We’re speeding along a gravel spit north of Barrow, Alaska, plumes of dust rising behind our four-wheelers, the Arctic sea ice blindingly bright, when archaeologist Anne Jensen hits the brakes. “Hang on,” she mutters, jumping off the machine. “I think I saw something.”

Jensen stands with her hands on her hips, peering down at a human femur that’s lying on the gravel as if dropped from the sky. In fact, the opposite is true: Just as cycles of freezing and thawing can churn up new rocks in a garden each spring, so the tundra occasionally spits out long-buried artifacts. Kneeling next to the bone, Jensen estimates it’s a thousand years old.

In most places, the old is constantly replaced by the new, the bones and tools of our ancestors devoured by soil bacteria or washed into the sea. But in the Arctic, long winters, frozen soil and a dry climate have preserved remnants of the past in startling completeness. Jensen, senior scientist for the Ukpeaġvik Iñupiat Corporation, once unearthed an 800-year-old girl in Barrow whose skin, hair and stomach contents were perfectly intact. In comparison, she once tried to exhume a pair of draft horses in Pennsylvania that had been buried 70 years earlier and recovered only a bit of brass from their harnesses. “Not even tooth enamel — nothing!” she says, incredulous.

Courtesy Anne Jensen

The organic remains that linger in Alaska’s Arctic have allowed archaeologists to sketch a rough picture of early North Americans’ migration routes and daily lives. Yet because the summers are brief and the distances so vast, much remains undiscovered. Where did Paleo-Eskimos come from, for example, and why did they disappear? Why did some groups survive climatic swings while others died out?

Jensen has spent decades chasing such questions. There were always more sites to excavate than there was money or manpower, but she spent winters working contentedly in her lab, knowing that whatever she didn’t get to one summer could be explored the next. “If something stays frozen, it can be incredibly long-lived,” she explains. “But once permafrost goes away, that kind of stuff will rot in no time at all.”

Today, with climate change accelerating the rate of thawing permafrost and coastal erosion, archaeological sites across the Arctic are being exposed — “incredible amounts of information that we didn’t know existed 10 years ago,” Jensen says. But just as quickly as they emerge, the sites are lost again, rotting into earth or tumbling into the sea. Jensen is forced to perform triage, deciding which to ignore and which to save.

Gently wrapping the femur in her parka, Jensen scans the broken ice, which floats scattered across the ocean even in mid-June. The temperature is in the upper 30s and feels even colder, but it’s warm by Arctic standards: Every year, the shelves of floating ice form later and melt earlier, and 2015 is shaping up to be among the lowest sea-ice years on record.

With less ice to break their force, waves build up and storms hit the coast with greater ferocity, accelerating erosion. Craig Tweedie, an Arctic ecologist and colleague of Jensen’s, has transposed aerial photos from the 1940s onto modern maps and calculated that the coast where we’re now standing is eroding at a rate of 10 to 12 meters a year. Not only that, but storm frequency may be increasing, and sea levels, of course, are rising. “Anne is running in crisis mode all the time,” Tweedie says. “It’s like she’s literally trying to catch these things as they’re falling off the cliff.”

Florencia Mazza Ramsay

In 2013, Jensen was at a conference in Iceland, giving a presentation about how climate change is impacting Arctic archaeology, when she got an email: A group of ATVers from Barrow had found a sod house crumbling out of a bluff at an old hunting camp called Walakpa. Jensen had known that Walakpa existed — it was partially excavated in the 1960s —but the photos showed a more extensive, deeper and more intact site than anyone had imagined. Plus, because its layer-cake stratification spanned millennia, Walakpa had the potential to provide a unique record of daily life during the time when the Birnirk culture was dying out and the ancestors of today’s Inupiat rose to dominance. Had there been war? Some natural disaster? Or did one culture -gradually transition into the other?

As soon as she got home, Jensen rushed out to Walakpa. She found artifacts spilling out of the dirt: carved ivory harpoons, a baleen spoon, waterproof bags made of skin. There were whale skulls, walrus tusks and thousands upon thousands of bones left over from successful- hunts.

Jensen believed that artifacts from Walakpa could help determine whether the Birnirk people, who disappeared around 1000 A.D., were directly related to today’s Inupiat — which could strengthen the tribe’s argument for the repatriation of Birnirk remains from the Smithsonian. But she never got the chance to find out. Before she could secure funding from the National Science Foundation, a storm sent a 30-foot chunk of the site — including the oldest layers — crashing into the Chukchi Sea. “Three thousand years of pre-contact history, gone,” she says, shaking- her head.

As places like Walakpa disappear, we’re losing a record of not only ancient humans, but also the environment they lived in. Because of Walakpa’s location at the confluence of freshwater, marine and terrestrial hunting grounds, it offers an environmental record similar to an ice or bog core, but with samples from a wider variety of places — the next best thing to a time machine, Jensen says. Plant pollen, for example, can offer clues to how wet or dry the weather was. Walrus tusks reveal where the animals were feeding. Ring seals’ body size directly corresponds to how long they spend on ice as juveniles, so bigger seal bones indicate periods of more stable sea ice. Combined with existing climate reconstructions, this knowledge can help scientists create more reliable climate models: the better such models simulate the past, the more confidence we gain in their predictions for the future.

“Ancient people have left behind these signals to tell us what their environment was really like,” Tweedie says. “They could provide one of the only mechanisms we have to validate modern-day science.”

Out on the gravel spit, the wind is picking up and the temperature dropping. Jensen’s cheeks are red. Using a bungee cord, she secures the wrapped femur next to the polar bear gun on her ATV, and we start back toward town. On the way, Jensen thinks she sees another bone. But when we stop to look for it, there’s nothing there but shifting gravel and the endless wind.