Die gute Nachricht: Grönländische Schneefallmengen haben sich in den letzten 100 Jahren erhöht und gleichen einen Teil der Eisschmelze aus

Die im letzten Jahrhundert stark gestiegenen Temperaturen in Grönland setzen auch dem Inlandeis zu, das an den Rändern heftig schmilzt. Was in der ganzen Diskussion gerne vergessen wird, ist der Import-/Export-Charakter der Eisbilanz. Auf der einen Seite wird Eis durch Schmelzen und Eisbergabbrüche verloren, auf der anderen Seite kommt aber auch immer neuer Schnee durch Niederschläge hinzu. Und gerade in Punkto Schnee-/Eiszufuhr hat sich in den letzten 100 Jahren kräftig etwas verändert. Laut einer Studie von Sebastian Mernild und Kollegen, die im Februar 2015 im International Journal of Climatology erschien, haben die Niederschläge in Grönland seit 1890 spürbar zugenommen. Zudem lassen atlantische Ozeanzyklen (NAO, AMO) die Schneefälle im Maßstab von mehreren Jahrzhenten oszillieren. Hier die Kurzfassung der spannenden Arbeit, über die sich die Presse jedoch leider ausschwieg:

Greenland precipitation trends in a long-term instrumental climate context (1890–2012): evaluation of coastal and ice core records
Here, we present an analysis of monthly, seasonal, and annual long-term precipitation time-series compiled from coastal meteorological stations in Greenland and Greenland Ice Sheet (GrIS) ice cores (including three new ice core records from ACT11D, Tunu2013, and Summit2010). The dataset covers the period from 1890 to 2012, a period of climate warming. For approximately the first decade of the new millennium (2001–2012) minimum and maximum mean annual precipitation conditions are found in Northeast Greenland (Tunu2013 c. 120 mm water equivalent (w.e.) year−1) and South Greenland (Ikerasassuaq: c. 2300 mm w.e. year−1), respectively. The coastal meteorological stations showed on average increasing trends for 1890–2012 (3.5 mm w.e. year−2) and 1961–2012 (1.3 mm w.e. year−2). Years with high coastal annual precipitation also had a: (1) significant high number of precipitation days (r2 = 0.59); and (2) high precipitation intensity measured as 24-h precipitation (r2 = 0.54). For the GrIS the precipitation estimated from ice cores increased on average by 0.1 mm w.e. year−2 (1890–2000), showing an antiphase variability in precipitation trends between the GrIS and the coastal regions. Around 1960 a major shift occurred in the precipitation pattern towards wetter precipitation conditions for coastal Greenland, while drier conditions became more prevalent on the GrIS. Differences in precipitation trends indicate a heterogeneous spatial distribution of precipitation in Greenland. An Empirical Orthogonal Function analysis reveals a spatiotemporal cycle of precipitation that is linked instantaneously to the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation and with an ∼6 years lag time response to the Greenland Blocking Index.

Ein ähnliches Ergebnis wurde im April 2014 im Journal of Glaciology auch von einer Gruppe um Robert Hawley berichtet. Entlang einer Traverse fanden sie eine Zunahme des Schneefalls von 10% während der letzten 52 Jahre. Die Autoren schlussfolgern, dass die wärmere Luft eine erhöhte Aufnahmefähigkeit von Wasserdampf ermöglicht, die sich nun offenbar in gesteigerten Schneemengen widerspiegelt. Hier die Kurzfassung der Studie:

Recent accumulation variability in northwest Greenland from ground-penetrating radar and shallow cores along the Greenland Inland Traverse
Accumulation is a key parameter governing the mass balance of the Greenland ice sheet. Several studies have documented the spatial variability of accumulation over wide spatial scales, primarily using point data, remote sensing or modeling. Direct measurements of spatially extensive, detailed profiles of accumulation in Greenland, however, are rare. We used 400 MHz ground-penetrating radar along the 1009 km route of the Greenland Inland Traverse from Thule to Summit during April and May of 2011, to image continuous internal reflecting horizons. We dated these horizons using ice-core chemistry at each end of the traverse. Using density profiles measured along the traverse, we determined the depth to the horizons and the corresponding water-equivalent accumulation rates. The measured accumulation rates vary from ∼0.1 m w.e.a–1 in the interior to ∼0.7 m w.e.a–1 near the coast, and correspond broadly with existing published model results, though there are some excursions. Comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10% increase in accumulation rates over the past 52 years along most of the traverse route. This implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of Greenland.

