PerspectiveClimate Change

Greenland Rumbles Louder as Glaciers Accelerate

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Science  24 Mar 2006:
Vol. 311, Issue 5768, pp. 1719-1720
DOI: 10.1126/science.1124496

Ice sheets are often assumed to respond slowly to climate change, with dynamic response times measured in centuries to millennia. On page 1756 of this issue, however, Ekström et al. (1) describe a dramatic increase in glacial seismicity over the past several years, which coincides with the acceleration of many of Greenland's major outlet glaciers (24). The rapidity of these changes counters the view of a sluggishly responding ice sheet and indicates that outlet glacier dynamics can respond swiftly to climate change with consequent increases in sea level. Over the past decade alone, glacier acceleration has increased Greenland's contribution to sealevel rise by more than 0.3 mm year−1 (2).

Using teleseismic data from 1999 and 2000, Ekström et al. (5) previously identified earthquakes associated with glacial flow—“glacialquakes.” Their more-recent observations cover more than a decade (1993 to 2005) and reveal a seasonal signal, with summer seismicity nearly five times greater than in winter. They also found a modest increase in seismicity in the late 1990s that was followed by a rapid increase from 2002 onwards, with 2005 producing nearly as many events as the combined total for 1993 through 1996.

The glacialquake magnitudes range from 4.6 to 5.1, yielding products of the displaced mass and slip of 0.1 × 1014 to 2.0 ×1014 kg m. This means that the Hellheim Glacier's 26 glacialquakes each resulted from roughly 0.16 to 3.7 m of slip if the displacement occurred along the full 14-km length of its fast-moving trunk, which has a mass of ∼6.1 × 1013 kg (6) and flows at a rate of ∼20 m day−1. This finding suggests that the glacialquakes make up only a small fraction of glacial motion.

The glacialquakes all originated on fastmoving (>2 km year−1) glaciers, with Greenland's three largest glaciers—Kangerdlugssuaq, Jakobshavn Isbrae, and Helheim—accounting for 72% of the events. Few glacialquakes were detected from Antarctica (5), possibly because the larger spatial extents of Antarctic ice streams produce slower events, with periods well beyond the 150-s detection threshold. For example, much of the Whillans Ice Stream moves primarily during twice-daily 0.5-m slip events lasting several minutes, with mass-slip products of roughly 3 × 1015 kg m (7).

On the move.

Retreat of Jakobshavn Isbrae since the Little Ice Age (10) marked by colored lines at different yearly intervals. (Inset graph) Summer (June, July, and August) temperatures (points) at several coastal stations shown in the silhouette map (9, 11). The solid lines are 5-year averages; color indicates measurement station.

The Kangerdlugssuaq, Jakobshavn Isbrae, and Helheim glaciers all accelerated by more than 50% over the period of increased seismicity, as have many of Greenland's smaller glaciers (24). The cause, seasonality, and the relation of the glacialquakes to these recent accelerations are not yet clear. Ekström et al. note that the drainage of summer melt to the bed through moulins (glacial conduits) may enhance lubrication at the bed to produce slip events. Other explanations may relate to the seasonal variation in calving, which for Jakobshavn Isbrae demonstrates an annual variability similar to that of the glacialquakes (8). Large calving events alone might yield mass displacements sufficient to produce glacialquakes. Alternatively, changes in glacier geometry after a calving event introduce a force imbalance, which may yield a slip event as a new force balance is established. Finally, the glacialquakes may be produced by stick-slip events that occur in the normal course of glacier sliding, with only brief (hours) periods of stick required to build enough elastic strain to produce detectable (magnitude >4.6) slip events.

The increased seismicity may be directly associated with the glacier accelerations if, for example, they are related to large calving events, which appear to precede acceleration (4). Alternatively, the increased incidence of glacialquakes may be a consequence of acceleration if seismicity scales with glacier speed. In either case, the ability to detect these events with teleseismic data provides a powerful new means for monitoring glacial activity. Furthermore, a new network of seismometers around Greenland might allow the detection of events on smaller glaciers, which may produce glacialquakes below the current detection threshold.

Although the reasons for increased incidence of glacialquakes and glacier acceleration are not clear, warming temperatures may be the underlying cause. Greenland undergoes regional warming and cooling (see the figure) on multidecadal times scales, with variability that exceeds global temperature trends (9). Summer temperatures from the late 1960s through mid-1990s tended to be cooler than average, which may have promoted glacier stability. For example, after decades of retreat, Jakobshavn Isbrae's calving front maintained its position during this cool period (8). Starting in about 1995, mean summer temperatures began to rise at coastal stations (9), reaching near-centurial highs. The correspondence between the rapid rise in temperature and the increased glacial activity (14) suggests that warming has a nearly immediate influence on glacier speeds. Although the duration of the recent warming is too short to determine whether it is an anthropogenic effect or natural variability, in either case, the data suggest that modest (∼1°C) changes in temperature can lead to large changes in discharge of glacial ice to the ocean. This sensitivity is not currently represented in ice-sheet models, which largely account for direct melt in response to climate change. Consequently, any further temperature increases may increase Greenland's contribution to sea level much more than anticipated.

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