We've already talked a bit about the mass balance of the Greenland ice sheet. What we realize is that the overall mass balance of Greenland is negative. What that means is that the gains in mass, primarily through snowfall are exceeded by the mass losses from summer melt and runoff. There's also some evaporation there and iceberg discharge into the Arctic Ocean. As a result of this, the Greenland ice sheet is contributing to sea level rise as our Arctic glaciers and ice caps, as we've learned earlier. Well, let's talk a little bit more about of mass balance of the Greenland ice sheet. If you want to understand the mass balance, it's useful to split it into two pieces, what we call the surface mass balance and what we call the dynamical mass balance or the dynamic mass balance. The surface balance is just the snow accumulation, which is largely in winter, but actually it snows all the time all year in parts of Greenland, minus the evaporation and summer runoff and these are like runoff of meltwater streams on the ice all the way out into the ocean. Then there's the mass losses from iceberg discharge. That's the dynamic mass balance or the total mass balance. We have to think of both of these things. Here, let's just focus first on the surface mass balance component. Now, what we find on the Greenland ice sheet is the annual surface mass balance is positive over the higher, that's the colder elevations and negative in the lower or warmer elevations. What that means is at the higher colder elevations, there's more mass gained from the snowfall, than there is loss from summer melt and evaporation. Whereas in the lower elevations, that's where you're having more mass loss from runoff and evaporation than you are getting gain from snowfall. That's what we're talking about there. It really depends on elevation. Now the one thing is it's hard to measure. It's a really hard thing to measure. There are some direct measurements, but one thing we've learned to rely on is complex models that can allow us to really get a good handle on what these mass balance characteristics are by knowing things like what the precipitation rates are, what the temperature rates are. Far from perfect but it's still given us a lot of insight into what is happening. It's hard to measure. Yeah. Now also the surface mass balance for the ice sheet as a whole is quite variable. It's quite variable, it's high and positive in some years, negative in some years. There's variability in both snowfall and the runoff, right? That's what's causing that variability in that surface mass balance, both the snowfall that you're getting and the melt and the runoff, those are both highly variable. As a result, the surface mass balance in the whole is quite variable. But what we're also seeing is the surface mass balance, it's starting to turn negative. In general, if the ice sheet is in an equilibrium, we think that the surface mass balance which is positive for the ice sheet as a whole is balanced by the iceberg discharge. In that sense that means that the total ice sheet is in mass balance. What we're finding now though is the surface mass balance. It's highly variable, but it is also itself starting to turn negative. Now, here's a photograph, looks like it's from a helicopter, at Summit Station that's at the top of the Greenland ice sheet. That is very high up there. It's very cold. There's not much snowfall, but it does occur, but it's only rare summer melt events and there's really no runoff at all. I mean, very, very rarely you might get a surface melt or something like this. So the surface mass balance at Summit Station is absolutely positive, you have gained from snowfall, but there's really not very many losses, then you get some from evaporation, also what we call sublimation. But there's not really any runoff, this maybe very rarely a surface melt event, it's been recorded a couple times. But that contrasts sharply with these low elevations where the snowfall can be high in some areas, but that's countered by strong summer melt and run off. A very different situation in the lower elevations. Here in the photograph on the left, you can see meltwater streams inside into the ice sheet. There you see on the right, there's melt ponds sitting on top of the ice sheet. Now, it could be some of these meltwater rivers, they're really are rivers, some of them. Here's a scientist putting some kind of instrument in one of these meltwater streams. I really call it a river. It's incised right into the ice. It's really remarkable. That's pretty dark cold water, not a place where you'd probably want to go swimming for very long. Now, here's an example of the surface mass balance for one year. This is basically looking at September 2018 through August of 2019. September 18, September is basically the end of the melt season. That's when we start to get more snowfall. Autumn comes on, right? Sun sets, autumn comes on. It gets to be winter and then, the next August that's gone through the previous summer. So we've had a lot of melt and things like that going on. This is looking overall for that year. Now, the figure on the left is just showing what happened over that year. What you see is most of the ice sheet is in these warm colors, in these yellows, these oranges and the reds. Those areas are showing where the surface mass balance was positive. So that means there was more snowfall gain than there was mass loss through runoff and melting, those sorts of things. So most of it is positive. You see over the South East part of the Greenland, quite positive, very red in that area. Now, the figure on the right is just showing the difference from average. If we look at it for a number of years, what's the difference from average? So you see over a lot of the ice sheets is sitting in those kind of pale blue. So it's not much of a departure from average. But you can see along the coast, especially along the Western coast, all those blues. That means it was well below average. That means for this year, there were a lot more losses than there were gains. South East Greenland, you see it was somewhat positive. That's in those oranges. That means that there was more mass gains than there were mass losses. What are the units here? These are in millimeters of water equivalent over the ice sheets. That's the unit that we're looking here. It ranges from like plus 2,000 down to minus 2,000, a pretty big