Arguments
Unprecedented Greenland Ice Sheet Surface Melt
Posted on 29 July 2012 by Neven
News from NASA regarding the Greenland ice sheet via Neven's Arctic Sea Ice blog. Note that similar Greenland ice sheet melt events have happened in the distant past, but perhaps not quite of this magnitude. On top of the natural influences, we now have the long-term human-caused Greenland ice sheet decline as well.
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.
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.
Researchers have not yet determined whether this extensive melt event will affect the overall volume of ice loss this summer and contribute to sea level rise.
"The Greenland ice sheet is a vast area with a varied history of change. This event, combined with other natural but uncommon phenomena, such as the large calving event last week on Petermann Glacier, are part of a complex story," said Tom Wagner, NASA's cryosphere program manager in Washington. "Satellite observations are helping us understand how events like these may relate to one another as well as to the broader climate system."
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.
"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."
In the meantime Dr. Jason Box reports on the Meltfactor blog:
Greenland ice sheet record surface melting underway
While the potential impact of wildfires on darkening the Greenland ice sheet surface remain to be resolved, there is mounting evidence of an extreme year 2012 melt.
Melt signatures from active microwave remote sensing are stronger than in recent years over the upper areas of the ice sheet. Dark areas indicate absorption of the microwave signal emitted by the satellite. While, year 2010 and 2011 are recognized as being record melt years (Tedesco et al. 2011, van As et al. 2011), year 2012 melting appears to be more extensive.
(...)
In my recently accepted albedo paper (Box et al. 2012, ACCEPTED VERSION), see abstract, the statement: “it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.” may be coming true already.
From ScienceDaily:
Surprising Link Between Ice and Atmosphere: GPS Can Now Measure Ice Melt, Change in Greenland Over Months Rather Than Years
ScienceDaily (July 24, 2012) — Researchers have found a way to use GPS to measure short-term changes in the rate of ice loss on Greenland -- and reveal a surprising link between the ice and the atmosphere above it.
The study, published in the early online edition of the Proceedings of the National Academy of Sciences, hints at the potential for GPS to detect many consequences of climate change, including ice loss, the uplift of bedrock, changes in air pressure -- and perhaps even sea level rise.
The team, led by earth scientists at Ohio State University, pinpointed a period in 2010 when high temperatures caused the natural ice flow out to sea to suddenly accelerate, and 100 billion tons of ice melted away from the continent in only 6 months.
They were able to make the measurement because Earth compresses or expands like a spring depending on the weight above it, letting them use the Greenland bedrock like a giant bathroom scale to weigh the ice atop it. As ice accumulates, the bedrock sinks, and as the ice melts away, the bedrock rises.
Measurements revealed that Greenland sank by about 6 mm (about one quarter of an inch) over the winter of 2010, and the researchers determined that half of the sinking (3 mm, or one eighth of an inch) was actually due to high air pressure above the ice, and the other half was due to ice accumulation.
Further, they determined that the bedrock lifted 11 mm (less than half an inch) over the course the summer. Air pressure appeared to affect the bedrock less during this time, so that the bounce-back appears to be mostly due to ice loss.
(...)
While shortening the detection time to six months is a substantial advance, Bevis thinks his team will soon do even better.
"Within the next year or so, we should be able to process the GPS data within a month of its being collected," he said, "and then we can monitor abrupt changes in ice mass only a month or two after they occur."
(...)
Although this study revealed a dramatic six-month period of melting in Greenland in 2010, that short-term ice loss isn't necessarily a sign of a long-term trend, Bevis cautioned.
"It is dangerous to assume that rates observed over even two or three years reflect a long-term trend. Rates are known to change. So, it would be even more dangerous to assume that the record breaking summer of 2010 is the new norm."
"That being said, the summer of 2011 was also very hot. And this summer is starting off hot, too. So, I do expect to see a sustained increase in uplift rates when we compare 2010-2012 to 2007-2009," he added.
Read the whole thing here.
"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...
