Greenland really has been melting, can someone tell Anthony Watts

Anthony Watts has decided once again that he, ex-weather announcer, knows all about ice in Greenland, and the real experts "don't know nuffin'". His headline (archived here) is: "Failed claim right out of the gate: Climate change altering Greenland ice sheet & accelerating sea level rise". And he took it upon himself, the ahem Greenland pseudo-scientist at WUWT, to continue:

From the “why worry, the 99.7% of the ice is still there” department and York University comes this climate claim that has to do with a natural event in 2012, and just doesn’t hold up as being driven by “climate change”. More details on that below.

If 20% of the ice in Greenland melted, there'd be a rise in sea level of about 1.2 metres or 4 feet. But leaving his "why worry" aside, Anthony was protesting a new paper in Nature Climate Change by Horst Machguth and a large team of scientists. The paper was related to the widescale melt of the surface, such as happened in 2012. It's also to do with the overall warming of Greenland and surface melting in general, particularly the implications for melt runoff and sea level rise.

The latest Arctic report card shows that last year (2015), more than half of the ice sheet had a surface melt, which was the most widespread melt since 2012, and was "above the 1981-2010 average on 54.3% of days (50 of 92 days)".

It's not just a random surface melt that ices up again quickly, either. Greenland is losing ice mass, as the chart below (from the same report) shows:

Fig 1. | Cumulative change in the total mass (in Gigatonnes, Gt) of the Greenland Ice Sheet between April 2002 and April 2015 estimated from GRACE measurements. Each symbol is an individual month and the orange asterisks denote April values for reference. Source: Arctic Report Card

Anthony Watts doesn't know that Greenland is melting

Down the bottom of his multiple copies and pastes, Anthony wrote:

So, while the University of York may be right about the firn/sponge issue not soaking up as much meltwater as before, climate change didn’t have one damn thing to do with the change in the firn. The same thing happened 150 years ago, in a few decades, the firn will be back to normal until the next 150 year melt event occurs.

Really? How he drew his conclusion from the papers he wrote about is a mystery. His first and second sentence are disconnected. The second sentence has no logical connection to Anthony's claim in the first - that climate change "didn't have one damn thing to do with the change in the firn". I gather that he thinks that there won't be any more widespread melting until 2162. He's wrong (see below). There was widespread melting in 2015. Not as widespread as in 2012, but more than half the surface of Greenland had surface melting on 4 July last year.

The porous firn layer is getting covered by impermeable ice

The latest work is, as you've probably guessed, about a change in the structure of the firn on the lower levels of the ice sheet in Greenland.

Firn can be described as old snow that has changed its crystalline structure from snow toward that of glacial ice. It's not yet the same structure as the underlying ice sheet, and neither does it have the structure of fresh snow. It's apparently quite hard.  The diagram below gives some idea of what it is:

Fig. 2 | The layers of the ice sheet. 
Source and credit: Centre for Ice and Climate, Niels Bohr Institute

The paper is about a study of changes to the firn, particularly in the lower reaches of the Greenland ice sheets, where melting is happening more quickly. The authors wrote how there is porous firn which is up to 80 metres thick under the surface where snow and ice are accumulating in Greenland. Until recently, much of the ice sheet only had limited melting. In recent years, the melting has been taking place over a much larger area. For example, you'll probably remember the reports when there was melting over the entire surface of the ice sheet, back in 2012.

Fig. 3 | Extent of surface melt over Greenland’s ice sheet on July 8 (left) and July 12 (right).
Source: NASA

The pore space in the firn is able to soak up meltwater as it runs off the glacier. This water can then freeze in the pore space. It's as if the firn acts like a sponge, so that the meltwater doesn't all run off the ice sheet. The less runoff goes into the sea, the slower will sea levels rise. So it's very useful to know how much the firn can soak up.

What these scientists found is that "surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled". They looked particularly at the areas where there was a lot of melting in 2010 and 2012. It turns out that the major melt events changed the structure of the firn, From the press release:

The team was surprised by what they found. An extreme melt that occurred in 2012 caused a layer of solid ice, several metres thick, to form on top of the porous firn in the low elevation areas of the ice sheet. "In subsequent years, meltwater couldn't penetrate vertically through the solid ice layer, and instead drained along the ice sheet surface toward the ocean," said Colgan. "It overturned the idea that firn can behave as a nearly bottomless sponge to absorb meltwater. Instead, we found that the meltwater storage capacity of the firn could be capped off relatively quickly."

As Machguth said, "Basically our research shows that the firn reacts fast to a changing climate. Its ability to limit mass loss of the ice sheet by retaining meltwater could be smaller than previously assumed."

