For assessing changes of glaciers and to calibrate models, we need measurements. We need measurements over long time periods. So, we need so-called monitoring. So, glacier monitoring is a very important element for glaciological research as it delivers important datasets for process understanding, but also for model calibration. So, glacier monitoring can be done regarding different variables that I'm shortly going to cover in this course. So, glacier mass balance is a direct response to climate change. If climate changes, glacier mass balance will respond immediately and therefore, it is a very good variable to follow the changes in glaciers. Glacier mass balance is defined as the change in the glacier volume or thickness over seasons, winter to summer, or over individual years. The measurement of glacial mass balance is not that simple as we need to go onto the glacier, when we follow the direct glaciological method, and perform the measurements at different locations on the glacier. In winter and the upper part of the glaciers, so-called snow pits need to be dark to measure the density of the snow and determine the snow water equivalent. This can be seen on the left side of this image. We also perform density measurements using snow cores as you can see here on this image. So, snow cores are taken to measure the weight of the snow and therefore to determine there's no water equivalent. And we also measure the movement of the ice using GPS at mass balance stakes. On these images, you see some of these mass balance stakes we use to measure the melting of the ice at individual positions. So these stakes are drilled into the ice. And after one year, these stakes are revisited, and we can measure how much ice has been melted during this time periods. So this is the basic concept of measuring glacial mass balance directly. So we obtain the mass balance at individual points. For a complete mass balance monitoring program, these individual measurement points are distributed across the entire glacier. So we have some mass balance points at the bottom of the glaciers in the ablation area and some point measurements in the upper part of the glacier in the accumulation area. If we extrapolate these point measurements over the entire glacier surface, different mythologies are possible. We can calculate the volume change of the glacier in individual years or seasons. If we consider the volume change over one year, this corresponds exactly to the water released from the glacier and therefore, this is an important hydrological variable as well. Besides the direct glaciological method, where we go directly onto the glacier and measure the mass balance at individual points, also indirect methods exist. The volume change of the glacier over longer time periods, for example, five to 10 years, can also be measured using the geodetic methods. What is a geodetic method? Digital elevation models of the glacier surface elevation are compared and differentiated, and this provides indirectly the volume change of the entire glacier. By combining these two different datasets, so the volume changes over long time periods based on digital elevation models, hence the temporarily highly resolved measurements using the direct methods. We can obtain highly accurate combined products as you can see here. So, In blue, we have the direct measurements from the glaciological methods, from going onto the glacier, providing the high temporal resolution. And in red, we have the long-term volume changes from the geodetic methods. And if these two theories agree with each other, we can be sure that the result is relatively accurate. If we extrapolate all measurements that we do in Switzerland for individual years to the entire country, we can calculate the mass loss of glaciers for individual years. And on this figure, you nicely see the glacial volume lost between 2011 and 2016. This means, for all glaciers in Switzerland, we have lost between 0.3 and 1.5 Kilometer cube of ice in single years over the last five years. This means that relating this values to the total volume of ice in Switzerland, we lost between 0.6 and up to three percent of all ice volume in Switzerland in one single year. And this indicates that climate change is really going on fast, and it is possible to remove up to three percent of the entire glacier ice volume that we still have left in Switzerland. Another variable that we have in glacier monitoring are the so-called length changes. Length changes can easily be measured by just accessing the glacial snout and monitoring how the snout is changing, so, if the snout of the glacier is advancing or retreating. This figure here shows the glacial length changes of different Swiss glaciers over the last 130 years, and it is quite easily visible that there are strong differences between the glaciers. Whereas the red line, great Aletsch glacier, retreats continuously with other glaciers like glacier Rhtriol that is advancing, retreating, advancing, retreating, advancing, and retreating. However, the climate has been the same for both glaciers. And here we see that the glacial response time is highly important for shaping the length change of the glacier. Whereas great Aletsch glacier is too large to respond to short-term climatic changes of one or two decades. Glacier Rhtriol, that is smaller and relatively steep, can react by advancing to a short time periods of better climatic conditions, like with more precipitation and cooler temperatures, as it occurred between the years 1960 to about 1980. So, glacial length changes are easy to be measured. However, are difficult to be interpreting in terms of climatic variations. We also have a third important variable in glacier monitoring. These are the glacier inventories. Glacier inventories provides outlines and areas for all glaciers within the mountain range. In Switzerland, we have three complete inventories for 1850, for 1973 and 2010. And these inventors are very helpful for inputs for regional modeling studies, as well as to just follow the evolution of a large set of glaciers over a long time period. With this glacial inventories, we see, for example, that over the last 40 years, glaciers in Switzerland have lost about 30 percent of their area.