degrees Fahrenheit this July. If significant quantities of permafrost start thawing early, that would only make things worse.

"This is a big deal," says Ted Schuur, a permafrost expert at Northern Arizona University. "In the permafrost world, this is a significant milestone in a disturbing trend-like carbon in the atmosphere reaching 400 parts per million."

Crossing a Threshold

Nearly a quarter of the Northern Hemisphere's landmass sits above permafrost. Trapped in this frozen soil and vegetation is more than twice the carbon found in the atmosphere.

As fossil-fuel burning warms the Earth, this ground is thawing, allowing microbes to consume buried organic matter and release carbon dioxide and shorter-lived methane, which is 25 times as potent a greenhouse gas as CO2.

Permafrost temperatures across the Arctic have been rising since at least the 1970s-so much that small-scale localized thawing is already underway in many places. But the vast majority of this frozen land is still insulated by an active layer of freezing and thawing ground above it.

Now signs are emerging that the annual freeze-up can quickly change.

Eleven miles downriver from where the Zimovs' started their drilling, Mathias Goeckede with Germany's Max Planck Institute for Biogeochemistry spends weeks each summer traversing crumbling boardwalks over spongy Siberian ground. He tracks carbon exchange between the earth and the atmosphere.

Measurements at his site show that snow depth there has roughly doubled in five years. When excessive snow smothers the ground, warmth below the surface may not dissipate during winter. Data from a drill hole on Goeckede's site appears to capture that phenomenon: In April, temperatures 13 inches below ground there increased roughly 10 degrees Fahrenheit in that same five-year period.

"This is just one site, and it's just five years, so this really should be considered just a case-study," Goeckede says. "But if you assume it's a trend or that it might continue like this, then it's alarming."

Thousands of miles away, Vladimir Romanovsky saw something similar. Romanovsky, a permafrost expert at the University of Alaska, Fairbanks, runs some of the most extensive permafrost monitoring sites in North America, with detailed records going back 25 years, and in some cases longer.

"For all years before 2014, the complete freeze-up of the active layer would happen in mid-January," he says. "Since 2014, the freeze-up date has shifted to late February and even March."

But this winter, Fairbanks, too, saw extremely heavy snow. And for the first time on record, the active layer at two of Romanovsky's sites didn't freeze at all.

"This is really a very important threshold," he adds.

Reasons to Be Skeptical

Of course, Arctic weather is famously variable. A few years of heavy snow in some regions could give way quickly to a long stretch of dry cold years.

Some scientists are also torn about work by the Zimovs, which isn't as rigorous as many western researchers are accustomed to. The Zimovs' findings didn't include temperature data, nor could they point to long-term records. Many of the sites they examined also had been disturbed by human activity or non-native animals, which makes soil more susceptible to warming.

"Digging holes in a handful of places is hardly rigorous science," says Matt Sturm, a snow expert at the University of Alaska, Fairbanks.

Charles Koven, a permafrost expert at Lawrence Berkeley National Laboratory, sees cause for skepticism and more research. "I don't know what to think without knowing more about the history of these sites," he says. "On the other hand, we don't want to ignore warning signs if they're there."

What's more, compared to Romanovsky and Goeckede, who are measured and methodical researchers, Sergey Zimov is something of a catastrophist-philosopher, who leans toward pessimistic projections and grand gestures. He and his son are the pair behind Pleistocene Park, a region in their stretch of Siberia roamed by imported large mammals, from bison to yaks and horses. It is part of an experiment to mimic the mammoth steppe ecosystem that ended 12,000 years ago to see how permafrost responds.

At the same time, Sergey Zimov was also one of the first scientists anywhere to show that Siberia contains enormous reserves of especially carbon-rich permafrost. And he has worked in Cherskiy for more than 40 years and is held in high regard by many researchers.

"He knows that landscape so well that he is very rarely wrong," says Katey Walter Anthony, an associate professor at the University of Alaska, Fairbanks, who studies methane in Arctic lakes. "For him to believe a process is important is valuable."

