Warming of the climate system is unequivocal, as is now evident from the wide range of observations—from the increase in global average air and ocean temperatures to the widespread melting of snow and ice in the polar regions to the global sea level rise. The cryosphere responds to these changes and provides clear signals about any alterations in the global climate; about 10% of the Earth’s surface is covered by glaciers, ice caps and ice sheets. The available data suggests that, as a function of the representative concentration pathways (RCPs), the world’s glaciers will continue to shrink.

Starting in 2003, with the discovery of globally detectable seismic signals (glacial earthquakes) caused by Greenland’s short-term glacial ice movements by Columbia University’s Göran Ekström and Meredith Nettles, the flourishing field of cryo-seismology has proved to be a powerful tool for studying a variety of glaciological phenomena, including crevasse formation, basal shear sources, iceberg calving, the rifting process in ice shelves, sea ice dynamics, precursory signs of unstable glaciers in real time, and beyond. These observations offer an invaluable foundation for understanding the short-term response of glaciers and ice sheets to a warming world. The techniques stand in sharp contrast to the traditional methods of previous decades, when glaciologists relied on photographs, satellite images, and direct measurements to document large-scale, long-term ice movements.

It is worth mentioning that this rapidly growing body of geophysical knowledge is also applicable to completely different environments like the icy moons of the solar system, such as Europa and Enceladus.

A recent review of cryoseismology published in Reviews of Geophysics by Evgeny A. Podolskiy of the Arctic Research Center (Hokkaido University, Sapporo) and Fabian Walter of Laboratory of Hydraulics, Hydrology and Glaciology (ETH Zürich) provides a timely and comprehensive reference for future directions and research frontiers on the subject, as well as a tutorial for anyone wanting to get into the rapidly developing field of cryoseismology.

Here the authors highlight the important results that have emerged from their research and some of the important questions that remain.

—Fabio Florindo, Editor, Review of Geophysics; email: [email protected]

Why is this topic timely and important?

Cryoseismology is an emerging and rapidly growing geophysical discipline. Numerous glaciological and seismological projects have recently been acquiring large volumes of new seismological data associated with dynamic processes in the cryosphere. In view of ongoing climate-induced cryosphere changes, seismology has therefore become a valuable monitoring tool. However, few research groups have fully focused on the interdisciplinary divide between glaciology and seismology. Members of the glaciological community are typically confronted with the challenge of mastering new analysis tools for treating, interpreting, and comparing seismic signals. On the other hand, glacier-related seismograms demand that seismologists usually working on tectonic seismicity quickly gain an overview of seismogenic processes in the cryosphere. Therefore, there has been a need for a “navigation map” to this interdisciplinary topic. Our review aims at meeting this need.

Moreover, seismic emissions in the cryosphere remain one of the least explored parts of the seismic wavefield. We hope that the paper will become: 1) a stepping stone for glaciologists and seismologists working on seismicity in the cryosphere, and 2) a bridge between two scientific disciplines, which will enhance scientific interaction and spark ideas for new research techniques. On one hand, the article provides what we call a “seismic portrait of the cryosphere”, i.e., an overview of ice-related processes, which generate seismic signals at much higher rates than tectonic environments. On the other hand, we present novel seismological methods to glaciologists studying ice dynamic processes in various contexts, such as Alpine glaciers, ice sheets, and fast flowing ice streams.

A video by the author of Icequakes and Glacier Calving, Bowdoin Glacier, Greenland.

What recent advances in seismology in particular are leading to a new understanding of glacial or ice sheet dynamics? Are we seeing any trends yet with the data that we have?

The analysis tools in glacier seismology have yet to catch up with their tectonic counterparts. High-resolution hypocenter locations, source process modeling, and a comprehensive picture of seismic background noise on ice sheets are still relatively new accomplishments in glacier seismology. On the other hand, glaciologists have yet to harness seismic interferometry, which over recent years has revolutionized the study of the solid Earth’s structure and its temporal changes. Glacier environments are subject to processes on rather small time scales, such as seasonal or diurnal changes of subglacial hydraulics beneath high-melt regions, unstable glacier collapses, or sudden slip episodes. Seismic interferometry could elucidate these processes in an unrivaled way by detecting and quantifying associated englacial and subglacial structural changes.

Glacier seismology is benefiting from improved seismometer installations in direct contact with the ice. Such deployments provide much cleaner data than networks installed on rock next to glacier ice. Moreover, real-time data communication via satellite or the portable phone network is making seismology a more and more effective monitoring tool.

How can we further improve the seismic study of glaciers and ice sheets? Are more local arrays needed? What major developments do you expect in the next few years?

With future improvements in sensor technology, expansion of polar networks, and data handling, we expect seismology to soon provide new key observations of ice dynamic processes, for which few or no alternative monitoring techniques exist. To our mind, a key step is to move the focus away from individual sites and instead investigate similar glacial bodies parallel, such as fast ice streams or mountain glaciers. This will produce seismic catalogues, reflecting processes that are characteristic for specific glacial environments.

Local on-ice arrays provide the high-quality data necessary to understand seismic source mechanisms particularly related to basal sliding, which is one of the most important but least understood forms of ice flow. Finally, long-term observations spanning multiple seasons can potentially target the evolution of basal hydraulic processes at a superior temporal and spatial resolution.

—Evgeny A. Podolskiy, Arctic Research Center, Hokkaido University, Sapporo; email: [email protected]