If you love snow and ice, one of the places you really need to go is the Greenland ice sheet, because there's a lot of snow, and certainly there's a lot of ice. I was only there once in my life, only for a short time. I hope to get back there again. Well, what are some of the aspects of the Greenland ice sheet? First of all, Greenland is the largest island in the world. Now it's administered by Denmark, but they have home rule. There are people on Greenland, its population of about 60,000, and most of Greenland is covered with in ice sheet. That ice sheet is up to 3200 meters thick at its thickest. So you were talking about something around 10,000 feet of ice. It's a hell of a lot of ice. Now the other thing about the Greenland ice sheet is it losing mass and this is contributing to sea level rise. No doubt part of the observed sea level rise is due to the loss of ice from Greenland, both from melt and through the discharge of icebergs. Now here's a question, how many ice sheets does our planet have? Glaciers and ice caps and ice sheets tend to be drained by large glaciers, but we really only have two ice sheets on our planet. That wasn't always the case, back at the height of the last Ice Age, about 25,000 years ago, we had several other ice sheets. Notably North America, a lot of North America was covered by what was called the Laurentide Ice Sheet, which was a very, very big ice sheet way, way, way bigger than Greenland. But right now we've only got two. Now, what does an ice sheet look like? Well, how does it work? This figure I'm showing here is kind of a simple schematic. If we think about an ice sheet well, at the higher elevations is what we call the accumulation zone. And that's where the gains of mass primarily from snow, exceed the summer losses of mass by either from melting, also sublimation can be occurring all the time. And that's a situation where you go from a solid like ice right into a vapor, that can actually happen. Now at the lower elevations is what we call the ablation zone. And that's where the summer mass losses basically by melt, but also iceberg discharge through these big outlet glaciers exceeds the winter gain. And the line dividing the tools will be called the equilibrium line. Now what is happening for the ice sheet to maintain its basic profile is there has to be a constant motion of ice, basically downward and outward. Does an ice flow from the higher into the lower elevations. And as I mentioned, these big ice sheets, Greenland ice sheet the Antarctic ice sheet are they tend to be great drained by a number of big glaciers that then a many of them just discharge into the ocean. Now, here's a shot of the summit station. Now there's a station of the very top agreement, pretty much at the top, called summit, appropriately named. Very much in the accumulation zone, rarely does melt ever occur in that region, once in a blue moon it might. There's a station up there. Scientists are up there measuring all kinds of things, measuring properties of the ice sheet, measuring aspects of the atmospheric circulation. So you see, this tracks of snow machines and like that, so there's a number of people that work up on summit station. It's a hard life, but I bet the food is actually pretty good, kind of has to be in a situation like that. Now, this is a photograph from the ablation zone. That's where the summer losses exceed the Winter Games. And one of the things you see is ablation zone, is these big meltwater streams and things like that. You can see this one rushing out into in the background, a big melt pond, look at that. It looks almost like a lake, [INAUDIBLE]. It's actually a melt pond that is sitting on top of the ice. Now, here's a photograph of some scientists studying the ablation zone. Looks like they've got some instruments out there to measure wind speed and direction. I suspect they're also measuring things like incoming solar radiation and albedo, of course that's reflected solar radiation. They look pretty busy. How did they get there? I suspect they were probably dropped off there by helicopter, or maybe they got their order snow machines, it's not clear. Now, Greenland ice sheet, it's a cold place, it's certainly a cold place. Now what I'm showing here in these three side by side images are average temperatures on the surface of the Greenland ice sheet for 2014, just the year 2014. A fairly typical year for January on the left, April in the middle, and July on the right. Now January, at the higher elevations of the ice sheet towards the interior, that would be, you're talking about average temperatures -40, -50 degrees C, darn cold. But it's a dry cold, of course, you can manage it if you're a well dressed. Of course, it can get windy at times. Now we go to April is warmer, but it's still relative thing. It's still pretty darn cold, you're still talking about temperatures -15, -20 even lower than that -25. Now the one on the right is looking at July, that's the warmest month and you see some of those oranges and yellows down near the lower elevations or down to the South. Now still, those are slightly below average, slightly below freezing temperature. But at the top it's still -10 degrees C, something like that. At the sort of summit area is the highest elevations of the Greenland ice sheet, it doesn't show much melt, it does once in awhile though. Now the ice sheet is drained by a number of very, very big outlet glaciers. Most end up in the ocean and they discharge iceberg. So when we think about the mass loss from Greenland is not just summer melt, it's icebergs that discharge into the ocean. And the Greenland ice sheet is the primary source of icebergs in the North Atlantic. If you think about the iceberg that sank the Titanic, very shortly, I think it was sunk by an iceberg that came from Greenland. One of those really big glaciers that drain the ice sheet is Jakobshavn, a very, very notable biggie. Drain 6.5% of the ice sheet, just that one glacier, and it turns out it spawns about 10% of Greenland iceberg. Titanic was sunk by icebergs from Jakobshavn could be, it could be, will never know. Here's a satellite image of Jakobshavn