Scientists long thought that the ground beneath the northern coasts of Alaska was permanently frozen. That was good news; permafrost stores large amounts of carbon, methane and other planet-warming gases, and coastal permafrost was thought to be a critical buffer against both global warming and coastal erosion.

That model could be very wrong. A new study documents an absence of permafrost along a coastal site in northeastern Alaska — and warns that coastal permafrost is more fragile than once thought. The study in the journal Science Advances documents efforts to map the subsurface of the Kaktovik Lagoon, a shallow bay at the edge of a large tundra underpinned by permafrost.

But electrical resistivity imaging of the beach and seafloor shows that both were ice-free to at least 65 feet.

That leaves the area vulnerable to coastal erosion, since the land near the shore is not protected by the freeze.

Another surprise was that the tundra, thought to be underpinned by a deep layer of permafrost, only had it down to about 16 feet.

Permafrost covers about 24 percent of the Northern Hemisphere’s exposed land surface and is defined as any rock or soil that remains at 32 degrees or below for two or more years. It covers about a third of Earth’s total coastline.

The researchers aren’t sure why the ice-free zone exists. Perhaps it never froze; perhaps it thawed abruptly.

Regardless of why it’s there, it has big implications for both local people and Earth. Indigenous communities that live nearby risk losing their homes. And, say the researchers, “permafrost stores an atmosphere’s worth of carbon.”

Helped along by a warming climate, permafrost melt could release even more greenhouse gases and fuel a vicious cycle of warming and melt. Arctic permafrost elsewhere is already melting. According to National Oceanic and Atmospheric Administration, it’s releasing between 300 million and 600 million tons of net carbon per year.

The research, which was carried out by scientists from the University of Texas at Austin, suggests that there’s more to learn about coastal permafrost — but with melting already underway, there’s no time to waste.