schools, cracked pipelines, and collapsed ice
cellars where Arctic hunters store walrus meat
and bowhead whale blubber. Warm summers
are already warping life for Arctic residents.
What the Zimovs were documenting in 2018,
though, was something different, with impli-
cations beyond the Arctic: a wintertime thaw.
The culprit, paradoxically, was heavy snow.
Siberia is dry, but for several winters before
2018, thick snow had
smothered the region.
The snow acted like a
blanket, trapping sum-
mer heat in the soil.
At a research site 11
miles from Cherskiy,
Mathias Goeckede
of Germany’s Max
Planck Institute for
Biogeochemistry found that snow depth had
doubled in five years. By April 2018 tempera-
tures in the active layer had risen 10 degrees
Fahrenheit.
The phenomenon wasn’t limited to Siberia.
Vladimir Romanovsky, a permafrost expert
at the University of Alaska Fairbanks, had for
years watched the active layer freeze completely
by mid-January at some 180 research sites in
Alaska. But as those places also faced a recent
period with heavy snow, the freezing slipped
first to February, then to March. In 2018, eight of
Romanovsky’s sites near Fairbanks and a dozen
on the Seward Peninsula, in western Alaska,
never fully froze at all.
Globally, permafrost holds up to 1,600 giga-
tons of carbon, nearly twice what’s in the atmo-
sphere. No one expects all or even most of that
to thaw. Until recently, researchers presumed
permafrost would lose at most 10 percent of its
carbon. Even that, it was thought, could take as
much as 80 years.
But when the active layer stops freezing in
winter, things speed up. The added warmth lets
microbes chomp organic material in the soil—
and emit carbon dioxide or methane—year-
round, instead of for just a few short months
each summer. And the winter warmth spreads
down into the permafrost itself, thawing it faster.
“A lot of our assumptions are breaking down,”
said Róisín Commane, an atmospheric chem-
ist at Columbia University who tracks carbon
emissions by airplane. She and her colleagues
have discovered that the amount of CO 2 coming
off Alaska’s North Slope in early winter has
increased by 73 percent since 1975. “We’ve been
trying to understand what’s going on in the Arc-
tic by relying on summer,” Commane said. “But
after the sun goes down—that’s when the real
story begins.”
A few snowy winters don’t make a trend; this
past winter there was less snow in Cherskiy, and
the soil cooled again considerably. Fairbanksalso got little snow. Yet at some of Romanovsky’s
sites in Alaska, the active layer again retained
enough heat to keep from completely freezing.
“This is truly amazing,” said Max Holmes,
deputy director of Massachusetts’s Woods
Hole Research Center, who has studied the
carbon cycle in both Alaska and Cherskiy. “I’ve
largely imagined permafrost thaw as a slow and
steady process, and maybe this is an odd five-
year period. But what if it’s not? What if things
change much more quickly?”ND WHAT IF THE CHANGE
becomes self-reinforcing—
as it already is, for exam-
ple, in the case of Arctic
sea ice? Sea ice reflects
the sun’s rays, keeping
the ocean below it cold.
But as sea ice melts, the
dark ocean absorbs that heat, which then melts
more ice.
As a rule, the tipping points at which such
feedback loops kick in are tricky to predict. “We
know there are thresholds we don’t want to cross,”
said Chris Field, director of Stanford University’s
Woods Institute for the Environment. “But we
don’t know precisely where they are.”
With permafrost, there’s just too much we
can’t see. It covers an area more than twice the
size of the United States, inhabited by half as
many people as New York City, in some of the
world’s least accessible terrain. Little of it is
monitored directly. Scientists instead studyTHE ZIMOVS FOUND SOMETHING DIFFERENT,
WITH IMPLICATIONS BEYOND THE ARCTIC:
A WINTERTIME THAW.
THE THREAT BELOW 81