Historisches Eisschmelzdrama in Grönland: Vor 5000 Jahren gab es viel weniger Eis als heute

Das grönländische Inlandeis schmilzt derzeit. Das ist auch keine große Überraschung angesichts der Erwärmung der letzten 150 Jahre nach Ende der Kleinen Eiszeit. Was allerdings nur wenige wissen ist, dass der grönländische Eisschild vor ein paar tausend Jahren viel kleiner war als heute. Eine Arbeit von Briner et al. 2016 in den Quaternary Science Reviews fasste die Entwicklung des Grönlandeises schön zusammen:

Holocene climate change in Arctic Canada and Greenland
This synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the Holocene in Arctic Canada and Greenland. Our synthesis includes 47 records from a recently published database of highly resolved Holocene paleoclimate time series from the Arctic (Sundqvist et al., 2014). We analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records, mostly from datasets that did not fit the selection criteria for the Arctic Holocene database. Combined, we review evidence from a variety of proxy archives including glaciers (ice cores and glacial geomorphology), lake sediments, peat sequences, and coastal and deep-marine sediments. The temperature-sensitive records indicate more consistent and earlier Holocene warmth in the north and east, and a more diffuse and later Holocene thermal maximum in the south and west. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene. The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C on average (n = 11 sites). The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 ka, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time. The spatial pattern of temperature change through the Holocene was likely driven by the decrease in northern latitude summer insolation through the Holocene, the varied influence of waning ice sheets in the early Holocene, and the variable influx of Atlantic Water into the study region.

In der Arbeit wird auch eine Kurve der Flächenveränderungen des grönländischen Eisschildes während der vergangenen 10.000 Jahre gezeigt, basierend auf einer Modellierung von Larsen et al. 2015 (Abb. 1). Schön zu erkennen das Schrumpfen des Eises nach der letzten Eiszeit. Vor etwa 5000 Jahren erreichte das Eis dann seine geringste Ausdehnung und wuchs in der Folge bis heute wieder an. Das heutige Abschmelzen wirkt dagegen eher untergeordnet.

 

Abbildung 1: Flächenveränderungen des grönländischen Eisschildes während der vergangenen 10.000 Jahre. Graphik: Briner et al. 2016 nach einer Modellierung von Larsen et al. 2015.

 

Jason Briner war auch Teil einer Eiskernstudie von Thomas et al. 2016, die interessante Hintergrundinformationen liefert. Als das Eis vor 6000-4000 Jahren besonders stark abschmolz und die globalen Temperaturen ein Maximum erreichten, nahm auch der Schneefall zu. Warme Zeiten sind also schneereiche Zeiten in Grönland. Hier die Pressemitteilung der University at Buffalo vom 23. Mai 2016:

A history of snowfall on Greenland, hidden in ancient leaf waxes
A surprising trove of data yields indications of increased Arctic snowfall in times of warming

The history of Greenland’s snowfall is chronicled in an unlikely place: the remains of aquatic plants that died long ago, collecting at the bottom of lakes in horizontal layers that document the passing years. Using this ancient record, scientists are attempting to reconstruct how Arctic precipitation fluctuated over the past several millennia, potentially influencing the size of the Greenland Ice Sheet as the Earth warmed and cooled. An early study in this field finds that snowfall at one key location in western Greenland may have intensified from 6,000 to 4,000 years ago, a period when the planet’s Northern Hemisphere was warmer than it is today. While more research needs to be done to draw conclusions about ancient precipitation patterns across Greenland, the new results are consistent with the hypothesis that global warming could drive increasing Arctic snowfall — a trend that would slow the shrinkage of the Greenland Ice Sheet and, ultimately, affect the pace at which sea levels rise.

“As the Arctic gets warmer, there is a vigorous scientific debate about how stable the Greenland Ice Sheet will be. How quickly will it lose mass?” says lead researcher Elizabeth Thomas, PhD, an assistant professor of geology in the University at Buffalo College of Arts and Sciences who completed much of the study as a postdoctoral fellow at the University of Massachusetts Amherst. “Climate models and observations suggest that as temperatures rise, snowfall over Greenland could increase as sea ice melts and larger areas of the ocean are exposed for evaporation. This would slow the decline of the ice sheet, because snow would add to its mass,” Thomas says. “Our findings are consistent with this hypothesis. We see evidence that the ratio of snow to rain was unusually high from 6,000 to 4,000 years ago, which is what you would expect to see if sea ice loss causes snowfall to increase in the region.” The research was published on May 23 in Geophysical Research Letters, a journal of the American Geophysical Union.