I’ve looked at the Greenland ice core data for the last 10,000 years. You can see it here:
http://www.gisp2.sr.unh.edu/DATA/alley1.gif
It’s true that if one divides the total number of melt incidents over that 10,000 year record, it does average out to ~1 melt every 150 years or so.
But it’s also true that only one melt event has happened in the last 800 years or so (1889).
Am I missing something here? From that graph, this 97% July melt looks anything but 'typical'.
When one examines the insolation table above, the data shows good evidence that current temps are now forced well above HCO temps relative to available insolation (especially given that temps are a long way from equilibria).
The takeaway I see is that, even at the summit, a very warm year puts the GIS at near-total ablation-zone status. Given the near-unprecedented, anthropogenic-derived warming currently being experienced and yet in the pipeline, coupled with a meandering and increasingly-sticky polar jet, total-melt-zone status will be a regular occurence in the very near future (per Box 2012). Like 2012, 2013...
"those that would have us debate the existence of gravity try to average-away the decline"
Unfortunately, that 'averaging' was made by a Goddard glaciologist, not a climate skeptic. No 150-year melt cycle exists in the Greenland ice core record, and implying that this melt is 'right on time' is highly misleading.
I see absolutely no discussion of this in any of the scientific reporting (most frustrating). The 'this is right on time' statement completely masks both the non-periodic nature Greenland melts in general, and the extraordinary anomoly of the July 2012 melt.
Which is why I left my original comment here. My hope is that someone will cover it in more detail, and that it gets the attention it deserves.
You can't just look at the data in isolation.
Perhaps I haven't made my point clear.
I am aware of Arctic sea ice trends, the 'sticky' jet stream, the numerous warm high pressure ridges that formed over Greenland beginning in May (culminating in early July), and other atypical phenomena that led to this startling GIS melt event.
I've also read Box (2012).
To anyone following this closely, the July melt should not be too much of a surprise.
But even scientific reporting on the melt infers that it could be part of a 'natural 150-year cycle', which does not actually seem to be accurate. My comments are not intended to request clarification on the drivers behind this melt, but rather to inspire SkS authors to explain that Koenig's comment is inaccurate and misleading.
I've seen no one do that to date, and I believe it's important. Perhaps I'm wrong.
Climate Progress
The NASA results are clearly unprecedented in the satellite record (and this is obviously what was being referred to), and come at the tail end of a strong increasing trend in summer surface melt area (as seen in data from the Steffen and Tedesco groups).
However, we know Greenland was warmer than today at many intervals in the past – the Early Holocene (from isotopes and borehole temperatures), the last interglacial, the Pliocene etc. so there is no claim that this is something that has never happened in the history of the planet.
Furthermore, the ‘every 150 years’ quote is very strange. The data on Summit melt layers – (discussed in the paper you reference
http://www.igsoc.org/annals.old/21/igs_annals_vol21_year1995_pg64-70.pdf )
and more easily visible here:
http://www.gisp2.sr.unh.edu/DATA/alley1.html
– indicates that the [1889] event was actually the only event in the last ~700 years, and there have only been 6 in the last 2000 years (4 of which were associated with the Medieval Climate Anomaly btw 750 and 1200AD). Hardly a frequently recurring ‘cycle’!
The all-Holocene average that Koenig is referring to includes the warmer Early Holocene where orbital variability was driving warmer northern high latitude summers — and which is not relevant to the expected frequency in today’s climate.
But I've revised that view. The 1,000 average has been running at 250 years which isn't that big a difference from 150 and there have been big gaps similar in size to the 600+ one in previous millennia. And with this recent 2012 event, hasn't the running 1,000 year average risen to 200?
So it wasn't very exact to claim 150 years. Indeed it was sloppy using that 10,000 year average. But to call it flat wrong? I don't think so.
It's good to know that Gavin and Joe are already on this.
Rob,
No harm, no foul. I've done the same thing myself - more than once. And while I've been reading SkS regularly for some time now, I haven't commented here before.
