<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//NLM//DTD Journal Publishing DTD v2.3 20070202//EN" "journalpublishing.dtd">
<article article-type="editorial" dtd-version="2.3" xml:lang="EN" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">
<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Earth Sci.</journal-id>
<journal-title>Frontiers in Earth Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Earth Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-6463</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">641710</article-id>
<article-id pub-id-type="doi">10.3389/feart.2020.641710</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Earth Science</subject>
<subj-group>
<subject>Editorial</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Editorial: Observational Assessments of Glacier Mass Changes at Regional and Global Level</article-title>
<alt-title alt-title-type="left-running-head">Thomson et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">Editorial: Observational Glacier Mass Change Assessments</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Thomson</surname>
<given-names>Laura</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="http://loop.frontiersin.org/people/690370/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Brun</surname>
<given-names>Fanny</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<uri xlink:href="http://loop.frontiersin.org/people/478482/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Braun</surname>
<given-names>Matthias</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<uri xlink:href="http://loop.frontiersin.org/people/233200/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zemp</surname>
<given-names>Michael</given-names>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
<uri xlink:href="http://loop.frontiersin.org/people/323131/overview"/>
</contrib>
</contrib-group>
<aff id="aff1">
<label>
<sup>1</sup>
</label>Department of Geography and Planning, Queen&#x2019;s University, <addr-line>Kingston</addr-line>, <addr-line>ON</addr-line>, <country>Canada</country>
</aff>
<aff id="aff2">
<label>
<sup>2</sup>
</label>Department of Physical Geography, Universit&#xe9; Grenoble Alpes, <addr-line>Saint Martin d&#x2019;H&#xe8;res</addr-line>, <country>France</country>
</aff>
<aff id="aff3">
<label>
<sup>3</sup>
</label>Department of Geography and Geosciences, Friedrich-Alexander-University of Erlangen-Nuremberg, <addr-line>Erlangen</addr-line>, <country>Germany</country>
</aff>
<aff id="aff4">
<label>
<sup>4</sup>
</label>Department of Geography, University of Zurich, <addr-line>Zurich</addr-line>, <country>Switzerland</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited and reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/215800">Michael Lehning</ext-link>, &#xc9;cole Polytechnique F&#xe9;d&#xe9;rale de Lausanne, Switzerland</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Laura Thomson, <email>L.Thomson@queensu.ca</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Cryospheric Sciences, a section of the journal Frontiers in Earth Science</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>01</day>
<month>02</month>
<year>2021</year>
</pub-date>
<pub-date pub-type="collection">
<year>2020</year>
</pub-date>
<volume>8</volume>
<elocation-id>641710</elocation-id>
<history>
<date date-type="received">
<day>14</day>
<month>12</month>
<year>2020</year>
</date>
<date date-type="accepted">
<day>23</day>
<month>12</month>
<year>2020</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2021 Thomson, Brun, Braun and Zemp.</copyright-statement>
<copyright-year>2021</copyright-year>
<copyright-holder>Thomson, Brun, Braun and Zemp</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<related-article id="RA1" related-article-type="commentary-article" xlink:href="https://www.frontiersin.org/researchtopic/9957" ext-link-type="uri">
<bold>Editorial on the Research Topic</bold>
<article-title> <bold>Observational Assessments of Glacier Mass Changes at Regional and Global Level</bold>. </article-title>
</related-article>
<kwd-group>
<kwd>glaciers</kwd>
<kwd>mass change</kwd>
<kwd>climate change</kwd>
<kwd>cryosphere</kwd>
<kwd>sea-level rise</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<p>Glaciers represent a measurable indicator of the spatial and temporal patterns of global climate variability. Those distinct from the Greenland and Antarctic Ice Sheets cover an area of approximately 706,000&#xa0;km<sup>2</sup> globally (<xref ref-type="bibr" rid="B3">RGI Consortium, 2017</xref>), with an estimated total volume of 170&#x20;&#xb1; 21&#x20;&#xd7; 10<sup>3</sup>&#xa0;km<sup>3</sup>, or 0.43&#x20;&#xb1; 0.06&#xa0;m of potential sea-level rise equivalent (<xref ref-type="bibr" rid="B1">Huss and Farinotti, 2012</xref>). Retreating and thinning glaciers are icons of climate change and affect the local hazard scenario, regional water resources and glacier runoff as well as changes in global sea&#x20;level.</p>
