Greenland Ice Darkening
Dark surface of Greenland icecap in summer, photo by Jason Box, PhD.
The ends of the ice ages were triggered when earth's wobble placed the Arctic in position to receive maximum summer time solar heating. The relatively small effects of the orbital variations were amplified by the melting of snow and ice which reflect sunlight back to space. Because rock and water take up heat from sunlight, ice loss adds heat to the environment, leading to more ice loss. Ultimately, this feedback loop led to the melting of large continental glaciers ten thousand feet thick. Today the same process is taking place at a rapid rate as the area of Arctic sea ice rapidly declines, the ice of Greenland darkens and the snows of Siberia melt weeks earlier in spring.
Greenland June ice darkened rapidly in 10 years. image by Jason Box, PhD.
Just as sandy, windblown deserts develop stony pavements by the wind removing fine particles, ablation of Greenland's ice produces a dark pavement of particles. Warming of the atmosphere has increased the rate of ablation of Greenland's ice, exposing more dark light adsorbing particles, increasing the heating of the icecap. Moreover, melting and refreezing causes ice grain sizes to grow. Larger ice grains reflect less light. Granular ice is less reflective than fresh snow. Low snowfall amounts last winter may also play a part in causing this June's reflectivity to drop.
Ice sheet reflectivity this year has been the lowest since accurate records began in March, 2000. In this condition, the ice sheet will continue to absorb more solar energy in a self-reinforcing feedback loop that amplifies the effect of warming. It’s not a runaway loop, just an amplifier. A record setting melt season is likely if this pattern keeps up this year.
Perhaps most remarkable about the 2012 pattern is how much darker the snow and ice is becoming, not only at the lowest elevations around the ice sheet periphery where melting is always most intense, but in the higher elevation net snow accumulation area. June monthly average reflectivity is below the 2000-2011 average across the southern-central area where surface elevations are above 2,000 m (6,561 feet). A purple area about 1/4 the distance north of the ice sheet southern tip at an elevation of 2,400 m (7,874 ft) has reflectivity 7% below the already declining 2000-2011 June (12 year) average.
Consistent with the low albedo anomaly at high elevations is the shift of the summer radiation balance from negative (cooling) to positive (heating) (Box et al. 2012). In the 12 years between 2000 and 2011 the high elevation ice sheet net radiation (sum of upward and downward solar and infrared radiation) approached positive values. What I expect we will see if these low albedo conditions persist is 100% surface melting over the ice sheet. This would be a first in observations. It may not happen this year, but the trajectory the ice sheet is on, along with amplified Arctic warming, will have the ice sheet responding by melting more and more.
The jet stream has gone north of Greenland this summer
Greenland's 10,000 foot altitude is an obstacle to the atmospheric circulation. Usually the jet stream solves this problem by staying to the south of Greenland, keeping it cold at the summit all year round. This summer temperatures have risen above freezing at the summit station 10,000 feet above sea level. These temperatures are the highest ever measured at the summit. The atmospheric anomalies over Greenland are greater than the anomalies that caused record heat over the United States. The shocking dome of warm air over Greenland produced one bizarre cold anomaly as it forced the jet stream south on its east coast towards England. English climate change deniers are having a field day in the cold and wet as the rest of the northern hemisphere bakes and melts. The jet stream has contracted to a shocking degree from its normal extent over the past 90 days. This bizarre contracted circulation pattern is leading to record ice melt across the Arctic and unprecedented melting in Greenland.
Greenland's ice sheet is now melting from top to bottom, shocking scientists
Extent of surface melt over Greenland’s ice sheet on July 8 (left) and July 12 (right). Measurements from three satellites showed that on July 8, about 40 percent of the ice sheet had undergone thawing at or near the surface. In just a few days, the melting had dramatically accelerated and an estimated 97 percent of the ice sheet surface had thawed by July 12. In the image, the areas classified as “probable melt” (light pink) correspond to those sites where at least one satellite detected surface melting. The areas classified as “melt” (dark pink) correspond to sites where two or three satellites detected surface melting. The satellites are measuring different physical properties at different scales and are passing over Greenland at different times. As a whole, they provide a picture of an extreme melt event about which scientists are very confident. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth
For several days this month, Greenland's surface ice cover melted over a larger area than at any time in more than 30 years of satellite observations. Nearly the entire ice cover of Greenland, from its thin, low-lying coastal edges to its two-mile-thick center, experienced some degree of melting at its surface, according to measurements from three independent satellites analyzed by NASA and university scientists.The record melting has caused unprecedented flooding on coastal rivers.
On average in the summer, about half of the surface of Greenland's ice sheet naturally melts. At high elevations, most of that melt water quickly refreezes in place. Near the coast, some of the melt water is retained by the ice sheet and the rest is lost to the ocean. But this year the extent of ice melting at or near the surface jumped dramatically. According to satellite data, an estimated 97 percent of the ice sheet surface thawed at some point in mid-July. ...snip NASA press release...
Son Nghiem of NASA's Jet Propulsion Laboratory in Pasadena, Calif., was analyzing radar data from the Indian Space Research Organisation's (ISRO) Oceansat-2 satellite last week when he noticed that most of Greenland appeared to have undergone surface melting on July 12. Nghiem said, "This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?"