Es lohnt sich also durchaus, in die jüngere Klimageschichte Grönlands hineinzuschauen. Nicht alles sieht so düster aus, wie es einem viele Medien vorgaukeln. Die österreichische Tageszeitung Der Standard berichtet vergleichsweise ausführlich über das Klimathema. Erfreulicherweise finden immer wieder auch relativierende Studien den Weg in das Blatt. Am 16. Dezember 2015 berichtete Der Standard über den Versuch, die grönländische Eisschmelze für die letzten 100 Jahre zu rekonstruieren:

Eisverlust Grönlands mit historischen Luftaufnahmen rekonstruiert
[...] Nun gelang es einem internationalen Forscherteam um Kristian Kjeldsen und Kurt Kjær von der Universität Kopenhagen, die gesicherte Datenlage ein gutes Stück zu erweitern: In ihrer Studie in “Nature” zeichnen die Wissenschafter den Eisverlust Grönlands seit Beginn des 20. Jahrhunderts unter Zuhilfenahme historischer Fotografien nach. Dazu werteten sie Luftaufnahmen aus den 1970er- und 1980er-Jahren mit modernen fotogrammetrischen Methoden aus. Kombiniert mit späteren und heutigen Messdaten kartierten sie die jeweilige Gletscherausbreitung und errechneten das Massevolumen und dessen Entwicklung in den vorangegangenen Jahrzehnten.

Ganzen Artikel im Standard lesen.

Es handelt sich dabei um einen Artikel von Kjeldsen et al. (2915), der in Nature erschien:

Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900
The response of the Greenland Ice Sheet (GIS) to changes in temperature during the twentieth century remains contentious1, largely owing to difficulties in estimating the spatial and temporal distribution of ice mass changes before 1992, when Greenland-wide observations first became available2. The only previous estimates of change during the twentieth century are based on empirical modelling3, 4, 5 and energy balance modelling6, 7. Consequently, no observation-based estimates of the contribution from the GIS to the global-mean sea level budget before 1990 are included in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change8. Here we calculate spatial ice mass loss around the entire GIS from 1900 to the present using aerial imagery from the 1980s. This allows accurate high-resolution mapping of geomorphic features related to the maximum extent of the GIS during the Little Ice Age9 at the end of the nineteenth century. We estimate the total ice mass loss and its spatial distribution for three periods: 1900–1983 (75.1 ± 29.4 gigatonnes per year), 1983–2003 (73.8 ± 40.5 gigatonnes per year), and 2003–2010 (186.4 ± 18.9 gigatonnes per year). Furthermore, using two surface mass balance models10, 11 we partition the mass balance into a term for surface mass balance (that is, total precipitation minus total sublimation minus runoff) and a dynamic term. We find that many areas currently undergoing change are identical to those that experienced considerable thinning throughout the twentieth century. We also reveal that the surface mass balance term shows a considerable decrease since 2003, whereas the dynamic term is constant over the past 110 years. Overall, our observation-based findings show that during the twentieth century the GIS contributed at least 25.0 ± 9.4 millimetres of global-mean sea level rise. Our result will help to close the twentieth-century sea level budget, which remains crucial for evaluating the reliability of models used to predict global sea level rise1, 8.

Siehe auch dazugehörige Pressemitteilung der Universität Kopenhagen.

Ein Jahr zuvor, im Dezember 2015, wies die University of Colorado Boulder in einer Pressemitteilung auf eine wichtige Schmelzphase in Grönland während der 1930er Jahre hin, die sogar intensiver ausfiel als das aktuelle Schmelzen der letzten 15 Jahre. Spannend. Basis waren wiederum alte Luftbilder:

Surprising findings in Greenland’s melt dynamics

A combination of new tools and old photographs are giving scientists a better view of Greenland’s ice, and recent discoveries promise to improve forecasts of the region’s future in a warmer world. Overall, the findings show Greenland’s ice is vulnerable to periods of rapid change including vicious cycles of warming promoting further warming.

“In the next century, Greenland melt may raise global sea level by one to three feet,” said Mike MacFerrin, a researcher with CIRES, the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. “As melting increases in Greenland, we’re discovering that melt water interacts with the ice sheet in unexpected ways. Understanding these mechanisms is crucial to predicting how Greenland’s ice responds to a warming climate, now and in the future.”

MacFerrin spoke during a news briefing at the fall meeting of the American Geophysical Union in San Francisco, California. There, four experts on Greenland highlighted several new findings related to water and ice on the northern island. Some emerged from the discovery and analysis of historic photographs of coastal glaciers; others from hard work dragging ground-penetrating radar across the ice sheet and a series of new imaging techniques innovated during NASA’s Operation IceBridge mission.