glacier. On the right you see where's labeled Jakobshavn glacier, you see the crevasses and things like that. And then once marked in there is what they called the calving front. And that's where icebergs calve into the ocean. In this case, into Baffin Bay and Jakobshavn Glacier, this is mild, in miles across, it's a huge, huge glacier. Now you see in front of that calving front is what's called ice melange, and what this is, is a melange of icebergs and sea ice, kind of all jumbled up avalanche, is they call it kind of a mixture. Lots and lots of icebergs get discharged from Jakobshavn, and some of these other did glaciers that drain the ice sheet. Now let's think about melt, surface melt. Now of course, it basically occurs only in summer, and it's a mostly the lower elevations that ever see melt because these are the lower elevations. These are the warmer parts of the glacier. Now there has been a general upward trend in melt intensity and the extent of melt since we've been monitoring things, and that can go back about 1979. Big notable events and in 2012 and 2019, and 2012, there was an event that nearly the entire ice sheet at least saw some melt, lower elevation saw quite a bit. 2019 was another big one. Now, as I mentioned, you can get these surface melt ponds, you get surface melt at the higher elevations. It's just a little bit of melt, maybe it trickles into the snow, maybe not even that. But the lower elevations, lots of melt and you get melt streams incised into the ice itself and many big melt ponds. And some of those melt ponds will drain quickly down at the base of the ice sheet through what are called moulins, which are basically holes that develop in the ice sheet. They go all the way from the surface down to the bottom. Now here's the illustration of the melt that occurred in 2019. This is from January 1st to November 10th. And anything that's not on the white is where there was melt observed in the areas in the yellows and the orange is where the melt was stronger. Basically more melt days is what I'm showing here. Now, of course, it's mostly in the summer, so January there's not much going on. So really this melted something that maybe in some parts began in May or something like that. But the point is, if you look at 2019, most of the ice sheet did in fact see some surface melt even the higher elevations. Some of the higher elevations, very highest elevation, some parts in the North didn't see any, but a lot of the areas certainly did see melt. One way to look at this, we can track this from satellite, and what I'm showing here is a graph on the x axis. It's showing month, April, May, June, July, August, September, October. And on the y axis, I'm showing the melt extent, that is the That is the percentage of the ice sheet that saw melt and what you see in the red, the red is what was going on in 2019. And that sort of blue line with the dashed line, that's sort of the media. That's what you typically see in a year, and what you see in 2019 is these big spikes. We had one in early June. You had one in late July, early August. These spikes, the one in August, it looks like 60% of the ice sheet was showing some melt. But it's very spiky and what that is is actually the effects of individual weather systems that are moving through, advecting or bringing in warm air that causes melt and then that storm passes on and maybe it gets colder. And the big melt single goes away. This is typical of what you see, so you see this overall increase in the extent of melt through time, but then you see big events like this and big spiky events. That's kind of how the system works. Now, I mentioned these melt ponds and here's a photograph. It looks like it's from a helicopter of a camp, a scientist on their camp, out near one of these surface melt ponds so you can see that melt pond in the back in the blue. And here we have brave intrepid scientists. Some might say foolish scientists, taking their rubber raft out to measure aspects of this melt pond. So what are they measuring? I'm not quite sure. Probably measuring things like the depth of it, the temperature, but I bet the temperature is pretty low and pretty darn cold. Maybe some other properties of it. But there they are in their little life raft, taking measurements of what was going on, and the danger of being out on the melt pond is that, as I mentioned, these things can catastrophically drain through a moulin which is a hole in the ice. So here's a picture of a moulin, looks like whatever lake was there had already drained. My understanding is those scientists who are out on that melt pond, they told me that overnight, after they had left, the thing indeed did catastrophically drain in a matter of hours. So it's a good thing they weren't out in their life raft when that particular thing happened. Just a few words on the mass balance of the Greenland ice sheet. The mass balance is negative. What I mean by that if we look at all of the winter gain and all of the summer loss, overall it's negative, meaning that the Greenland ice sheet is contributing to sea level rise. How much? 260 to 270 gigatons per year since not 2002. That's a lot, that's a lot of gigatons, a lot of mass, more summer melt going on, more iceberg discharge. And also Arctic glaciers and ice caps are also shrinking as they are worldwide, so the Greenland ice sheet is only part of what is going on. That's an illustration of the mass balance from the GRACE instrument. It actually measures gravitational anomalies, kind of a fascinating instrument, going back through 2002 through 2016, and what you can see is mass balance is definitely becoming more and more negative. You see ups and downs and ups and downs and that, that's the winter gains versus the summer losses. So gaining in winter, losing mass in the summer. But we see clearly, overall there is a clear mass loss. This one only goes through 2016. But well, if we update this through other data sources, it continues to go down, so the Greenland ice sheet is certainly losing mass. I hope we've learned a little bit about the Greenland ice sheet and will learn a little bit more in some future videos.
