Thick debris paradoxically controls the &#8216;anomalous&#8217; thinning of debris-covered glaciers in High Mountain Asia
Authors: Anderson Leif , Scherler Dirk 
Published: March 2020

&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;Thick debris &lt;/span&gt;&lt;span&gt;cover&lt;/span&gt;&lt;span&gt;, greater than about 5 cm, insulates ice and reduces melt rates. &lt;/span&gt;&lt;span&gt;Despite this melt-suppressing effect, &lt;/span&gt;&lt;span&gt;glaciers &lt;/span&gt;&lt;span&gt;often thin rapidly under thick debris cover&lt;/span&gt;&lt;span&gt;. In High Mountain Asia, the European Alps, and Alaska many debris-covered and debris-free glacier tongues are thinning at similar rates &lt;/span&gt;&lt;span&gt;(e.g., K&amp;#228;&amp;#228;b et al., 2012)&lt;/span&gt;&lt;span&gt;. This apparent paradox is known as the &amp;#8216;debris-cover anomaly&amp;#8217; (Pellicciotti et al., 2015). Two mechanisms have been proposed to explain this behavior, &lt;/span&gt;&lt;span&gt;which are not mutually exclusive&lt;/span&gt;&lt;span&gt;. First, glacier thinning under thick debris is enhanced by melt hotspots (lakes, ice cliffs, &lt;/span&gt;&lt;span&gt;and streams&lt;/span&gt;&lt;span&gt;) within otherwise continuous debris cover. Second, the decline in ice flow from upglacier leads to thinning under thick debris (e.g., Vincent et al., 2016). &lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;W&lt;/span&gt;&lt;span&gt;e &lt;/span&gt;&lt;span&gt;propose &lt;/span&gt;&lt;span&gt;a new &lt;/span&gt;&lt;span&gt;mechanis&lt;/span&gt;&lt;span&gt;m &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;expla&lt;/span&gt;&lt;span&gt;in&lt;/span&gt;&lt;span&gt; why thinning amplifies under thick debris&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;It appears &lt;/span&gt;&lt;span&gt;that&lt;/span&gt;&lt;span&gt; debris &lt;/span&gt;&lt;span&gt;cover&amp;#8212;&lt;/span&gt;&lt;span&gt;through its &lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;ffect&lt;/span&gt;&lt;span&gt; on the melt &lt;/span&gt;&lt;span&gt;pattern&lt;/span&gt;&lt;span&gt;&amp;#8212;&lt;/span&gt;&lt;span&gt;controls &lt;/span&gt;&lt;span&gt;glacier &lt;/span&gt;&lt;span&gt;geometry &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;i.e., patterns of &lt;/span&gt;&lt;span&gt;ice &lt;/span&gt;&lt;span&gt;thickness and &lt;/span&gt;&lt;span&gt;surface &lt;/span&gt;&lt;span&gt;slope)&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;A characteristic&lt;/span&gt; &lt;span&gt;debris-perturbed &lt;/span&gt;&lt;span&gt;driving stress pattern &lt;/span&gt;&lt;span&gt;results which&lt;/span&gt; &lt;span&gt;in turn&lt;/span&gt;&lt;span&gt; control&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; where dynamical thinning amplifie&lt;/span&gt;&lt;span&gt;s, &lt;/span&gt;&lt;span&gt;often in the upper reaches of debris-covered tongue&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;Our&lt;/span&gt;&lt;span&gt; explanation is supported &lt;/span&gt;&lt;span&gt;with&lt;/span&gt; &lt;span&gt;data&lt;/span&gt; &lt;span&gt;from &lt;/span&gt;&lt;span&gt;a suite of&lt;/span&gt; &lt;span&gt;glaciers &lt;/span&gt;&lt;span&gt;in the Himalaya&lt;/span&gt; &lt;span&gt;and &lt;/span&gt;&lt;span&gt;with &lt;/span&gt;&lt;span&gt;simulations&lt;/span&gt;&lt;span&gt; from &lt;/span&gt;&lt;span&gt;a numerical debris-covered glacier model &lt;/span&gt;&lt;span&gt;responding to climate change&lt;/span&gt;&lt;span&gt; (Anderson and Anderson, 2016).&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In all numerical simulations, the zone of maximum glacier thinning initially occurs upglacier from the debris cover. This zone of maximum thinning then propagates downglacier into the debris-covered portion. We explain how this zone of maximum thinning can be spatially pinned and amplified at different locations relative to the terminus depending on debris thickness, bed slope, glacier size, and glacier topology. This seemingly paradoxical mechanism in which debris itself controls thinning under thick debris is further supported by an analysis of published &lt;span&gt;thinning data from glaciers across High Mountain Asia.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;