The researchers discussed the implications of newly discovered ice layers perched just underneath the surface high on the ice sheet: they likely contributed to damaging coastal floods in 2012 and are poised to contribute more in the future. Firn aquifers, recently found beneath porous snow layers, store substantial amounts of liquid water year round and represent a vast reservoir within the ice. This water contributes to a complex hydrologic system within the ice, both storing and releasing water. And surface lakes that hold liquid water through Greenland’s frigid winters are likely warming the ice sheet, priming it for further melt during summer.

“Many of these discoveries are clears signs of a warming ice sheet,” MacFerrin said. “New tools are allowing us to see these subsurface processes for the first time. If we’re going to understand Greenland’s melt contribution to sea-level rise, we need to understand these new melt features and dynamics.”

Old photos, new insights

Greenland’s glaciers retreated rapidly between 1900 and 1930 as the Little Ice Age lost its grip on the region and temperatures climbed. By analyzing early photos of Greenland paired with contemporary ones, researcher Anders Bjork with the Natural History Museum of Denmark has for the first time mapped out the retreat of those glaciers over time.

“Satellites obviously do not cover the early 1900s, when the region experienced a rapid increase in temperatures,” Bjork said. But with time constraints provided by historic photographs, he and his colleagues recorded a remarkably quick ice response between 1900 and 1930, more rapid than seen in the last 15 years, he said. The new data promise to help researchers understand how quickly glaciers can react to temperature changes, which is important today as the Arctic climate warms again.

Unfrozen

Across wide areas of Greenland researchers are finding, that water can remain liquid, hiding in layers of snow just below the surface, even through cold, harsh winters. The discoveries—made by teams including Rick Forster of the University of Utah and Lora Koenig of the National Snow and Ice Data Center—mean that scientists seeking to understand the future of the Greenland ice sheet need to account for relatively warm liquid water retained in the ice. This discovery also means that the surface hydrologic system, once thought to freeze solid during the winter, can remain active year-round.

Using airborne radars flown during NASA’s Operation IceBridge, Koenig and her colleagues were surprised to see the signature of liquid water under snow. They now report these “buried lakes” are common and extensive on the western margins of the Greenland Ice Sheet. The volume of water retained in buried lakes is small compared with the total mass of water melting from the ice sheet every year, but the lakes can warm the ice and prime the system for melt in spring and summer.

While Koenig was studying persistent “buried lakes” in Western Greenland, Forster was using similar radars and satellite measurements to show extensive water retention in a large aquifer concentrated in southeastern Greenland.

Together these findings present a picture of water remaining just below the surface year round around nearly the entire perimeter of the ice sheet. “More year-round water means more heat is available to warm the ice,” Koenig said. “Simply put, for ice sheet stability, lots of water is not good.”

Ice lenses focus runoff

Two years ago, CIRES graduate student Michael MacFerrin was studying snow compaction on the southwest Greenland ice sheet when their drill hit something completely unexpected: dense layers of ice more than 15 feet thick just under the surface. This high on the ice, the researchers expected to find mostly firn (porous, partially compacted snow) with thin, patchy ice layers or “lenses” scattered within. Such firn acts as a sponge of sorts, soaking up surface meltwater and preventing runoff from high up on the ice sheet.

MacFerrin and his colleagues wondered if the ice layers became thick enough to block surface meltwater, how long might it take for meltwater to pool at the surface and run off toward the coast? Two months later, during the record-breaking melt of July 2012, they got an answer: Landsat 7 satellite images showed unprecedented lakes and rivers forming and draining westward. Meltwater poured into the Watson River 90 miles away, contributing to the worst flooding on record and destroying major portions of a bridge in Kangerlussuaq that had spanned the river for 50 years.

MacFerrin returned to Greenland the following year, armed with the tools needed to survey these ice layers on a larger scale. He and his colleagues dragged a ground-penetrating radar system for over 100 miles behind a snowmobile, and have pored over IceBridge radar data from the ice sheet to find where else in Greenland these thick subsurface layers appear. They now report that continuous, thick ice lenses extend dozens of miles further inland than ever recorded before and cover more than 27,000 square miles, the approximate size of New Jersey, New Hampshire and Vermont combined. Recent record-breaking warm summers (2002, 2005, 2007, 2010, and 2012) appear to have generated large amounts of meltwater, which trickled down, refroze, and fattened once-thin ice layers.

With continued warming in Greenland, more melt water will be generated, adding to the processes recently discovered. “Every few years, the ice sheet surprises us, doing something we never knew it could do,” MacFerrin said. “As melt water expands and feeds all these mechanisms, it’s anybody’s guess what we might discover within the next several years. Using the tools we currently have, we’re doing our best to keep up right now.”

Siehe auch Beitrag “Greenland retained 99.7% of its ice mass in 20th Century!!!” auf WUWT.