The Dark Snow Project is staring up again, it being almost summer(ish) in Greenland.
The results in the study of the odd 2012 winter are now in. That year, there was a huge spike in melting on the surface of Greenland. (Discussed here.) One idea is that a good part of this melting was caused by extra soot from extensive wildfires in North America, which increased the amount of solar energy collected on the ice surface.
The results confirm this, and the Dark Snow team is returning this year to collect more information.
A paper came out in today’s Nature about glacial melting and its contribution to sea level rise. This paper does not present new research, but rather summarizes and evaluates the last several years of research on modeling and measuring contiental glaciers and their dynamics.
From the Abstract:
Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain
There are many difficulties with measuring and understanding the dynamics of melting of large continental glaciers, the large ice sheets that cover Antarctica and Greenland. As ice melts from these glaciers, they grow lighter and this allows the underlying bedrock to rise up, and conversely, if snow is added to the surface this increases the amount of depression of the underlying bedrock. For this reason you can’t just measure the surface of the ice to estimate how much has been added or removed. When ice melts on the surface, some of it travels down into the glacier and some comes right off the surface. The ice that goes into the glacier may cause deeper ice to melt, or it may provide lubrication to the base of moving streams of ice. As a glacier loses mass at the edge through calving of ice bergs, and the margin retreats away from the sea, the degree of calving, which is an ice-ocean interaction effect probably decreases. Large masses of ice are “grounded” at the outer margin on a “grounding line” beyond which is floating glacier (not sea ice, but large masses of ice undercut by the sea). The grounding line can move towards the sea or away from it, and the dynamics of this movement are complex and difficult to model or measure. Many of the Antarctic grounding lines occur on surfaces that slope downwards in the inland direction, which makes the dynamic a bit more complicated to measure.
Major changes that have improved estimates include adding dimensions to some of the models, such as considering both vertical and horizontal forces along grounding lines. Also, newer models use a finer resolution. However, the increase in resolution is thought to be insufficient; current models are not calculated at fine enough resolution to include numerous smaller ice streams that are narrower than the sampling density of the models.
It appears that the range of uncertainty of ice-melting models has improved significantly over the years so greater confidence in their predictions may be warranted. The best estimates of future contribution to sea level rise of melting glaciers is still highly variable, however.
The current estimates of contributions to sea level rise in mm per year from various studies are between 0.59 and 0.82 from the major ice sheets, between 0.71 and 1.4 for ice caps and glaciers, about 1.1 for thermal expansion, and a negligible but positive amount from changes in terrestrial water storage. These modeled amounts sum to 1.66 mm per year or 3.11 mm per year depending on the set of sources that are used. The observed change in sea level rise over the period from 1993=2008 is 3.22, so there is good agreement though the models are a bit light.
But there is a problem. Several areas of uncertainty exist in the models that are currently in use, and my impression is that these areas of uncertainty could be associated with dramatic errors in sea level rise estimate. The dynamics of grounding line changes, the role of lubrication at the base of glaciers (which can cause ice streams to speed up on their way to the sea) and the effects of warm currents shifting their position in Antarctic to cause more melt at the boundaries are among those factors that are least known and that have the highest uncertainty. Also, the seaward edge of continental glaciers are not only held in place by their grounding line on the continent, but also by more distal parts of the floating segment of the glaciers being pinned on prominence. As far as I know the effects of pinning being disrupted or lost are not included in any of the models. Also, I’m pretty sure that the effects of sea level rise on grounding and pinning have not been adequately addressed.
That these issues may be a problem is empirically suggested. The paleo-record shows that continental ice melting and associated sea level rise may occur in fits and starts, with steady melting punctuated by brief periods of extreme melting. The current models don’t seem to predict this sort of event, though these events probably happen.
Hanna, E., Navarro, F., Pattyn, F., Domingues, C., Fettweis, X., Ivins, E., Nicholls, R., Ritz, C., Smith, B., Tulaczyk, S., Whitehouse, P., & Zwally, H. (2013). Ice-sheet mass balance and climate change Nature, 498 (7452), 51–59 DOI: 10.1038/nature12238
You know about Albedo. No not Libido, Albedo. Sunlight is to varying degrees reflected off the surface of the earth more or less back into space. That is Albedo. The vast regions of snow and ice covered glacial surfaces in the northern and southern Polar regions contributes to a good amount of the Earth’s Albedo. In the north, the biggest chunk of that is the ice-covered subcontinent of Greenland.
Over the last several years, Greenland’s Albedo has diminished. This was in part predicted by scientists who expected that warmer conditions would change the nature of ice and snow crystals on the glacier’s surface, thus darkening it and causing less reflection. But the rate of Albedo reduction in Greenland has been much more rapid than expected.
This could be because of increased deposition of soot from wildfires and possibly increased dust from aridification, both caused in large part by global warming. This means that global warming could be causing more global warming, and more importantly, that it could be causing it at a higher rate than previously expected.
There is a research project afoot that will look into this, but that is having a hard time getting funded, probably for bad reasons. The project is now asking for crowdsourcing funds. Please have a look at the following video, learn some interesting stuff, get scared, then click the link below the video to donate money to the project, then feel better. But not too much better.
We are becoming aware of two very important changes in the Arctic that you need to know about. These are separate thing but related, and both are almost certainly the outcome of anthropogenic global warming (AGW). They are:
The sea ice that covers much of the Arctic Sea during the winter is normally reduced during the northern summer, but this year, the reduction has been dramatic. There is less sea ice in the Arctic Circle than recorded in recent history.