Nghiem consulted with Dorothy Hall at NASA's Goddard Space Flight Center in Greenbelt, Md. Hall studies the surface temperature of Greenland using the Moderate-resolution Imaging Spectroradiometer (MODIS) on NASA's Terra and Aqua satellites. She confirmed that MODIS showed unusually high temperatures and that melt was extensive over the ice sheet surface.
Thomas Mote, a climatologist at the University of Georgia, Athens, Ga; and Marco Tedesco of City University of New York also confirmed the melt seen by Oceansat-2 and MODIS with passive-microwave satellite data from the Special Sensor Microwave Imager/Sounder on a U.S. Air Force meteorological satellite.
The melting spread quickly. Melt maps derived from the three satellites showed that on July 8, about 40 percent of the ice sheet's surface had melted. By July 12, 97 percent had melted.
This extreme melt event coincided with an unusually strong ridge of warm air, or a heat dome, over Greenland. The ridge was one of a series that has dominated Greenland's weather since the end of May. "Each successive ridge has been stronger than the previous one," said Mote. This latest heat dome started to move over Greenland on July 8, and then parked itself over the ice sheet about three days later. By July 16, it had begun to dissipate.
Even the area around Summit Station in central Greenland, which at 2 miles above sea level is near the highest point of the ice sheet, showed signs of melting. Such pronounced melting at Summit and across the ice sheet has not occurred since 1889, according to ice cores analyzed by Kaitlin Keegan at Dartmouth College in Hanover, N.H. A National Oceanic and Atmospheric Administration weather station at Summit confirmed air temperatures hovered above or within a degree of freezing for several hours July 11-12. (ed note: I examined the temperature record on line. High temperatures were a degree above freezing on consecutive days at the summit.)
The heat may also be speeding up the break up of the Peterman glacier. Much above normal water temperatures, as much as 5 degrees Celsius above normal, are undermining Greenland's outlet glaciers from below.
Previous Greenland Summit melt events
Multiple ice cores from the center of Greenland at or north of the summit station show previous melting events, the last occurring in 1889. The average time between these events is about 150 years.
The NASA press release includes the paragraphs:
Even the area around Summit Station in central Greenland, which at 2 miles above sea level is near the highest point of the ice sheet, showed signs of melting. Such pronounced melting at Summit and across the ice sheet has not occurred since 1889, according to ice cores analyzed by Kaitlin Keegan at Dartmouth College in Hanover, N.H. A National Oceanic and Atmospheric Administration weather station at Summit confirmed air temperatures hovered above or within a degree of freezing for several hours July 11-12.I left out the second paragraph because I strongly disagree with the statement that this melt event "was right on time". Atmospheric and oceanic circulation patterns don't show clockwork like patterns except for those tied to the solar cycle. Atmospheric circulation patterns are inherently chaotic. Milankovitch cycles associated with variations in the earth's precession and elliptical orbit, affecting glacial cycles, have a period of 21,000 years. Even the solar cycle has been irregular over the past 500 years. The Maunder minimum in sunspots and the "little ice age" disrupted solar activity cycles and any possibly associated weather cycles over the past 500 years.
"Ice cores from Summit show that melting events of this type occur about once every 150 years on average. With the last one happening in 1889, this event is right on time," says Lora Koenig, a Goddard glaciologist and a member of the research team analyzing the satellite data. "But if we continue to observe melting events like this in upcoming years, it will be worrisome."
Until recently it was thought that there were 28 cycles in the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784–1799) seems actually to have been two cycles, so that the average length is only around 10.66 years. Cycles as short as 9 years and as long as 14 years have been observed, and in the double cycle of 1784-1799 one of the two component cycles had to be less than 8 years in length. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.However, I found out, in the comments section of Neven's sea ice blog that Lora Koenig has blogged about the event.
I arrived at Summit Station on July 13th, while traveling with the Joint Science and Education Program (JSEP) for a short visit to the camp. When we arrived, Summit Station had been experiencing above freezing temperatures for multiple days prior to our arrival and a melt layer formed on the near surface snow. I have been studying the physical properties of the top layers of the ice, the firn, at Summit and NEEM for my Ph.D. research. Recently, I have been focused on the melt layers present in both firn cores because they occur very infrequently. At Summit, there is only one other melt layer besides the melt layer from this past week and this previous melt layer dates to 1889.She has also responded to questions in the comments section of her blog, helping to clarify what she was said in the NASA press release.
The most interesting part of being at Summit Station just after a melt event had occurred, is that the melt layer formation process could be observed. When studying a firn core, there is only a small cross section of the firn column that can be examined, which makes it hard to understand how the melt layer formed and how evenly distributed it is. Studying snow pits at Summit, including the recent melt layer, presents a unique opportunity for us to understand how previous melt events occurred. While at Summit density, stratigraphy, and permeability measurements have been taken and samples will also be brought back to the laboratory at Dartmouth, which will give us a clue about melt layers in the past.
Terry MoranOxygen Temperature proxy vs Time at Greenland Summit
I’m disturbed by the ” every 150 years” statement. Are there other cores going back further than 1750 that validate this? If there has only been one previous example how does one discern a trend?
on July 24, 2012 at 11:02 pm | Reply keegankm
Yes, in Greenland there have been many deep ice core drilling projects which drilled ice to the bedrock. At Summit, the deep ice core (GISP2) reaches a depth of about 3km with the oldest ice being about 123,000 years old. Alley and Anandakrishnan’s paper looks at the ice just below the firn, and into the last 10,000 years. In the past 10,000 years (the Holocene), there is on average a melt layer every 150 years.