There's just no doubt that the Greenland ice sheet mass balance has turned negative. It is losing mass an that means it is contributing to sea level rise. Now, as we've talked about before, we think about the mass balance of the Greenland ice sheet. There's really two parts that we have to consider. First is the surface mass balance, and that is the snow accumulation minus the summer melt runoff. And also there's some evaporation and sublimation that you'd have to work through. And then there's the dynamic mass balance, and those are the mass losses from iceberg discharge. Now if we think about the surface mass balance in a steady state climate, let's say the climate wasn't changing, that surface mass balance would be positive. And the dynamic mass balance, the iceberg discharge would be negative. But as we've seen, the surface mass balance is starting to itself turn negative. That means, yeah, you get lots of accumulation by snow, specially like over Southeastern Greenland. But that seems to be starting to be outweighed by the summer of losses due to, especially to melt and run off. So that's the surface mass balance. But what I want to focus on now is this dynamic mass balance. That's the mass losses from iceberg discharge. Now here's the key point. The Greenland ice sheet is drained by a number of very, very large glaciers. I mean, these are glaciers miles across, and these are lot of anyhow discharging icebergs into the ocean. That's a mass loss. The Titanic was probably sunk by an iceberg that came from Greenland. We don't know for sure, but it's probably very likely. Now some of these big drainage glaciers as there known as, they show accelerating flow. So they are discharging more icebergs, are moving faster. Now it's a complex issue why some of these glaciers are doing this. A meltwater lubrication effect, which we'll get to in a minute, and then thinning, will be called back pressure effects. These are factors which tend to slow the velocity of the ice of the glaciers. They work against basically the river flow. If you want to think of a glacier as of river of ice, which it kind of, it is wants to go that way, but we've got these other processes which want to push it back. And those back pressure effects as they are called are changing. Now, here's a satellite image, an Aster satellite image, high resolution to NASA satellite for a region over South East Coast of Greenland. And you can see on the left part of that figure it's where the ice sheet is, and then you see on the right some of these big big glaciers that are draining the ice sheet. So this is what you see if you look down from space or even if you were in a jet airliner looking down, you would be able to see these things if you could get a nice clear surface of Greenland, which is not always the case. It could be a cloudy place at the lower elevation, but that's what these things look like. Now, here's a Greenland iceberg, right? Discharge from one of these glaciers, somewhere, probably from Greenland. I assume that one is from Greenland. Remember the tip of the iceberg, right? Only 10% of it is sticking above the surface, 90% of it is below. There's a Coast Guard plane, it's a C130 part of the International Ice Patrol I believe, which Maps these icebergs, because even today they can represent real hazards to shipping. Now, about these accelerating glacier, one idea out there is this basil lubrication theory. It's sometimes called the Zwally effect because it was popularized by this guy J Zwally, a well known a NASA scientist. And the idea is this that we have these summer melt ponds and accumulate over top of the ice surface at the lower elevations, not of these high elevation but these low elevations and get these melt pods. And what they do is they quickly drain to the base of the ice sheet via moulins and crevasses in the ice. And if that melt water coming down through these moulins and crevasses really fast enough, there's a lot of water pressure that's generated at the base of the ice sheet. And this is sufficient to really basically lubricate the base of these glaciers so that they flow faster and they can discharge more icebergs into the ocean. Now here's some of these surface melt ponds. This is again a satellite image that I'm showing, so you see towards the left of the image that's where it's ice free. And then you see that kind of light, blueish shade. That's down on the ablation zone of the Greenland ice sheet, and you see those dark blue patches. Those are all surface melt ponds. And then you see to the right of that the higher elevations where these melt pond do not form, you're getting down to the right towards the accumulation zone of the ice sheet. Here's another shot of a surface melt pawn. This is clearly an aerial photograph. So, you can see what they look like if you were to fly over one, they're quite pretty quite blue, right? Very pretty things. And this is a little cartoon here showing this drainage through moulins and crevasses. So you see label, dairy, moulin and crevass. So you get these melt pawn that form and also these crevasses can fill with a lot of water too. And sometimes they could basically bore the by the water pressure itself it boars its way down to the bottom of the ice sheet and just drains down very quickly. Sometimes it can lubricate the base of that ice sheet. Now here the ice flow is from left to right. It's kind of shown there left to right. The idea of this lubrication at the bottom is it makes it slide faster from left to right. Now here's a question. How big can these melt pawns in the accumulation zone of the Greenland ice sheet get? The answer is up to about six meters deep and several square kilometers. So, these things can be pretty big six meters deep. That's 20 feet of water, right? So it's not like the Great Lakes or anything like that, but it's still a lot of water and several square kilometers, so that's kind of how big they can get fairly reasonably sized now. Here's a photograph