Romanovsky knows the Zimovs, too, and says that while he wishes their work included temperature data, checking freeze depth is a sound approach. "That's still a convincing method," Romanovsky says. "For me, it just means it's not 100 percent."

It's also not clear how widespread a region Romanovsky's and the Zimovs' findings represent. It is a small sample size.

But Romanovsky says his sites were chosen because they fairly represent central Alaska.

"So, we assume that freeze-up didn't happen this winter within large areas in the Alaskan Interior," he says.

And even scientists uncomfortable with the limited data say the possibility that something so fundamental could change so quickly gives them pause.

"It's worrisome," says Sue Natali, a permafrost expert, also with Woods Hole, who saw an active layer not re-freeze recently during a research trip to Alaska's Yukon region. "When we see things happening that haven't happened in the lifetime of the scientists studying them, that should be a concern."

The stakes are high. If a region's active layer stops freezing consistently, consequences can be swift. Once unfrozen, soil microbes in the active layer can decompose organic material and release greenhouse gases year-round-not just in summer. And it exposes permafrost below to more heat so that layer, too, can begin thawing and releasing gases.

In ice-rich soils, such as in Siberia, the ground may slump. That can buckle roads and buildings and cause ice cellars to collapse. Such depressions also alter the landscape by forming troughs and bowls where snow can accumulate, making the ground even warmer in winter. Those troughs can fill with rain and snowmelt, forming new wetlands and tundra lakes, both of which expel large amounts of methane.

And the movement of all this water, above and below ground, can transport large amounts of heat, hastening thawing. Permafrost collapse can begin feeding on itself, releasing more greenhouse gases, which fuel more warming.

No one expects permafrost will ever release all its stored carbon. Most models suggest just 10 to 20 percent at most would escape even at high human emissions scenarios.

But more than a dozen Arctic climate scientists contacted by National Geographic agree that this year's active-layer data highlights the limitations of global climate models. The sophisticated computer programs that forecast future climate scenarios often used by government decision-makers simply can't capture major changes in permafrost.

"When we simulate these things there are a number of processes the models don't include-processes that multiply the transfer of heat," says Daniel Fortier, an associate professor of geography with the University of Montreal. "I think it's safe to say that things are happening faster than we were expecting."

For example, scientists have long known that loss of sea ice and rising temperatures will lead to more Arctic snow over time, which models are able to incorporate. But those same simulations are far less reliable when trying to track the cascading shifts in soil types, surface vegetation, melting ice, and the flow of water that will come from rising temperatures and all that snow, all of which could substantially hasten permafrost thaw.

"The models can't handle those landscape-scale changes, all of the processes that could lead to rapid change," says David Lawrence, a permafrost modeler with the National Center for Atmospheric Research in Boulder. "And it's going to be a long time before they can."

By the time some changes are detected, a significant transition may be underway, he says. That means the public and policymakers may not grasp the real risks.

"Most models don't project major carbon releases until beyond 2100," Walter Anthony says. That may be the case. But it's also possible, she says, that they "could actually happen in my children's lifetime-or my own."

This story was originally published on August 20, 2018, and was updated on January 16, 2019 with a note about a new study.

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Exclusive: Some Arctic ground no longer freezing-even in winter

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On January 16, 2019, a new global study published inNature Communications confirmed that permafrost is thawing quickly across much of the world. Between 2007 and 2016, permafrost temperature increased by 0.29 ± 0.12 °C globally. The greatest warming was seen in parts of Siberia, up to 0.93 °C. Significant warming was also seen in Antarctica, and less in mountain regions.

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As the global thermostat rises more and more permafrost will thaw, releasing planet heating emissions. With those planet heating emissions trapping more warmth, more permafrost will thaw releasing more carbon dioxide and methane into the atmosphere; trapping us in cycle where the output enhances the original stimulus.