<p>Techniques for measuring and monitoring changes to glaciers over the last century have expanded from <italic>in situ</italic> point measurements of snow accumulation and ice ablation to large regional- and global-scale surveys employing remote sensing and modeling approaches, which have shaped our understanding of world-wide glacier changes. Today, the Gravity Recovery and Climate Experiment (GRACE) mission can be used to derive monthly regional mass changes (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00096">Wouters et&#x20;al.</ext-link>) and has proven to be particularly effective in detecting glacier mass changes over regions with extensive ice cover (Alaska, Canadian Arctic, Russian Arctic, Svalbard, Iceland, the Southern Andes, and High Mountain Asia). However, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00096">Wouters et&#x20;al.</ext-link> note that GRACE cannot resolve the signal from peripheral glaciers of the Greenland and Antarctic ice sheets and struggles to detect statistically significant signals in mountain ranges with smaller glacier covers due to weaker signals and relatively greater background noise and uncertainty.</p>
<p>Increasingly, geodetic mass-balance records are filling the spatial-scale gap between coarse-resolution gravimetry and point-based glaciological mass-balance records. Advances in digital elevation model creation, automation, and analysis from historic and contemporary sources are driving a notable increase in the availability of geodetic mass-balance records from around the world. Indeed, original works within this Research Topic alone represent 9,908 new geodetic mass balance contributions to the World Glacier Monitoring Service (WGMS) and the IPCC AR6, notably from regions of Greenland (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00035/full">Huber et&#x20;al.</ext-link>), the European Alps (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link>), Iceland (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00163/full">Belart et&#x20;al.</ext-link>), Northern Tien Shan (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00259/full">Kapitsa et&#x20;al.</ext-link>) and the Patagonian Andes (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00326">Falashi et&#x20;al.</ext-link>), as well as &#x223c;8,000 updated geodetic records from the Central Andes (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.530997/full">Ferri et&#x20;al.</ext-link>). Advances in digital photogrammetry and improved accuracies in the 3D alignment of elevation models have rekindled the scientific value inherent to historical maps, aerial photography and declassified spy satellite imagery as exemplified by <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00326">Falashi et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00035/full">Huber et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00163/full">Belart et&#x20;al.</ext-link>, and <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00259/full">Kapitsa et&#x20;al.</ext-link> These advances extend the temporal reach of geodetic mass-balance estimates, offering new life to early imagery and resulting in notably reduced uncertainties in geodetic mass-balance estimates. However, issues persist in high-elevation, snow-covered regions where low contrast between images results in data voids, leading researchers to devise strategies for void-filling (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00035/full">Huber et&#x20;al.</ext-link>, <xref ref-type="bibr" rid="B2">McNabb et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B5">Seehaus et&#x20;al., 2020</xref>).</p>
<p>The global collection of glaciological (<italic>in situ</italic>) mass-balance records available through WGMS and national monitoring programs provide important cross-validation for gravimetry (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00096">Wouters et&#x20;al.</ext-link>) and geodetic mass-balance methods (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00259/full">Kapitsa et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00156">Schuler et&#x20;al.</ext-link>). Furthermore, these glaciological mass-balance records capture interannual and seasonal mass balance amplitudes and expand our understanding of the regional climatologies (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00302/full">Braithewaite and Hughes</ext-link>) and glacier morphologies (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link>) responsible for the global and regional variability in glacier response.</p>