of brave scientists on a melt pawn scientist go and study these things. That was, I'm not sure if this was taken by my colleague Twyla Moon, or she is one of the people out there on the raft and so there are probably measuring things like the depth of the ice, maybe contaminants in the ice. I'm not quite sure, but we know it's pretty darn cold water. And here's a shot of a muilan on now this is this is probably sometime in the spring before the melt water started, so you could just kind of look down at right and it goes all the way to the base of the ice sheet. Don't want to fall down into one of these things. And if you were to. One of these melt ponds that can suddenly catastrophically drain. Well you might go right down to the bottom of the ice sheet. Maybe not to be found for several 100 years when you come out on the other side. As a matter of fact my colleague Twyla tells us that one of these melt pawns that she was studying some time ago after they had left that night it catastrophically drained. So it's good thing they weren't out there on their little raft. Now I mentioned these back pressure effects as well. The issue here is we have fiord walls and bedrock sills and the weight of the glacier on the bedrock itself that impeads the glacier flow. So, the glacier want's to flow like that okay. But these back pressure effects work the other way. A bedrock sill basically, if you have a bedrock it can basically stick in the ice and keep it from flowing. The idea is that as things warm up, we're getting a thinning of some of these big big outlet glaciers of these big drainage glaciers and what that does is reduce these back pressure effects. And so what happens is the glaciers tend to flow faster and that can foster further thinning by extensional flow and also a strong retreat of the front of the glacier itself. Instantly know we're really talking here about is this retreat were talking about glaciers that drain or extend into the ocean. And what we found is that probably these back pressure effects are more important than the Zwally Effect. But both effects are there. Were finding at the back pressure effects maybe are more important. Now ere's a satellite image two side-by-side satellite images of the Helheim Glacier. Now this is one of those big Greenland glaciers, and the one on the left was taken in May of 2001, and there you see, there's the glacier on the left and on the right you can see the calving front worth calving icebergs into the ocean. Now the one on the right was taken in June of 2005 and you can see how this glacier really retreated back. And what's happening here is its thinning and it's also accelerating at the same time, but backing up at the same time. In other words, the glacier front is backing up, discharging lots of icebergs into the into the ocean. And of course that's contributing to sea level rise, so sea level rise okay, what's going on here in terms of the relative roles of the dynamic mass balance? That's this glacier flow and the iceberg discharge and the surface mass balance. And that's just that, balance between the accumulation by snow and the losses through summer melfan evaporation sublimation well now both are contributing at this point to sea level rise from what we Can gather at present the contributions to the Mass Lawson total from the dynamic mass valid and the surface mass balance seemed to be about roughly equal in magnitude. Now it's a hard thing to get at, and we have to also have models to help us look at this sort of thing. And so here's a combination of satellite data and modeling that we can use to try and assess this sort of thing. And this is going back to 1980, so there on the x-axis is year 1980 going through about 2018. And there's the mass change on the y-axis in gigatons, right, gigatons. It's a lot of tons. And so basically back in the early 1990s, we were pretty much in balance between the effect of the, I should say that the ice sheet was pretty much in a balance. That it was really not losing mass. But as we've gone through time, what we found is that it is losing mass. Now the darker blue is showing the total from the dynamical mass balance. And from the surface mass balance and the other two colors are showing the contributions individually from the dynamic mass balance and the surface mass balance. And they're roughly equal in magnitude. But there's uncertainty here. There's certainly, we don't have all these numbers pinned down, but it looks right now that both of them are contributing. So dynamic mass balance, what we've got is more glaciers being dumped into the ocean. Surface mass balance, what we have is really a situation now with the effects of the summer melt and the run off are starting to dominate over the winter gains. It was really just the gains by snow. Of course, it can snow over a lot of the ice sheet during the summer, as well. But it's just saying that the summer melt is starting to dominate over the accumulation by snow. So here's one more question. So which do we think is going to play the more dominant role in the total mass balance through the 21st century? The surface mass balance or the dynamic mass balance? The answer is, we're not sure. The picture seems to still be kind of fuzzy, right? And so this is one of the wild cards out there. We don't know everything about the future. I think it's a sure bet that the Greenland ice sheet is going to continue to lose mass as we move through the 21st century. It will continue to contribute to sea level rise as will the melt of glaciers and ice caps around the world. Including those in the Arctic and the ocean thermal expansion. That is the ocean sea level rise due to the ocean warming up itself because temperature has an effect on the density and warmer water is less dense. All of these things are at work. But for the Greenland ice sheet, which is going to dominate the effect of the surface mass balance or the dynamic mass balance, we are still not sure, thank you.