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In Bethel, Alaska, walls are splitting, houses are collapsing, and the main road looks like a kiddy rollercoaster. In the coastal town of Kongiganak, sinking cemeteries prevent Alaskans from burying their dead in the ground. The village of Shishmaref, located on an island five miles from the western Alaska mainland, has eroded so much that it is contemplating total relocation. These communities are being plagued by permafrost that is thawing.

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Why Thawing Permafrost Matters

In Bethel, Alaska, walls are splitting, houses are collapsing, and the main road looks like a kiddy rollercoaster. In the coastal town of Kongiganak, sinking cemeteries prevent Alaskans from burying their dead in the ground. The village of Shishmaref, located on an island five miles from the western Alaska mainland, has eroded so much that it is contemplating total relocation. These communities are being plagued by permafrost that is thawing.

Permafrost is ground that remains frozen for two or more consecutive years. It is composed of rock, soil, sediments, and varying amounts of ice that bind the elements together. Some permafrost has been frozen for tens or hundreds of thousands of years.

Found under a layer of soil, permafrost can be from three feet to 4,900 feet thick. It stores the carbon-based remains of plants and animals that froze before they could decompose. Scientists estimate that the world's permafrost holds 1,500 billion tons of carbon, almost double the amount of carbon that is currently in the atmosphere.

Unfortunately, when permafrost warms and thaws, it releases carbon dioxide and methane into the atmosphere. As the global thermostat rises, permafrost, rather than storing carbon, could become a significant source of planet-heating emissions.

Permafrost is already thawing in some places, and if the problem spreads, scientists worry it could initiate a runaway process of global warming.
The Warming Arctic
Permafrost covers about 24 percent of the exposed landmass of the Northern Hemisphere-about 9 million square miles. It is found at high latitudes and high altitudes, mainly in Siberia, the Tibetan Plateau, Alaska, Northern Canada, Greenland, parts of Scandinavia and Russia. The continental shelves below the Arctic Ocean, which were exposed during the last ice age, also contain permafrost.

However, polar and high altitude regions are some the most climate-sensitive places on the planet. The Arctic is warming twice as fast as the rest of the planet, at a rate of temperature change that has not been observed in at least the last 2,000 years, according to the National Oceanic and Atmospheric Administration (NOAA). In 2016, annual average surface temperatures were 3.5 degrees Celsius warmer than they were at the start of the 20 th century. That year, permafrost temperatures in the Arctic were the warmest ever recorded.

In Alaska, permafrost temperatures have warmed as much as 2˚C in the last few decades. A recent study projects that with every 1˚C increase in temperature, 1.5 million square miles of permafrost could be lost through thawing.
When Permafrost Thaws
O. Roger Anderson, a biologist at the Earth Institute's Lamont-Doherty Earth Observatory, explained why permafrost releases carbon as it thaws.

The 'active layer' of soil on top of the permafrost, which may be two to 13 feet deep, thaws each summer and can sustain plant life. This layer releases carbon from the roots of plants that respire out CO2, and from microbes in the soil. Some microbes break down the organic matter into CO2. Others, called archaea, produce methane instead, when conditions are anaerobic-when the soil is saturated with water or no oxygen is available. Methane is 20 to 30 times more potent than carbon dioxide at exacerbating global warming, but it remains in the atmosphere for less time.

As permafrost thaws, the active layer deepens. The microbes become active and plant roots can penetrate further down, resulting in the production of more CO2. The amount of methane generated depends on how saturated the ground is.

Scientists don't know the relative proportions of carbon dioxide and methane emissions that might result from largescale thawing permafrost, said Anderson, because this has never happened in human history. However, research on the upper layer of the tundra (treeless plains overlying the permafrost) suggests that the average carbon dioxide emissions are about 50 times higher than those of methane.

"And we know that for every 10 degrees Celsius that the soil warms up, the emission of CO2 will double," said Anderson.

A 2017 study estimated that if global temperatures rise 1.5˚C above 1861 levels, thawing permafrost could release 68 to 508 gigatons of carbon. Without factoring in human activity, this carbon alone would increase global temperatures 0.13 to1.69˚C by 2300. Since we may have already locked in 1.5˚C of warming above pre-industrial levels, this amount of additional warming could result in catastrophic impacts of climate change.