Aquatic plant leaf waxes as a record of snowfall

Thomas’ research looks to understand how precipitation changed in the past, with an eye toward better predicting how modern warming will affect the Earth. “We are using the past to see what might happen in the future,” she says. Aquatic leaf waxes are a relatively new tool for completing this work. They reveal information about the seasonality of precipitation — how amounts of ancient summer rain compared to amounts of ancient winter snow. To understand how aquatic leaf waxes function as a historical record, you need to know a little about aquatic plants. In the Arctic, these organisms survive on lake water, and use hydrogen atoms from this water to produce wax coatings on leaves.

These hydrogen atoms are the key to studying precipitation: In years when the ratio of summer rain to winter snow in a region is high, lake water and aquatic leaf waxes end up containing high levels of a rare form of hydrogen called deuterium, which is heavier than “normal” hydrogen. (This is because summer rain holds more deuterium than winter snowfall.) In contrast, in years when snow is relatively abundant, aquatic plants start producing waxes with less deuterium. This is what Thomas and her colleagues saw when they extracted a long, cylindrical sediment sample from a lake bottom in western Greenland. The mud contains ancient leaf waxes, with the oldest at the base of the column and the youngest at the top.

By dating and analyzing thin slices of the sample, the team determined that aquatic leaf waxes had low levels of deuterium from 6,000 to 4,000 years ago. This is exactly what researchers would expect to see if the warm temperatures of that time had fueled sea ice loss, leading to increased Arctic snowfall and a decline in deuterium in lakes, Thomas said. She acknowledged that it’s possible that a drop in summer rainfall accounted for the changes in deuterium, but says a rise in winter snowfall is the more likely explanation, as scientists have found independent evidence that the region was wetter 6,000 to 4,000 years ago.

The research team included UB Associate Professor of Geology Jason Briner; undergraduate student John J. Ryan-Henry from Brown University, the University of Rhode Island and the Roger Williams University School of Law; and Professor Yongsong Huang from Brown University and the Chinese Academy of Sciences. The study was funded by the National Science Foundation and research grants from the Geological Society of America.

Das Schmelzen des Grönlandeises läuft wie nicht anders zu erwarten in jahreszeitlichen Zyklen ab. Im Sommer schmilzt das Eis und im Winter wächst es dann wieder. Allerdings verschiebt sich das Niveau momentan jedes Jahr nach unten. Der winterliche Eiszuwachs kann die sommerliche Schmelze nicht voll ausgleichen. Harig & Simons haben 2016 die Entwicklung detailliert dokumentiert:

 

Abb 2. Veränderung der grönländischen Eismasse während der letzten 15 Jahre. Quelle Harig & Simons 2016.

 

Hier der Abstract der Arbeit:

Ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Archipelago: Seasonal cycles and decadal trends
Over the past several decades mountain glaciers and ice caps have been significant contributors to sea level rise. Here we estimate the ice mass changes in the Canadian Archipelago, the Gulf of Alaska, and Greenland since 2003 by analyzing time-varying gravimetry data from the Gravity Recovery and Climate Experiment. Prior to 2013, interannual ice mass variability in the Gulf of Alaska and in regions around Greenland remains within the average estimated over the whole data span. Beginning in summer 2013, ice mass in regions around Greenland departs positively from its long-term trend. Over Greenland this anomaly reached almost 500 Gt through the end of 2014. Overall, long-term ice mass loss from Greenland and the Canadian Archipelago continues to accelerate, while losses around the Gulf of Alaska region continue but remain steady with no significant acceleration.

Das grönländische Inlandeis hat sich in den letzten Jahrzehnten zunehmend verdunkelt, so dass die Rückstrahlkraft (Albedo) abgenommen hat. Hierdurch verstärkt sich die Schmelze, wie Columbia University’s Lamont-Doherty Earth Observatory am 3. März 2016 per Pressemitteilung bekanntgab:

Greenland’s Ice Is Getting Darker, Increasing Risk of Melting

Greenland’s snowy surface has been getting darker over the past two decades, absorbing more heat from the sun and increasing snow melt, a new study of satellite data shows. That trend is likely to continue, with the surface’s reflectivity, or albedo, decreasing by as much as 10 percent by the end of the century, the study says. The culprits are two feedback loops that are created by the melting itself. One of those processes isn’t visible to the human eye, but it is having a profound effect.

“You don’t necessarily have to have a ‘dirtier’ snowpack to make it dark,” said lead author Marco Tedesco, a research professor at Columbia University’s Lamont-Doherty Earth Observatory and adjunct scientist at NASA Goddard Institute of Space Studies. “A snowpack that might look ‘clean’ to our eyes can be more effective in absorbing solar radiation than a dirty one. Overall, what matters, it is the total amount of solar energy that the surface absorbs. This is the real driver of melting.”

