Rock glaciers, large tongue-shaped flows of frozen mountain debris, are significant indicators of permafrost conditions and serve as an essential climate variable. Approximately 11% of Earth's land surface is characterized by permafrost, with mountain permafrost areas being substantial. Changes in rock glacier velocity are sensitive to climate change; warmer frozen debris moves faster due to reduced viscosity and the effects of meltwater. While a limited number of long-term time series documenting these changes exist (mostly from the European Alps), North America lacked such systematic data despite the significant presence of rock glaciers. This research addresses this gap by creating long-term (60-70 years) time series of rock glacier speeds for 16 rock glaciers in the western contiguous United States, significantly expanding the global dataset. The study's importance lies in its contribution to understanding the impacts of climate change on permafrost dynamics and associated environmental changes in North America, a region with substantial permafrost coverage and a high number of active rock glaciers.

Existing research confirms the susceptibility of rock glacier velocities to climatic changes. However, long-term time series documenting these changes are scarce, with most located in the European Alps. While velocity measurements from North America exist for shorter time intervals, there was no systematic long-term data for the continent prior to this study. Previous studies highlight the influence of air temperature and snow cover on permafrost temperatures and rock glacier deformation rates. The reduced viscosity of warming frozen materials, and the impact of meltwater from snow and ground ice, are identified as key drivers of acceleration. However, the relationship between rock glacier motion and climate change is complex and influenced by various local factors such as material composition, slope, thickness, water content, and mass input from rockfalls and avalanches. This study aimed to expand the dataset of long-term rock glacier velocity changes, particularly in North America, to improve understanding of the complex interplay between climate change and periglacial processes.

The study utilized historical aerial photographs from the U.S. Geological Survey (USGS) archive to reconstruct 60-70-year timelines of surface speeds for 16 rock glaciers across the Rocky Mountains and the contiguous western United States. The selected rock glaciers spanned a range of latitudes and longitudes and showed clear signs of modern motion in Sentinel-1 radar interferograms. USGS airphotos from approximately the 2000s onward are ortho-rectified; older photos required ortho-rectification using camera calibration protocols and ground control points extracted from the most recent high-resolution USGS orthoimages and the Copernicus global DEM. Digital image matching was applied between pairs of orthoimages to measure horizontal surface displacements. To ensure accuracy and address the lower resolution and radiometric quality of older images, only measurement locations that yielded matches for all measurement periods were retained. Each individual time series was normalized by its average speed before summarization. Uncertainty in the results was expressed in three ways: 1) notches between time-step median speeds; 2) standard error of stable ground offsets after orthoimage co-registration; and 3) a statistical significance test for changes between time steps using the Wilcoxon rank sum test. Air temperature data was collected from NOAA and SNOTEL stations, and snow water equivalents were assessed from SNOTEL stations and ERA5 reanalysis. The researchers compared their findings with existing geodetic and photogrammetric measurements to validate their results.

The study found significant variations in average rock glacier speeds, ranging from 8 cm per year to 70 cm per year. Twelve out of the 16 rock glaciers showed statistically significant acceleration over the measurement period, with several exceeding twice their initial speed. One rock glacier even tripled its surface speed. The accelerations were generally consistent over time for some rock glaciers, while others exhibited a more rapid increase in recent decades. Deceleration, when observed, was slight. The observed rock glacier accelerations correlate strongly with the increase in air temperatures, particularly summer temperatures, in the region. High-elevation meteorological stations showed even more pronounced warming. Snow water equivalents showed less clear trends and greater spatio-temporal variability. Correlations between average speeds and static rock glacier attributes (from a previous inventory) were highest for temperature-driven factors such as mean annual maximum and mean annual temperatures. Correlations between speed change factors and static attributes were generally weak, with the highest correlation found with slope. Most speed time series showed high similarity to mean annual or winter temperatures, but the correspondence was less clear with summer temperatures. The relationship between rock glacier speed and precipitation was inconsistent. The acceleration factors observed are in line with theoretical predictions based on temperature-dependent ice deformation and a global set of rock glacier speeds. However, significant spatial variability in speed and speed changes highlights the complex influence of local factors.

The findings strongly suggest a link between the observed rock glacier acceleration and regional increases in air temperatures. This acceleration is likely driven by a decrease in the viscosity of the rock glacier material due to ground warming and the impact of meltwater. The significant spatial variability in speed and speed changes among individual rock glaciers underscores the complex interplay of various local factors, including slope gradient, thickness, material composition, water content, and mass input. While the study shows a strong correlation between air temperature rise and rock glacier acceleration, local factors must be considered for a complete understanding. The observed accelerations are consistent with trends on other continents, although some local rock glaciers exhibit acceleration factors at the higher end of the global spectrum. The contrast between the acceleration of rock glaciers and the typical slowing of mountain glaciers under atmospheric warming is noteworthy and likely reflects the different mechanisms governing their behavior. The absence of acceleration or deceleration in some rock glaciers despite regional warming might be linked to thickness changes or ground ice loss.

This study more than doubled the global number of long-term rock glacier velocity time series by providing the first such data for North America. The observed substantial acceleration of rock glaciers across the western contiguous U.S. over the past 60-70 years is strongly linked to increased air temperatures. The results indicate intensification of periglacial activity and potential long-term changes in ground temperatures, debris and ground ice fluxes, landscape development, mountain hazards, hydrology, and water resources. Further research is needed to investigate the specific roles of various local factors and to monitor ongoing changes in rock glacier dynamics and their implications for the environment.

The study is limited by the availability of suitable historical aerial photographs. The lower resolution and radiometric quality of older photographs resulted in a reduced number of successful displacement measurements. The study focused on a relatively limited number of rock glaciers, potentially limiting the generalizability of the findings to other regions. While the study considered air temperature and snow cover, other climatic variables could also be investigated for their influence on rock glacier speed changes. Further studies are needed to confirm these results and to investigate the role of other factors.