Although a warmer Arctic could support more plants, and plants absorb carbon dioxide through photosynthesis, the new growth is projected to offset only about 20 percent of the permafrost's carbon release.
What Protects Permafrost
One thing that protects permafrost from the impacts of climate change is peat, the partly decayed vegetation that accumulates in water-saturated environments with no oxygen. Found in much of the low Arctic, peat can overlie or encompass the whole active layer or be frozen as permafrost.

Ben Gaglioti, a postdoctoral research scientist at Lamont-Doherty Earth Observatory, studied lake sediment records in northern Alaska to determine how much carbon the permafrost released in response to warming periods at the end of the last ice age. It turns out that the permafrost was much more sensitive-meaning it released more carbon-during past warming events, with gradually less response over time. In the most recent 150 years of warming, there has been relatively little response.

"Our hypothesis is that the buildup of organic matter or peat in the watershed has occurred because of a relatively warm and stable climate since the ice age," said Gaglioti. "That peat, which started accumulating around 13,000 years ago, does a great job at insulating the underlying ground from thaw, so we think it's buffering the permafrost."

Gaglioti's models show that the sensitivity of the underlying permafrost is highly dependent on the thickness of the peat. "The fate of the underlying permafrost rests in some ways on the stability of the peat layer," he said.
An Irreversible Cycle?
Wildfires can peel back the peat and make permafrost more sensitive to climate change-and they're expected to increase in tundra regions, Gaglioti said. Warmer and drier summers make vegetation more combustible. Warmer temperatures also bring about more thunderstorms and lightning strikes that can spark wildfires.

Fires not only release CO2 as they burn; afterwards, the blackened ground absorbs more solar radiation and warms further. And once fire removes the peat and vegetation that shades the ground, the landscape can become too well-drained to regenerate the peat.

Many scientists are concerned that thawing permafrost could be a tipping point that triggers an irreversible cycle: When permafrost releases its carbon as CO2 or methane, it will accelerate warming, which will then precipitate more permafrost thaw, and so on. There will be nothing humans can do to stop it.

The regions where permafrost is frozen year-round are already shifting northwards; and in some areas, the tundra now freezes later in the fall, allowing more time for microbes to decompose organic matter and for plants to respire.
Impacts of Thawing Permafrost
When the ice in permafrost melts, the ground becomes unstable and can slump, causing rock and landslides, floods and coastal erosion.

The ground has collapsed 280 feet deep in some parts of Siberia. The buckling earth can damage buildings, roads, power lines and other infrastructure.

It can also harm natural ecosystems. Thermokarst lakes-depressions formed when thawing permafrost collapsed and filled with meltwater-are important for wildlife and provide water to local communities. But if the underlying permafrost continues to thaw, lakes and wetlands can drain completely, destroying these biologically important resources.

As the sediments from landslides muddy streams and lakes, they affect plant life at the base of the food chain and potentially all the creatures that depend on it. Changes in the landscape can alter caribou breeding and migration patterns. And as the Arctic warms, beavers are moving northward. Their dams flood new areas, creating boggy stretches that allow for more warm water to thaw permafrost further.

Thawing permafrost can release more than carbon emissions. In 2016, a young boy died and dozens were hospitalized after contracting anthrax on the Yamal Peninsula in Siberia. An anthrax-infected reindeer carcass that froze 75 years earlier became exposed when the permafrost thawed. Anthrax spores entered the soil and water, and eventually the food supply, infecting the humans.

People and animals and their diseases have been frozen in the permafrost for hundreds of years, but bacteria and viruses can survive in permafrost for hundreds of thousands of years-scientists recently revived a 30,000-year-old virus that infects amoebas. Diseases like the Spanish flu, smallpox or the plague that have been wiped out might be frozen in the permafrost. As the Arctic warms, more activity, like mining for rare earth or precious metals, could potentially put us in contact with them again.