The feedback loops work like this: During a warm summer with clear skies and lots of solar radiation pouring in, the surface starts to melt. As the top layers of fresh snow disappear, old impurities, like dust from erosion or soot that blew in years before, begin to appear, darkening the surface. A warm summer can remove enough snow to allow several years of impurities to concentrate at the surface as surrounding snow layers disappear. At the same time, as the snow melts and refreezes, the grains of snow get larger. This is because the meltwater acts like glue, sticking grains together when the surface refreezes. The larger grains create a less reflective surface that allows more solar radiation to be absorbed. The impact of grain size on albedo – the ratio between reflected and incoming solar radiation – is strong in the infrared range, where humans can’t see, but satellite instruments can detect the change.

“It’s a complex system of interaction between the atmosphere and the ice sheet surface. Rising temperatures are promoting more melting, and that melting is reducing albedo, which in turn is increasing melting,” Tedesco said. “How this accumulates over decades is going to be important, because it can accelerate the amount of water Greenland loses. Even if we don’t have a lot of melting because of atmospheric conditions one year, the surface is more sensitive to any kind of input the sun can give it, because of the previous cycle.”

The findings, published in the European Geosciences Union journal The Cryosphere, have global implications. Fresh meltwater from Greenland’s land-based ice raises sea level and could affect ocean ecology and circulation.

Past and future changes

The study used satellite data to compare summertime changes in Greenland’s albedo from 1981 to 2012. The first decade showed little change, but starting around 1996, the data show that due to darkening, the ice began absorbing about 2 percent more solar radiation per decade. At the same time, summer near-surface temperatures in Greenland increased at a rate of about 0.74°C per decade, allowing more snow to melt and fuel the feedback loops.

A likely cause for the large shift around 1996 was a change in atmospheric circulation, Tedesco said. The North Atlantic Oscillation, a large-scale natural weather cycle, went into a phase in which summer atmospheric conditions favored more incoming solar radiation and warmer, moist air from the south. Later records show those conditions shifted in 2013-2014 to favor less melting, but the damage was already done – the ice sheet had become more sensitive. In 2015, melting spiked again to reach more than half of the Greenland  ice sheet. While new snowfall can make the ice sheet brighter again, the dark material built up during the melt years is waiting just below the surface, preconditioning the surface to future melting, Tedesco said.

The scientists also ran a computer model to simulate the future of Greenland’s surface temperature, grain size, exposed ice area and albedo. Over the current century, the model projects that the average albedo for the entire ice sheet will fall by as much as 8 percent, and by as much 10 percent on the western edge, where the ice is darkest today. Those are conservative estimates – the change could be twice that, Tedesco said.

The role of wildfires

The scientists looked into the hypothesis that soot from forest fires in China, Siberia and North America could be driving the increased darkening of the ice sheet. Using the Global Fire Emissions Database, they analyzed trends in black-carbon emissions from wildfires in those regions and Europe. While the amount of black carbon released by fires varied year to year, the scientists found no statistically significant increase during 1997-2012 to match the darkening of Greenland.

The study also raises questions about whether Greenland’s high plateau is darkening as previous reports have suggested. The scientists found no long-term trend of darkening at the top, and they suspect that the Terra MODIS satellite sensor that has detected darkening in the past may actually be degrading, as previous studies have suggested. At lower elevations, the signal is much stronger.

“It is a very good paper which provides valuable new insights about the physical processes controlling the change in reflectivity of the Greenland ice sheet and specifically its darkening over time,” said Eric Rignot, a senior research scientist at NASA’s Jet Propulsion Laboratory who studies ice sheets but was not involved with the new study. “I also find it particularly interesting that the darkening indicated earlier by satellite sensors is now confirmed to be less, which is good news for the ice sheet. Yet the darkening of Greenland around its periphery remains a source of concern because it contributes to making the ice sheet melt away faster.”

Warming ahead

The feedback loops could be stopped with lots of snowfall and less melting, but that doesn’t seem likely given the increasing concentration of greenhouse gases in the atmosphere, Tedesco said.  And while warming is expected to increase precipitation, that precipitation includes increasing rainfall, which speeds up melting. Melting is also moving to higher elevations as global temperatures warm.  “As warming continues, the feedback from declining albedo will add up,” Tedesco said. “It’s a train running downhill, and the hill is getting steeper.” Co-authors of the paper are Sarah Doherty of the University of Washington; Xavier Fettweis of University of Liege; Patrick Alexander of NASA Goddard Institute of Space Studies and City University of New York; Jeyavinoth Jeyaratnam of the City College of New York; and Julienne Stroeve of University of Boulder.