<p>While the collection of glaciological mass-balance measurements can be logistically demanding, new potential exists for continuous, remote monitoring of surface mass-balance components (accumulation and ablation) given advances in automated field instrumentation and data telemetry. This was demonstrated over a 10-year period (2009&#x2013;2019) by <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00251">Fausto et&#x20;al.</ext-link> using a pressure transducer assembly complemented by sonic ranging systems as part of the Program for Monitoring of the Greenland ice sheet (PROMICE). In the time of COVID-19 and restrictions on field-related travel, the automated measurement and remote-delivery of mass balance measurements and related climate variables is likely to become increasingly popular.</p>
<p>Creative approaches that bring together the more abundant geodetic mass-balance estimates, spanning multiple years, and the relatively small sample of glaciological mass-balance records, providing interannual variability, has also enabled the modeling of annual mass balances for glaciers lacking field observations (e.g. <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00163/full">Belart et&#x20;al.</ext-link>). It was demonstrated by <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link> that annual snowline altitudes in combination with multi-year geodetic records may alternatively provide the annual temporal variability signal required for the estimation of annual mass balances of unmonitored glaciers in the Alps. <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.523646/full">A&#xf0;algeirsd&#xf3;ttir et&#x20;al.</ext-link> revealed that the combination of multiple data sources can even support the estimation of mass-balance conditions dating back to the Little Ice Age (estimated as &#x223c;1890 for Iceland).</p>
<p>Despite these advances, challenges remain regarding the estimation of glacier mass changes. While glaciological mass balances remain an invaluable indicator of temporal variability and important ground-validation for remote sensing and modeling techniques, several studies acknowledge the need for caution using these records. For instance, a slight negative bias has been found to be associated with glaciological records that may be attributed to the relatively shallower slopes of monitored glaciers (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link>), or due to bias in stake distribution across elevations (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00259/full">Kapitsa et&#x20;al.</ext-link>). <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00156">Schuler et&#x20;al.</ext-link> also highlight the need to extend glaciological mass-balance efforts into under-sampled, often logistically challenging areas to reduce regional biases. In high-latitude and highly glacierized regions, the greater potential for surge dynamics, calving, as well as internal accumulation by melt and refreezing in cold accumulation zones adds uncertainty to mass balance estimates (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00156">Schuler et&#x20;al.</ext-link>). In low-latitude and arid regions, challenges associated with the monitoring of debris-covered ice and rock glaciers persist (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.530997/full">Ferri et&#x20;al.</ext-link>), however the consideration and inclusion of these features in glacier inventories have proven to be particularly valuable in drought-susceptible regions where ice-rock complexes serve as important water resources (<xref ref-type="bibr" rid="B4">Schaffer et&#x20;al., 2019</xref>).</p>
<p>Independent of technique, the original works presented in this Research Topic indicate mass loss to be the dominant signal observed from glaciers distinct from the ice sheets through the 19th, 20th and early twenty-first centuries. Ice mass changes detected by GRACE over regions with extensive ice coverage confirm widespread losses but cannot detect an acceleration in these losses over the period of record (2002&#x2013;2016; <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00096">Wouters et&#x20;al.</ext-link>). Geodetic mass-balance surveys spanning multiple epochs, including the pre-satellite era, generally demonstrate greater thinning rates toward the end of the 20th century (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00326">Falaschi et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00149/full">Davaze et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.523646/full">A&#xf0;algeirsd&#xf3;ttir et&#x20;al.</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00302/full">Braithwaite and Hughes</ext-link>, <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00156">Schuler et&#x20;al.