Building on permafrost is problematic, not only because the ground is unstable, but because the heat of buildings and pipes themselves can warm permafrost. Structures must be built on wood piles or based on thick gravel pads. Water and sewer pipes must be placed above ground. Some Bethel roads and airport runways are outfitted with liquid-filled pipes that transfer heat away from the permafrost, and the hospital has installed machines that keep the ground constantly refrigerated.

In Inuvik, a town in northwestern Canada, researchers are experimenting with new varieties of pilings to stabilize buildings, but they haven't yet found a perfect solution. They say it's hard to know what will work best because no one in the Arctic has ever experienced the rate of permafrost change that is occurring today.
Many Questions Remain
While the most recent Intergovernmental Panel on Climate Change's report acknowledged that permafrost is warming, its climate models did not account for these emissions when making climate projections.

That's because ultimately, how much the planet warms from thawing permafrost will depend on how much carbon is released, how quickly, and whether it is in the form of CO2 or methane. But to get a fuller understanding of this phenomenon and to be able to make more accurate climate projections, scientists need to be able to better assess the vulnerability of permafrost to thawing and its many consequences for the Arctic and the planet.

"Scientific research is so important in understanding," said Anderson. "We just can't predict from what's already known because we've never had permafrost thawing previously to this extent. It's only through more careful scientific research that we can answer these questions."

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In Bethel, Alaska, walls are splitting, houses are collapsing, and the main road looks like a kiddy rollercoaster. In the coastal town of Kongiganak, sinking cemeteries prevent Alaskans from burying their dead in the ground. The village of Shishmaref, located on an island five miles from the western Alaska mainland, has eroded so much that it is contemplating total relocation.

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The Earth's surface contains many forms of snow and ice, including sea, lake, and river ice; snow cover; glaciers, ice caps, and ice sheets; and frozen ground. Climate change can dramatically alter the Earth's snow- and ice-covered areas because snow and ice can easily change between solid and liquid states in response to relatively minor changes in temperature. This chapter focuses on trends in snow, glaciers, and the freezing and thawing of oceans and lakes.

Why does it matter?

Reduced snowfall and less snow cover on the ground could diminish the beneficial insulating effects of snow for vegetation and wildlife, while also affecting water supplies, transportation, cultural practices, travel, and recreation for millions of people. For communities in Arctic regions, reduced sea ice could increase coastal erosion and exposure to storms, threatening homes and property, while thawing ground could damage roads and buildings and accelerate erosion. Conversely, reduced snow and ice could present commercial opportunities for others, including ice-free shipping lanes and increased access to natural resources.

Such changing climate conditions can have worldwide implications because snow and ice influence air temperatures, sea level, ocean currents, and storm patterns. For example, melting ice sheets on Greenland and Antarctica add fresh water to the ocean, increasing sea level and possibly changing ocean circulation that is driven by differences in temperature and salinity. Because of their light color, snow and ice also reflect more sunlight than open water or bare ground, so a reduction in snow cover and ice causes the Earth's surface to absorb more energy from the sun and become warmer.

Summary of Key Points

Arctic Sea Ice. Part of the Arctic Ocean is covered by ice year-round. The area covered by ice is typically smallest in September, after the summer melting season. The annual minimum extent of Arctic sea ice has decreased over time, and in September 2023 it was the 5th smallest ever recorded. The length of the melt season for Arctic ice has grown, and the ice has also become thinner, which makes it more vulnerable to further melting.

Antarctic Sea Ice. Antarctic sea ice extent in September (annual maximum) and February (annual minimum) has increased slightly overall since 1979, though it has decreased in the last few years. Slight increases in Antarctic sea ice are outweighed by the loss of sea ice in the Arctic during the same time period.

Ice Sheets. Since 1992, the giant ice sheets that cover Greenland and Antarctica have each lost more than 100 billion metric tons of ice per year on average. The total amount of ice lost from 1992 to 2018 was enough to raise sea level worldwide by an average of roughly seven-tenths of an inch. This accounts for about one-quarter of the total observed sea level rise during that time period.

