The Unstable Foundation
In Mohe, China's northernmost city, the ground is not permanent as it seems. The city is built on permafrost, which is ground that remains at or below 0°C for at least two consecutive years. This frozen layer is part of the vast Eurasian permafrost belt, and Mohe sits on its southern edge. Here, the permafrost is classified as "warm," with mean annual ground temperatures between -1.5°C and 0°C, making it particularly susceptible to changes in climate. The average thickness of the permafrost in this part of the Greater Khingan Mountains is around 47 meters.
One part of this system is the "active layer"—the topsoil that thaws during the summer and refreezes in the winter. The health of the underlying permafrost depends heavily on this layer. Monitoring has shown a clear trend: the active layer is getting deeper. This indicates that summer thaws are penetrating further into the ground, a process known as permafrost degradation. In undisturbed areas near Mohe, the permafrost table begins between 1.65 and 2.0 meters below the surface, but this is changing. The deepening of the active layer fundamentally alters the thermal balance that has kept the deeper ground frozen for millennia.
A Warming Region
The consequences of thawing permafrost are visible across Mohe and the surrounding region. As the ice-rich ground thaws, the land surface can collapse and sink, a process called thermokarst. This ground instability poses a direct threat to infrastructure. Buildings, roads, and pipelines that rely on the frozen ground for a solid foundation are at risk of damage. Studies show that urbanization exacerbates the problem; the "heat island" effect of a city, combined with heat from buildings, accelerates thawing. In the urban center of Mohe, the permafrost table has been measured at depths exceeding 15 meters, very different from the much shallower depths in the surrounding forests.
The local ecosystem is also responding to the thaw. The dominant tree species is the Dahurian Larch (Larix gmelinii), a tree uniquely adapted to the thin active layer with a shallow root system. While a deeper active layer might initially benefit some trees by providing more room for roots, it also leads to drier soil conditions and an unstable landscape, which can stress the forest. As the organic matter locked within the permafrost thaws, it begins to decompose, releasing greenhouse gases like carbon dioxide and methane into the atmosphere. This creates a feedback loop, where emissions from thawing permafrost contribute to further global warming.