The paper provides more detail. At the lower levels where there's more melting anyway, the surface melt itself formed a layer of ice when it refroze, which stopped any later meltwater from getting into the porous layer, so more of it just runs off into the ocean.  At higher elevations the firn is becoming more dense. From the paper:

Our conclusions refer to the impact of near-surface ice layers under the extraordinary melt conditions of summer 2012; partitioning of surface migration and percolation could be different during gentle melt seasons. Comparing 2013 and 2015 stratigraphy at the 1,840m site (Supplementary Fig. 4 and Methods) indicates that melt generated during the moderate melt seasons of 2013 and 2014 has partly accreted as superimposed ice on top of the ice layer. Melt rivers originate at high elevations in both summers (Supplementary Fig. 2), providing further evidence that even limited quantities of water now mainly migrated along the surface. 

As the Greenland climate warms, this effect is likely to continue. The authors finished by writing about how this is likely to play out over time:

Provided persistent climate warming, the firn runoff  regime will probably migrate to higher elevations. Owing to amplifying feedback processes, surface runoff from the zone of thick near-surface ice layers is anticipated to increase beyond its 2012 contribution (11+/-4% of total regional runoff). First, the ice sheet becomes flatter as elevation increases, exposing greater areas to runoff  given a step change in the upper boundary of the firn runoff  regime. Second, the related emergence of slush fields and runoff  channels decreases surface albedo, creating additional melt in these newly saturated regions. The combination of these factors provides firn transition with powerful leverage on Greenland's future mass balance and contribution to sea level rise. 

What about Anthony Watts' 150 years?

Remember when Anthony got the idea into his head that the change in the structure of the firn only happens once every 150 years? He probably got this notion from something he read in the paper last year.  He wrote:

And this: A researcher said it was a recurring 150 year event that was ‘right on time‘:
“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.

Well that was very misleading. There has been no melting event of that type every 150 years. (In any case, 2012 minus 1889 is only 123 years.) The 1889 melt was from a study published last year. The research was first alluded to at the AGU Fall Meeting in 2011, and the research was later published in PNAS. The AGU presentation was before the big melt event of 2012. The 2014 paper referred to the 2012 melt and to the melt event in 1889, but there was no mention of any similar event "every 150 years". At the time of the "150 year" quote, there was a bit of a kerfuffle around the climate blogs - such as at Joe Romm's ClimateProgress, DailyKos and Andy Revkin's DotEarth. And something of a discussion on the blog of the lead author, Kaitlin Keegan.

It appears that the 150 average was based on a paper by Richard Alley and Sridhar Anandakrishnan in the Annals of Glaciology back in 1995. The "average" is meaningless, because there hasn't been a widespread melt every 150 years by a long shot. Below are two charts showing melt, based on the GISP2 ice core. I've marked with an arrow the third closest melt in time, after the 2012 melt and the 1889 melt, which occurred around 700 years ago. Click to enlarge it.

Fig. 4 | Melt against age for the GISP2 site. Years containing melt features are shown by green lines. The red line is the 100-a running mean of melt frequency (number of melt features per 100 years), and the heavy black line is the 1000-a running mean. Source: University of New Hampshire - GISP2

Here's another chart, from a paper by Meese et al in 1994 in Science, which is also based on GISP2. The arrows mark the melt layers, about which the authors say "These melt layers are extremely thin and affect the signal of only one summer". Apart from the single arrow which would be for the year 1889, the previously most recent melt was prior to 1250, 765 years ago.

Fig. 5 | The 100-year smoothed accumulation record from the GISP2 core for the period AD 500 to the present. The arrows show locations of visually identified melt layers in the ice core.
Source: Fig 2 in Meese et al (1994) 

As you can see, even if there was a melt "on average" every 150 years since whenever, it hasn't been melting anything like every 150 years.

Looking at Figure 5 above, nine melts in 1515 years is one every 168 years, but seven of those melts were over a period spanning less than 500 years, or once every 71 years. If you look at the chart above it, Fig 4, you can see that by far the most melt years were during the Holocene Climatic Optimum from around 6,000 to 8,000 years ago.

While Anthony posted the abstract of a paper by Kaitlin M. Keegan and co from 2014, and a part of a press release about a paper by Edward Hanna and co, he doesn't appear to have read what he copied and pasted, and insists the 2012 melt was a "weather event not a climate event". He seems to think it won't happen again for 150 years, without giving any sign that he's bothered to explore the issue.

The Keegan team attributed the melts of 2012 and 1889 to the combined effect of black carbon from forest fires and rising temperatures. They noted that "although 1889 and 2012 were relatively warm years, there have been even warmer years over the past century when the snow in the coldest part of the ice sheet did not melt."

In regard to the high temperature, they wrote:

During most summers in the 20th and 21st centuries, it has been too cold to permit snowmelt in the dry snow region of the GIS. Accordingly, the widespread melting in 1889 and in 2012 occurred during periods that were unusually warm....Temperatures were warmer in 1785, for example, but melting in the dry snow region did not occur in that year (Fig. 2A). Similarly, widespread melting in the dry snow region did not occur during the most recent record-breaking melt extent years of 2002, 2007, or 2010 (10−12). Thus, high temperatures alone are often not enough to cause widespread melt.