Glaciers. Glaciers in the United States and around the world have generally shrunk since the 1960s, and the rate at which glaciers are melting has accelerated over the last decade. The loss of ice from glaciers has contributed to the observed rise in sea level.

Arctic Glaciers. Rapid changes are occurring across the Arctic, where air temperatures are warming twice as fast as the global average temperature. Of the eight Arctic glaciers with long-term measurements, seven have steadily lost ice since the mid-20th century. The Arctic has been the dominant source of global sea-level rise since at least 1972.

A Closer Look: The Glaciers of Glacier National Park. Visitors from far and wide are drawn to Glacier National Park in Montana for its dramatic beauty and its glaciers. The total surface area of the 37 named glaciers in Glacier National Park decreased by about 34 percent between 1966 and 2015. Every glacier was smaller in 2015 than in 1966.

Lake Ice. Lakes in the northern United States are freezing later and thawing earlier compared with the 1800s and early 1900s. Freeze dates have shifted later at a rate of roughly half a day to one-and-a-half days per decade. All of the lakes studied were also found to be thawing earlier in the year, with spring thaw dates growing earlier by up to 24 days in the past 114 years.

Great Lakes Ice Cover. Parts of the Great Lakes typically freeze every winter. Since the early 1970s, all five of the Great Lakes have experienced a long-term decrease in the maximum area that freezes each year, but the decrease is only statistically meaningful in one lake (Superior). The number of frozen days per year has also decreased for all five lakes since the early 1970s.

Community Connection: Ice Breakup in Three Alaskan Rivers. Regions in the far north are warming more quickly than other parts of the world. Three long-running contests on the Tanana, Yukon, and Kuskokwim rivers in Alaska-where people guess the date when the river ice will break up in the spring-provide a century's worth of community-based evidence revealing that the ice on these rivers is generally breaking up earlier in the spring than it once did.

Snowfall. Total snowfall-the amount of snow that falls in a particular location-has decreased in most parts of the country since widespread records began in 1930. One reason for this decline is that nearly 80 percent of the locations studied have seen more winter precipitation fall in the form of rain instead of snow.

Snow Cover. Snow cover refers to the area of land that is covered by snow at any given time. Between 1972 and 2020, the average portion of North America covered by snow decreased at a rate of about 1,870 square miles per year, based on weekly measurements taken throughout the year. There has been much year-to-year variability, however. The length of time when snow covers the ground has become shorter by nearly two weeks since 1972, on average.

Snowpack. The amount of snow on the ground (snowpack) in early spring decreased at 93 percent of measurement sites in the western United States between 1955 and 2022. Across all sites, snowpack declined by an average of 23 percent during this time period. Peak snowpack is happening earlier in the season at the majority of sites, as higher temperatures cause snow to melt sooner. Snowpack season length decreased at about 86 percent of sites analyzed, decreasing by an average of more than 18 days.

Permafrost. About 80 percent of Alaska's land is underlain by permafrost, which refers to rock or soil with ice that stays frozen for two or more years. Between 1978 and 2020, permafrost temperatures increased at 14 out of 15 long-term monitoring sites in Alaska. Permafrost has warmed most quickly in northern Alaska. These increases in permafrost temperatures are driven mostly by long-term air temperatures and are consistent with changes observed in other parts of the Arctic.

Freeze-Thaw Conditions. The number of days per year with unfrozen ground has increased at an average rate of about four days per decade in both the contiguous 48 states and Alaska. Unfrozen days have generally increased across North America, with some variability by region.

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Climate Change Indicators: Snow and Ice | US EPA

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Polar and high altitude regions are some the most climate-sensitive places on the planet. Because of this, in 2016 average annual temperatures had risen 3.5 degrees celsius since the start of the 20th century.

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When permafrost warms and thaws, it releases carbon dioxide and methane into the atmosphere. As the global temperature rises, permafrost could become a significant source of planet-heating emissions.

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"Permanently" frozen ground existing in cold regions of high altitudes and latitudes.