On the response of rockglacier creep to surface temperature increase

摘要

Besides its thermal characteristics creeping mountain permafrost is substantially defined by its kinematics. Due to the - in general considerable - ice content of rockglaciers, their dynamics respond sensitively to climate forcing. Questions arise how rockglaciers react to the current or recent climatic changes, and what the further consequences of such reactions could be. Using a one-dimensional thermomechanically coupled numerical model we simulate the potential response of rockglacier creep to a change in surface temperature (Section 2). It turns out that variations in temperature could indeed affect rockglacier creep in the currently observed order of magnitude. Other influences, however, clearly act as well. Among these, the occurrence and complex influence of liquid water in the frozen material might be the most important factor for permafrost close to 0℃, though difficult to model. As a next step in this contribution, we plot globally observed rockglacier speeds against mean annual air temperature (Section 3). In fact, air temperature can be statistically identified as a major factor determining rockglacier speed. The remaining scatter clearly points to other influences such as slope, debris content, column thickness or liquid water. In a further step, we summarize current monitoring results on rockglacier speed (Section 4). A surprisingly large number of Alpine rockglaciers showed an increase in speed during recent years. This large number points to other than solely local influences, but rather to some regional-scale impact such as the observed increase in air temperatures. Our monitoring and modelling work clearly shows that rockglaciers with ground temperatures close to 0℃ creep in general faster than colder ones. Furthermore, our findings suggest that the creep of permafrost close to 0℃ is more sensitive to thermal forcing than the creep of colder one. From this, we conclude that increasing rockglacier temperatures may lead to a marked, but both spatially and temporally highly variable speed-up, before a significant loss of ice content by melt-out is able to reduce the deformation rate of the frozen mass towards its entire deactivation. By means of three scenarios, we exemplify the possible consequences of an increase in rockglacier temperature and subsequent acceleration: (1) increasing sensitivity of rockglacier creep to seasonal influences, (2) activation of so far stable frozen debris slopes, and (3) rockglacier destabilization.