The massive continental glacier on Greenland, the largest glacial mass in the Northern Hemisphere, has undergone more melting this summer than observed over recent years. A few weeks ago you may have heard about a great melt in Greenland. This is not that. The July Melting event was interesting and I’ll discuss it below, but the news story breaking today is about something else.
The Arctic Sea Ice
Every summer the large areas of the Arctic Sea’s ice melts away, then it refreezes each winter. The minimum extent of ice is typically reached in about mid September. The extent of ice at this minimum has been getting steadily less over time. Direct and accurate measurements don’t go back far enough to track the effects of AGW over the entire time this has been happening, but we can pretty easily look at the last few decades. Have a look at this graph:
Note that the total amount of sea ice in an average year in the 2000’s decade is about one third less than the total amount of ice in the 1980s, at the minimum period in September. Below the 2000’s line are plotted the three most ice-free years in the dataset; those are the extreme years. The present year, 2012, is tracked through mid-August on this graph.
The present year, 2012, is on track for breaking all records for the Arctic Sea ice minimum.
Here’s an interesting side story. Notice how the red line for 2012 is much straighter in a downward direction than the other lines for the same time period for the last several days of measurement. My understanding is that large storms in the Arctic appeared, covering the sea ice from observation for several days, and when the storms cleared a whole bunch of the ice was gone. This is not that unusual. Storms hasten the disappearance of sea ice. But this was a more dramatic than typical example of this event.
In case you were wondering if it was storminess and not AGW that is causing this year’s ice to be less than the other years, and I’m sure that climate change deniers will make this claim, keep in mind that a) this recent storminess does not explain more than a small amount of the ice reduction compared to overall melting and b) AGW has caused there to be more storminess in the Arctic and more warmth in the Arctic.
The Greenland Melt-off of July
Before getting to the really big news from Greenland, I want to first remind you of an interesting event that happened in July, reported by NASA (I mentioned it here). Every summer, some of the ice melts on the surface of Greenland’s massive glacier, then much of that refreezes. However, the melting is usually spotty…here and there and rarely everywhere. The highlands are too cold to melt at all in some years. But July was very warm and there were a few days when virtually the entire surface of greenland melted. There were slushy puddles everywhere. Then, much of it refroze. This happens now and then. We can assume that widespread melting like this is more common in a warmer world, and will be part of the process of glacial wasting as the Greenland Ice Sheet turns over time into seawater. But this particular event was in and of itself not entirely unheard of.
But there is a neat graph that shows why glacial melting is both more important than one might thing and also more complicated than one might think. Have a look:
Just as sea ice extent in the Arctic reduces every summer, the albedo of the Greenland Ice sheet reduces. The white fresh frozen snow that falls over the winter is highly reflective…has a high albedo…and as it melts and gets slushy and mixes with water is has lower albedo. Plain water has very low albedo compared to snow. This is important because high albedo surfaces reflect a lot of sunlight (which provides heat) back into space while low albedo surfaces absorb sunlight, converting it to heat that adds to the local and ultimately global temperature.
There is a feedback mechanism at work here. Imagine that something happens to make for a late Canadian winter with a widespread heavy snow storms much later than usual. This could be caused by a combination of events happening just right in a given year. So, late in the spring there is a lot more snow cover than normal. This snow will reflect a lot of sunlight away so the beginning of the summer is cooler. If this effect lasts into fall, early snows may cause the subsequent winter to be even colder and snow to fall instead of rain, producing more Albedo. Etc. Conversely, something that reduces albedo in a given year may cause more melting of snow and ice, which means less albedo, and thus more melting. You get the picture.
In the days before we understood Orbital Geometry and before we had a very good ida of how air and sea currents really work, this feedback effect with albedo was considered as a candidate for what causes glacial periods to come on and go away. We now know that albedo related forcing and feedback is not the prime mover in climate change, but it is still an effect.
In the graph above, you see the line for the present year. Note that Greenland albedo is lower for the entire year than for any of the other years plotted. Then, in July, that melting event occurs. Then, the water freezes. Albedo is not like sea ice extent (compare to the two graphs). Sea ice extent is a slowly changing ponderous slow moving variable, while Albedo is al wiggly-wobbly and highly variable. A big snow storm, albedo goes up. A big rain storm, albedo goes down. So, the wobbliness of the line does not mean too much, but it is very cool to see the direct relationship between observed widespread melting and albedo. And, this effect will probably play a role as the Greenland Ice Sheet melts away.
And now, for the big news
The really big new that is coming out today is about greenland. From a press release covering the findings of Mardo Tedesco, professor of Earth and atmospheric sciences at The City College of New York:
Melting over the Greenland ice sheet shattered the seasonal record on August 8 – a full four weeks before the close of the melting season…
This year, cumulative melting in the first week in August had already exceeded the record of 2010, taken over a full season
“With more yet to come in August, this year’s overall melting will fall way above the old records. That’s a goliath year – the greatest melt since satellite recording began in 1979.” …
This spells a change for the face of southern Greenland, he added, with the ice sheet thinning at its edges and lakes on top of glaciers proliferating.
Professor Tedesco noted that these changes jibe with what most of the models predict – the difference is how quickly this seems to be happening.
To quantify the changes, he calculated the duration and extent of melting throughout the season across the whole ice sheet, using data collected by microwave satellite sensors.
This ‘cumulative melting index’ can be seen as a measure of the ‘strength’ of the melting season: the higher the index, the more melting has occurred.
This year, Greenland experienced extreme melting in nearly every region – the west, northwest and northeast of the continent – but especially at high elevations. In most years, the ice and snow at high elevations in southern Greenland melt for a few days at most. This year it has already gone on for two months.
Here’s the graph showing the relative amount of ice melt per year in Greenland for the last few decades.