Changing mountain permafrost from the 1970s to today - comparing two examples from Niwot Ridge, Colorado Front Range, USA

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 sufficient older data 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 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. (i) Air photographs and survey stakes were used to compare modern surface conditions of a solifluction lobe with those in the past. Despite high resolution of the air photographs (0.3 m), the error of position after geo-rectification was higher (± 1.0 m) than average displacement rates of gelifluction lobes (10 mm∗a-1), rendering this approach unsuitable. Replication of a 1963 -1967 study of soil movement from 2006 - 2009 yielded average movement rates of 11.4 mm∗a-1 compared to 9.4 mm∗a-1 in the 1960s. (ii) Temperature profiles 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 profiles document a complex annual curve that includes several weeks of unfrozen conditions, this finding is in contrast with the permafrost conditions reported from the 1970s. (iii) Electric resistivity profiles on a gelifluction 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 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 survey to other areas on Niwot Ridge. (iv) A simple 1 D-heat flow model was driven by the annual temperature variations of 1972 and 2008, resulting in several weeks of unfrozen conditions at various depths but with temperatures close to freezing during the 1970s. 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 suggests that, except on wet gelifluction 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 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 streams.