</ext-link>), with the exception of northern Tien Shan where very little difference in change rates was observed between 1958&#x2013;1998 and 1998&#x2013;2016 periods (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00259/full">Kapitsa et&#x20;al</ext-link>.). Interestingly, in some instances these studies note a slight decline in thinning rates since &#x223c;2010, including in regions of the Patagonian Andes (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00326">Falaschi et&#x20;al.</ext-link>) and Iceland (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00163/full">Belart et&#x20;al.</ext-link>). The large variability observed in the seasonal and annual mass-balance signal (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00302/full">Braithwaite and Hughes</ext-link>; <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2019.00096">Wouters et&#x20;al.</ext-link>) highlights the need for persistent efforts at all scales of glacier monitoring and continued &#x201c;openness and generosity with hard-won data&#x201d; within the glaciological community (<ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/feart.2020.00302/full">Braithwaite and Hughes</ext-link>).</p>
<sec id="s1">
<title>Author Contributions</title>
<p>The editorial was drafted by LT, with contributions from MZ, MB, and FB. The scope and contents of the editorial were discussed among all authors.</p>
</sec>
<sec sec-type="COI-statement" id="s2">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
</body>
<back>
<ack>
<p>The present Research Topic is a contribution to the working group on Regional Assessments of Glacier Mass Change (RAGMAC) of the International Association of Cryospheric Sciences (IACS; <ext-link ext-link-type="uri" xlink:href="https://cryosphericsciences.org/activities/wg-ragmac/">https://cryosphericsciences.org/activities/wg-ragmac/</ext-link>). It was enabled by support from the Federal Office of Meteorology and Climatology MeteoSwiss within the framework of the Global Climate Observing System (GCOS) Switzerland for&#x20;MZ.</p>
</ack>
<ref-list>
<title>References</title>
<ref id="B1">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Huss</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Farinotti</surname>
<given-names>D.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Distributed ice thickness and volume of all glaciers around the globe</article-title>. <source>J.&#x20;Geophys. Res.</source> <volume>117</volume>, <fpage>F04010</fpage>. <pub-id pub-id-type="doi">10.1029/2012JF002523</pub-id> </citation>
</ref>
<ref id="B2">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>McNabb</surname>
<given-names>R.</given-names>
</name>
<name>
<surname>Nuth</surname>
<given-names>C.</given-names>
</name>
<name>
<surname>K&#xe4;&#xe4;b</surname>
<given-names>A.</given-names>
</name>
<name>
<surname>Girod</surname>
<given-names>L.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Sensitivity of glacier volume change estimation to DEM void interpolation</article-title>. <source>Cryosphere</source> <volume>13</volume>, <fpage>895</fpage>&#x2013;<lpage>910</lpage>. <pub-id pub-id-type="doi">10.5194/tc-13-895-2019</pub-id> </citation>
</ref>
<ref id="B3">
<citation citation-type="journal">
<collab>RGI Consortium</collab> (<year>2017</year>). <comment>Randolph glacier inventory (v.6.0)</comment>. <article-title>A dataset of global glacier outlines. Global land ice measurements from space</article-title>. <pub-id pub-id-type="doi">10.7265/N5-RGI-60</pub-id> </citation>
</ref>
<ref id="B4">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Schaffer</surname>
<given-names>N.</given-names>
</name>
<name>
<surname>MacDonell</surname>
<given-names>S.</given-names>
</name>
<name>
<surname>R&#xe9;veillet</surname>
<given-names>M.</given-names>
</name>
<name>
<surname>Y&#xe1;&#xf1;ez</surname>
<given-names>E.</given-names>
</name>
<name>
<surname>Valois</surname>
<given-names>R.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Rock glaciers as a water resource in a changing climate in the semiarid Chilean Andes</article-title>. <source>Reg. Environ. Change</source> <volume>19</volume>, <fpage>1263</fpage>. <pub-id pub-id-type="doi">10.1007/s10113-018-01459-3</pub-id> </citation>
</ref>
<ref id="B5">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Seehaus</surname>
<given-names>T.</given-names>
</name>
<name>
<surname>Morgenshtern</surname>
<given-names>V. I.</given-names>
</name>
<name>
<surname>H&#xfc;bner</surname>
<given-names>F.</given-names>
</name>
<name>
<surname>B&#xe4;nsch</surname>
<given-names>E.</given-names>
</name>
<name>
<surname>Braun</surname>
<given-names>M. H.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Novel techniques for void filling in glacier elevation change data sets</article-title>. <source>Rem. Sens.</source> <volume>12</volume>, <fpage>3917</fpage>. <pub-id pub-id-type="doi">10.3390/rs12233917</pub-id> </citation>
</ref>
</ref-list>
</back>
</article>