Abstract

Melting mountain permafrost is reported from alpine areas around the world as a direct conse- Melting mountain permafrost is reported from alpine areas around the world as a direct consequence of rising air temperatures over the past decades. However, alpine sites that offer suf quence of rising air temperatures over the past decades. However, alpine sites that offer suffi cient older data cient older data to compare with recent conditions are rare. The study site Niwot Ridge, situated at ~ 3600 m a.s.l. in the to compare with recent conditions are rare. The study site Niwot Ridge, situated at ~ 3600 m a.s.l. in the Front Range of the Rocky Mountains, Colorado, USA, offers permafrost distribution data from the early Front Range of the Rocky Mountains, Colorado, USA, offers permafrost distribution data from the early 1970s. We used four different approaches to evaluate and compare the old data with recent conditions and to 1970s. We used four different approaches to evaluate and compare the old data with recent conditions and to
discuss consequences in how the old data should be considered. discuss consequences in how the old data should be considered.

(i) Air photographs and survey stakes were used to compare modern surface conditions of a (i) Air photographs and survey stakes were used to compare modern surface conditions of a soli sofl uction lobe with those in the past. Despite high resolution of the air photographs (0.3 m), uction lobe with those in the past. Despite high resolution of the air photographs (0.3 m), the error of position after geo-recti the error of position after geo-rectifi cation was higher ( cation was higher (1.0 m) than average displacement 1.0 m) than average displacement rates of geli rates of gelifl uction lobes (10 mm * a uction lobes (10 mm * a–1–1), rendering this approach unsuitable. Replication of ), rendering this approach unsuitable. Replication of a 1963 –1967 study of soil movement from 2006 – 2009 yielded average movement rates of a 1963 –1967 study of soil movement from 2006 – 2009 yielded average movement rates of 11.4 mm * a 11.4 mm * a–1–1 compared to 9.4 mm * a compared to 9.4 mm * a–1–1 in the 1960s. in the 1960s.

(ii) Temperature profiles of a three-year survey (2007– 2009) to depths of 7 m were compared with les of a three-year survey (2007– 2009) to depths of 7 m were compared with data from the 1970s from the same site. Modern temperature pro data from the 1970s from the same site. Modern temperature profi les document a complex an- les document a complex annual curve that includes several weeks of unfrozen conditions, this nual curve that includes several weeks of unfrozen conditions, this fi nding is in contrast with nding is in contrast with the permafrost conditions reported from the 1970s. the permafrost conditions reported from the 1970s.

(iii) Electric resistivity profi les on a geli les on a gelifl uction lobe, surveyed in different seasons during the year, uction lobe, surveyed in different seasons during the year, show the freezing front down to 2 m depth during the early winter, the melting process during show the freezing front down to 2 m depth during the early winter, the melting process during spring conditions and the complete melt of all ice lenses during the summer months. Geo- spring conditions and the complete melt of all ice lenses during the summer months. Geophysical results corroborate data from nearby temperature loggers and were used to extend the physical results corroborate data from nearby temperature loggers and were used to extend the survey to other areas on Niwot Ridge. survey to other areas on Niwot Ridge.

(iv) A simple 1 D-heat fl ow model was driven by the annual temperature variations of 1972 and ow model was driven by the annual temperature variations of 1972 and
2008, resulting in several weeks of unfrozen conditions at various depths but with tempera- 2008, resulting in several weeks of unfrozen conditions at various depths but with temperatures close to freezing during the 1970s. tures close to freezing during the 1970s. Our study documents that at present, on south facing slopes, permafrost neither exists at 2 m depth Our study documents that at present, on south facing slopes, permafrost neither exists at 2 m depth on wet sites nor at 4 m depth on dry sites as suggested during the 1970s. Our modeling approach further on wet sites nor at 4 m depth on dry sites as suggested during the 1970s. Our modeling approach further
suggests that, except on wet geli suggests that, except on wet gelifl uction lobes, it is likely that permafrost was not present at 3600 m during uction lobes, it is likely that permafrost was not present at 3600 m during 1970, if so, permafrost degradation on south facing slopes on Niwot Ridge was not driven by recent climate 1970, if so, permafrost degradation on south facing slopes on Niwot Ridge was not driven by recent climate change. However, north-facing slopes do cover permafrost, and certainly did in the 1970s, they are most change. However, north-facing slopes do cover permafrost, and certainly did in the 1970s, they are most probably affected by climate warming as already documented by a changing hydro-chemical signal in nearby probably affected by climate warming as already documented by a changing hydro-chemical signal in nearby
streams.

Citation

Leopold, M., Volkel, J., Dethier, D. P., and Williams, M.W. (2013): Changing mountain permafrost from the 1970s to today - comparing two examples from Niwot Ridge, Colorado Front Range, USA. Zeitschrift for Geomorphologie, Supplementary Issue. DOI: 10.1127/0372-8854/2013/S-00129

This Paper/Book acknowledges NSF CZO grant support.