Die Schwachstellen des Grönlandeises mit der meisten Schmelze sind übrigens keine Unbekannten. Bereits in den letzten Jahrtausenden hatten diese Regionen während warmer Zeiten kräftig Federn gelassen und waren geschrumpft. Die Universität Utrecht gab dies am 21. September 2016 per Pressemitteilung bekannt:

Current weak spots in Greenland’s ice sheet have been weak for thousands of years

Earlier recent work has shown that the east, southeast and northwest regions of the Greenland ice sheet have contributed to 77% of the total mass loss over the last century. Now, researchers have used gps data to show that the east, southeast and northwest also contributed significantly to ice mass loss in the past, over thousands of years: about 40% of the total loss of ice mass. The researchers, amongst which Utrecht University climate researchers Bert Wouters and Peter Kuipers Munneke, are publishing their results in Science Advances on 23 September.

The researchers also measured the changes in height of the land masses under the melting ice. When land ice melts, the land underneath rises, since the ice no longer weighs it down. The researchers found that land masses in the southeast of Greenland have risen by about 12 millimeters per year. The results indicate that these basins have solely contributed to an ice mass loss corresponding to global sea level rise of 1.5 m over the past millennia. It seems likely, therefore, that further destabilisation of these ice sheet regions will continue to be the main source of sea-level rise in the future, the authors say.

Past and present-day mass loss

Mass loss of the Greenland ice sheet has increased over the last twenty years, mainly because glaciers are accelerating and the surface of the ice sheet is melting faster and faster. However, due to a lack of in situ observations in key regions it is still difficult to accurately establish how much mass the ice sheet loses over a period of thousands of years and how much it contributes to global sea-level rise. The research also shows that some calculations of present-day mass loss of the Greenland ice sheet may be underestimated: the new measurements show that Greenland loses about 7 percent of ice per year more than previously assumed.

Publication:  Geodetic measurements reveal similarities between post Last Glacial Maximum and present-day mass loss from the Greenland ice sheet. Sci. Adv. 2, e1600931 (2016). S.A. Khan, I. Sasgen, M. Bevis, T. van Dam, J.L. Bamber, J. Wahr, M. Willis, K.H. Kjær, B. Wouters*, V. Helm, B. Csatho, K. Fleming, A.A. Bjørk, A. Aschwanden, P. Knudsen, P. Kuipers Munneke*

Und schließlich wäre da noch eine Studie der Universität Bristol, die den verborgenen Turboantrieb des schnellsten Gletschers Grönlands identifizierte: Der Jakobshavn Isbræ befindet sich in einem ehemaligen Flusstal, das vor der Vergletscherung von Flüssen zu einer regelrechten Gletscherautobahn ausgeschabt wurde. Pressemitteilung der Uni Bristol vom 13. Juni 2016:

Huge ancient river basin explains location of the world’s fastest flowing glacier

An ancient basin hidden beneath the Greenland ice sheet, discovered by researchers at the University of Bristol, may help explain the location, size and velocity of Jakobshavn Isbræ, Greenland’s fastest flowing outlet glacier.The research also provides an insight into what past river drainage looked like in Greenland, and what it could look like in the future as the ice sheet retreats.

Michael Cooper and colleagues from Bristol’s School of Geographical Sciences and Cabot Institute, and Imperial College London, studied the bedrock in Greenland using data collected mainly by NASA (through Operation Ice Bridge), as well as various researchers from the UK and Germany, over several decades. This data is collected by aircraft using ice penetrating radar, which bounces back off the bedrock underneath the ice (as ice is mostly transparent to radio waves at certain frequencies). Mr Cooper said: “The drainage basin we discovered shows signs of being carved by ancient rivers, prior to the extensive glaciation of Greenland (i.e. before the Greenland Ice Sheet existed), rather than being carved by the movement of ice itself. It has been remarkably well preserved – and has not been eroded away by successive glaciations. The channel network has never been seen before by humans – it was last uncovered around 3.8 million years ago.” 

The size of the drainage basin the team discovered is very large, at around 450,000 km2, and accounts for about 20 per cent of the total land area of Greenland (including islands). This is comparable to the size of the Ohio River drainage basin, which is the largest tributary of the Mississippi. The channels the team mapped could more appropriately be called ‘canyons’, with relative depths of around 1,400 metres in places, and nearly 12km wide, all hidden underneath the ice. As well as being an interesting discovery of great size, the channel network and basin was instrumental in influencing the flow of ice from the deep interior to the margin, both now and over several glacial cycles, as well as influencing the location and speed of the Jakobshavn ice stream. 

Palaeofluvial landscape inheritance for Jakobshavn Isbrae catchment, Greenland‘ by M. A. Cooper, K. Michaelides, M. J. Siegert and J. L. Bamber in Geophysical Research Letters