In addition the authors found that there were other years in which there was a high amount of black carbon from forest fires, but in those years there wasn't widespread melting because it wasn't as warm. They concluded that to get widespread melting, both factors were needed:

Unlike other years where BC peaks occurred but melting of the ice sheet was minimal, widespread melting of the entire GIS surface did occur in 1889 and 2012. Why did it occur in these years and not the others? Our data suggest that during these two years, abnormally warm summer temperatures combined with BC deposited on the ice sheet to reduce albedo below a critical threshold.

What Anthony didn't comment on was the authors conclusions regarding the impact of human-caused warming. The authors wrote:

Over the next century, climate change is predicted to raise both the average summer temperature and the frequency of forest fires. The Arctic mean summer temperature is predicted to increase 2–9 °C by the end of the century (24), and forest fire frequency is expected to at least double per 1 °C rise in temperature (25). 

They did the sums and suggested that in their most conservative scenario modeled, there would be widespread melting every six years on average, and "in the least conservative scenario, our projections suggest that the GIS should experience almost annual widespread melting by the year 2100."

It looks as if Anthony won't have to wait another 150 years for the next widespread melt. It also looks as if this signals climate change, not merely weather.

From the WUWT comments

Are you really interested in the silly thoughts of a bunch of conspiracy-theorising science deniers? Here are a few:

jorgekafkazar does a Lysenko

January 4, 2016 at 5:32 pm
The world has gone mad, and Science, our most important tool for maintaining living conditions, has been turned over to a bunch of Lysenkoists.

FJ Shepherd needs a lesson on averaging. If the average temperature were zero in summer, then half the time and/or places would be above zero.

January 4, 2016 at 3:59 pm
I am not sure where all this alleged melting is taking place on the Greenland ice sheet. The annual average temperature of Greenland is -16.8 degrees C, and even in the warmest month of July, the average temperature for the month is still below 0 degrees C.

I doubt daveandrews723 has ever applied the scientific method to anything, or understands what it is. He certainly hasn't in this case:

January 4, 2016 at 4:02 pm
It seems as though pre-conceived notions that must be “proven” regardless of the evidence are required in order to be a loyal climate scientist. Did these researchers ever learn the scientific method?  

Janice Moore gives what he or she thinks is a girlish giggle:

January 4, 2016 at 7:06 pm
Heh. Hi, Mod! #(:))… Tee, Tee, Tee….. (h)Eee. 

Patrick B clearly didn't read "their work", which was meticulously detailed. He just wanted to protest and was trying to sound clever:

January 4, 2016 at 4:06 pm
So did I miss the science part supporting their conclusion that the layer of ice created an impervious “cap” that forced some unidentified amount of melt water to be shed into the ocean? That theory seems to be completely unsupported by their work. Were there cracks in the “cap”? Was water transport through the glaciers actually different after the big melt? Did more melt water actually flow into the oceans in subsequent years? Etc. 

I don't know if Peter Miller thinks scientists should work for free, or if he is just against any and all scientific research:

January 4, 2016 at 4:29 pm
A classic case of grant addiction.

What a bunch of raving ratbags.

References and further reading

Horst Machguth, Mike MacFerrin, Dirk van As, Jason E. Box, Charalampos Charalampidis, William Colgan, Robert S. Fausto, Harro A. J. Meijer, Ellen Mosley-Thompson, Roderik S. W. van de Wal. "Greenland meltwater storage in firn limited by near-surface ice formation". Nature Climate Change, 2016; DOI: 10.1038/nclimate2899

• Climate change is altering Greenland ice sheet, accelerating sea level rise - press release in Science Daily

Arctic Report Card - Greenland Ice Sheet - report by M. Tedesco, J. E. Box, J. Cappelen, R. S. Fausto, X. Fettweis, K. Hansen, T. Mote, C. J. P. P. Smeets, D. van As, R. S. W. van de Wal, J. Wahr

Keegan, Kaitlin M., Mary R. Albert, Joseph R. McConnell, and Ian Baker. "Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet." Proceedings of the National Academy of Sciences 111, no. 22 (2014): 7964-7967. doi: 10.1073/pnas.1405397111 (open access)

Alley RB, Anandakrishnan S. 1995. Variations in melt-layer frequency in the GISP2 ice core: implications for Holocene summer temperatures in central Greenland. Annals of Glaciology 21: 64–70. (society members only)

Meese, Debra A., A. J. Gow, Pieter Grootes, Minze Stuiver, Paul A. Mayewski, G. A. Zielinski, M. Ram, K. C. Taylor, and E. D. Waddington. "The accumulation record from the GISP2 core as an indicator of climate change throughout the Holocene." Science 266, no. 5191 (1994): 1680-1682. DOI: 10.1126/science.266.5191.1680

Hanna, Edward, Xavier Fettweis, Sebastian H. Mernild, John Cappelen, Mads H. Ribergaard, Christopher A. Shuman, Konrad Steffen, Len Wood, and Thomas L. Mote. "Atmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012." International Journal of Climatology 34, no. 4 (2014): 1022-1037. DOI: 10.1002/joc.3743 (pdf here)