range. Now, here's a question. What we find here on average is that the surface mass balance is really strongly positive over the Southeastern part of the Greenland ice sheet. So here's a question. Why would that be? The answer is precipitation. Snowfall is really, really high in that region. It's an area with very, very high accumulation. Part of this is that there's all these storms coming up along the North Atlantic storm track. Basically, you get winds from the East, warm, moist winds from the East, they're riding up over the ice sheet over the higher elevations. As they ride up, the air is cooled and condensed and the moisture gets squeezed out, so you get a lot of precipitation in that region. It's not that it's cold. It's not that there's extensive cloud cover, although there would be with clouds with a lot of storms, but it's really that precipitation is really high in that region. Now, here's an annual cycle of the surface mass balance, and this is going from 2019-2020 as another example. The x-axis here is the month which is going from September through August. September, the end of summer through winter and to the end of the next summer, so September through August. On the y-axis, that's on the vertical part, that's the surface melts in gigatons per day. Gigatons is a lot of mass, right? It's a gigaton, right? What we're showing here is what was going on for the average as assessed over 1981 to 2010. Also some individual years including 2019 through 2020, which wasn't complete as this analysis had been done. Now, what we see if you look going from left to right, you see a lot of ups and downs and ups and downs and ups and downs. That's the variability in the system, natural variability. But you see that mostly, it's all above that zero line. So what we're seeing is mass gained through the season. Then we get towards like May and June and that's when summer really gets going. Then you see it turn negative. That's when we're seeing really at the lower elevations, primarily at the lower elevations where you're seeing a lot of melt and run off takeover. But remember, overall, the surface mass balance tends to be positive, whereas the dynamical mass balance, which is the iceberg discharge, that is negative. Now, in terms of the surface mass balance it's relation to melt, it's interesting to look at melt patterns over the ice sheet. This is something we can actually derive or look at directly from satellite data. This is showing two years. It's January 1st through September 15th of 2018, and January 1st to November 17 of 2019. This is the record. We have a little more on record there, and basically it's just showing the summer melt days, how many days we're actually seeing melt during the summer. What you see, it's quite variable. Two thousand and eighteen, most were in the lower elevations. That makes sense because it's warmer. But also seeing those blues there, a quite number of melt days. I think, 30, 40, 50 are a lot of those areas. Twenty nineteen was very unusual. There was a big, big, big melt event. Really most of the ice sheet you'll see here did show melt, the areas of the white are those that didn't show any melt, but you can see there's not much white on that figure on the right, and so there was a whole lot of melt. This is an example of the variability that you see. Now here's the surface melt area in a drift book to the different way for the ice sheet as a whole. What this is looking at the bottom is month and it starts out in April and it goes through October, and on the y-axis is the surface melt area and they're showing it for a number of different years. The point being made here is that we start out with basically no melt. Spring and summer comes along, the surface melt area increases and then it decreases as the sun starts to set. But you see how variable it is. Now I'm marking one peak there, July 2012, when essentially the entire ice sheet saw some melt. Even at Summit Station, they saw some melt. There wasn't any runoff or anything like that. It was just a little bit of surface melt, but there was surface melt, but you see how variable it is from year to year. Each of those spikes really corresponds to weather systems that are passing through, bringing in warm air, then the storm passes along and passes through and then it's replaced by cold air. That's what you're seeing here. Here is one example of what happened. This is the Watson River Bridge on Greenland. The coast of Greenland, West coast on July 11th, 2012 got washed out. I mean, there was so much melt going on really at the lower elevation that all this runoff was generated. Look at what happened there. I mean, this tremendous flood that occurred. These are kind of the dangers you've got if you're living next to an ice sheet. Now here's a question, which is more variable from year to year? The snowfall or the summer runoff. Both are variable we know because of natural climate variability, snowfall, how much snowfall is brought by storms? Summer runoff, how intense is that? How warm was that summer? These things all vary. Which is more variable from year to year, the snowfall or the runoff? Turns out it's the runoff. The runoff appears to be somewhat more variable than the snowfall. I might not have expected you to know this. I was surprised myself, but it really is the runoff that appears to be more variable. Now here's just a figure that shows the variability in each. What it's showing here is the surface mass balance total that's in these blue bars, and then it's looking at the part from snowfall, which is in the red bars, and the part from the melt water runoff, which is in the orange bars and the bottom axis, it goes by year 1980 and onwards, and the y-axis is the surface mass belt in gigatons per year. You can just see how variable all of the things are. The total surface mass balance is highly variable from one year to another. But that's because both the runoff and the snow accumulation are also highly variable. As I said, what we're starting to see now is hints that the surface mass balance itself seems to be starting to turn negative. That means that those losses from summer melt and runoff and also some evaporation are starting to exceed the snowfall gain. Now, the next video though, will talk about the dynamical mass balance and that relates to the discharge of the iceberg into the North Atlantic like the one that sank the Titanic. Thank you.