(I'm not sure that I phrased that correctly)
Thanks a lot for this, Physicist-retired. I didn't give it much thought, as I figured it was entirely irrelevant to the event itself, but now that I have thought about it, I find it amazing that Koenig has said this and even more amazing that the people over at NASA put this quote in. They handed the fake skeptics everything they needed to mislead and obfuscate on a silver platter.
Wow. Just wow.
I first caught wind of this on Peter Sinclair's site. How could JPL say in the headline "unprecedented" and then in the body of the artcle say that the last event was in 1889 and part of a cycle of 150 years? Sloppy and naive I thought. It gives the rabble something more to froth at the mouth about.
I went looking for the Alley paper without success so it is good to see it referenced above.
I think that Twemoran nailed it at your place:
"Undoubtedly accurate, but there are other ways of expressing how unusual the event is.
How about "Only once since Columbus's discovery of America" or "Only twice since the signing of the Magna Charta"
Probably cherry picked, but relating historic events to the data might make an effective headline.
The thing I drew from the chart was that averaging events that occur in clusters can be misleading. The most recent cluster occurred in the Viking Age with a long dry period following that."
I agree with his point that relating the melt event to a well known historical event help the less enumerate among us.
1 - The Lora Koenig's 150 years periodicity allegation, as notified before, so waiting for enlightenments.
2 - The SkS headline in which, as explained in the Joe Romm's article and on Carbonbrief : "the description of 'unprecedented' needs to be qualified - on the latest satellite measurements it's certainly the case, but longer-term records suggest this kind of melt may have happened before."
So shouldnt it be : "Satellites see unprecedented Greenland ice sheet surface melt" just like the NASA press release, or "Record Greenland ice sheet surface melt" to put it in a geological era context ?
3 - The missing perspective of this event, however begun by the Gavin Schmidt's comment :
"The NASA results are clearly unprecedented in the satellite record (and this is obviously what was being referred to), and come at the tail end of a strong increasing trend in summer surface melt area (as seen in data from the Steffen and Tedesco groups)."
Perhaps a graph, presenting maximum ice sheet surface melt extent and its trend like in Mernild et al. 2011, with this new 2012 value plotted, could be usefull ?
Just in order to explain us the diverse hypotheses :
- "whether this year's melting is a one off or the start of a trend".
- or even if it could be both : the consequences of an extreme weather pattern (noise) on a continued multi-decadal upwards surface melting (trend).
Misleading implication of a 150-year cycle resulting from averaging a non-uniform distribution. (Which I had guessed, but not confirmed; thanks Physicist-retired.)
Handing skeptics an out on silver platter, etc.
I also thought "will be worrisome" was an odd turn of phrase choice. The state of arctic ice isn't worrisome already? In the context of an accelerating ice mass loss, a strong warming trend globally, and polar amplification, I might have used 'expected'. I can't see why one more symptom on a terminal patient would change a state from not 'worrisome' to 'worrisome'.
It somewhat appears to me that Dr. Koenig was caught unprepared at an interview, and of course, the skeptics will give more credence to her casual reply than to the actual distribution of events in the data.
(1) The melt is ice sheet surface melt (i.e. the formation of meltwater on the surface of the ice), which is certainly a concern. The impression I get from the USA Today article is one of greater melting than is the case.
(2) (-Snip-)
Regarding the '150-year natural melt cycle' - any clarification on this from Dr. Box yet?
As an FYI, Dr. Box has a blog post update of relevance, here: http://www.meltfactor.org/blog/?p=677.
In which can be found this excellent graphic:
The warming of the GIS during the 1930's is notably less than that warming being experienced by the GIS today.
Plummer ends his article with:
“Currently, Greenland and Antarctica contribute approximately 1.3 millimeters to sea level rise each year, but this rate is increasing. Under the current rates of acceleration for ice sheet loss, we could expect 56 centimeters of sea level rise by 2100, from the ice sheets alone. Whether this month's extreme melt event was truly unprecedented, or part of a larger cycle, is not really the point. There exists many years of data, from multiple sources of sea level rise, to justify concern. We need not glob onto (nor dismiss) one extraordinary number to come to that conclusion.”
Ouch.