Reading — Test 44

Reading passage

Permafrost—ground that remains at or below 0°C for at least two consecutive years—underlies roughly a quarter of the exposed land surface in the Northern Hemisphere, spanning much of Siberia, Alaska, northern Canada, and parts of the Tibetan Plateau. Contrary to its name, permafrost is not defined by the presence of ice, but strictly by temperature; a layer of soil, rock, or sediment qualifies as permafrost even if it contains little or no water. Where moisture is present, however, the frozen ground can trap ice in forms ranging from thin veins to massive wedges tens of meters thick, features that develop over centuries as water seeps into cracks and refreezes. Above this frozen layer lies the active layer, a zone from several centimeters to a few meters deep that thaws each summer and refreezes each winter, supporting the shallow-rooted vegetation characteristic of tundra and boreal ecosystems. The formation of permafrost depends on a delicate balance between the heat radiating from Earth's interior and the heat lost to the atmosphere above. In regions where mean annual air temperatures remain sufficiently low, the ground loses more heat during winter than it regains in summer, and a permanently frozen layer gradually develops downward from the surface. Some permafrost, particularly in parts of Siberia, extends more than a kilometer below the surface and appears to have formed during the Pleistocene, persisting through subsequent warm intervals because thick, frozen ground responds only slowly to changes in surface temperature. This thermal inertia means that permafrost can serve as a kind of archive, preserving ancient plant matter, pollen, and even the remains of Ice Age mammals in a state of arrested decay. Carcasses of woolly mammoths recovered from Siberian permafrost, for instance, have retained soft tissue for tens of thousands of years, providing paleontologists with genetic material otherwise unobtainable from fossils alone. Beyond its scientific value as a repository of the past, permafrost plays an active role in the present-day climate system, largely because of the organic carbon locked within it. Over thousands of years, dead plant material in cold environments decomposes only partially before being incorporated into the frozen ground, where further decay is halted almost entirely. Researchers estimate that the upper three meters of Northern Hemisphere permafrost alone contain roughly twice the amount of carbon currently present in the atmosphere. As long as this material stays frozen, it remains largely inert. But when permafrost thaws, microbial activity resumes, and the organic matter begins to decompose, releasing carbon dioxide and, in waterlogged conditions, methane—a gas with a substantially greater warming potential over shorter timescales. Because Arctic regions are warming at more than twice the global average rate, a trend often described as Arctic amplification, sustained thawing raises the possibility of a feedback loop in which released greenhouse gases contribute to further warming, which in turn accelerates additional thaw. The physical consequences of thaw extend well beyond atmospheric chemistry. When ice-rich permafrost melts, the ground it once supported can subside unevenly, a process known as thermokarst, producing depressions, collapsed hillslopes, and shifting lake basins. Infrastructure built atop permafrost—roads, pipelines, airstrips, and building foundations—was typically engineered on the assumption that the ground beneath would remain stable indefinitely. Communities across Alaska and Siberia have already reported cracked foundations, buckled roadways, and tilting structures as previously frozen soil loses its load-bearing integrity. Some engineers now recommend elevating buildings on adjustable pilings or incorporating passive cooling systems, such as thermosiphons, that draw heat away from the ground during winter without requiring external power. These adaptations, however, are costly and cannot easily be retrofitted across the vast networks of existing infrastructure in affected regions. Predicting the pace of permafrost thaw remains one of the more difficult challenges in climate science, partly because thaw does not proceed uniformly. Abrupt thaw, triggered by the formation of thermokarst lakes or rapidly eroding coastlines, can destabilize ground within a matter of years, whereas gradual thaw driven by rising mean temperatures may unfold over decades. Current climate models, many of which were developed primarily to represent atmospheric and oceanic processes, have historically struggled to incorporate these abrupt permafrost dynamics with precision. Refining such models is considered essential, since the trajectory of permafrost carbon release will influence, and be influenced by, the broader pace of global climate change in the coming century.