Listening to the glaciers

Fanny Brun is a glaciologist at Grenoble's Institut des géosciences de l'environnement (IGE), specializing in glacier mass balance. In addition to her work in the field, she has developed satellite data analysis tools that have enabled her to produce 3D maps of glaciers worldwide. In 2024, she was awarded the Collège de France Award for Young Researchers for the excellence of her early career and her outstanding scientific contributions.

At the heart of our planet's mountains, glaciers are veritable water towers. Yet the stored water they contain represents only 1 % of all the ice on earth, the rest being distributed between the Greenland and Antarctic polar ice caps. Glaciers are no less important to the hydrology of certain regions, and are heavily impacted by global warming, of which they are both witnesses and victims. Fanny Brun, a glaciologist at Grenoble's Institut des géosciences de l'environnement (IGE), has been fascinated by these frozen colossi ever since she first encountered them as a youngster, hiking and mountaineering with her parents. Since then, she has worked to put glaciers at the heart of her work, a way for her to combine the intellectual effort of scientific research with her enjoyment of the great outdoors, and the mountains in particular.

Establishing a glacier health record

When she began her thesis in 2015, Fanny Brun was interested in the mass balance of glaciers, a veritable health record that tracks their change in volume or mass from one year to the next." To calculate this balance, we make the difference between the mass of snow gained, or accumulation, and the mass of ice lost, or ablation, from the glacier," explains Fanny Brun. To do this, the classic method is a field approach that involves going directly to the site of a glacier to measure its ice losses in the lower part and its snow gains in the upper part, by planting stakes in the ice that deliver accumulation and ablation values."  It's a very precise method, with annual or even seasonal resolution of mass balances, which is very important for understanding the interaction of glaciers with the climate ", stresses the glaciologist. The only problem is that this is a very time-consuming process, requiring teams of scientists to travel to high altitudes for weeks at a time, and is therefore very costly. What's more, it can only be applied locally, to a limited number of glaciers.

Another method consists of directly measuring the complete change in glacier volume using digital terrain models. The latter, three-dimensional maps of glaciers, are generated from satellite data or obtained from drones." The advantage of satellite images is that you can observe every nook and cranny of the earth, at any time ", notes Fanny Brun. However, during her PhD, she noticed a certain gap in her field ; there are very few glacier mass balances on the scale of large regions and the entire planet. While field measurements abounded, particularly for glaciers in Europe and North America, estimates of regional mass changes based on satellite observations were, at the time, rather crude and did not allow for case-by-case studies of individual glaciers. With her colleagues, Fanny Brun developed a means of automatically processing large quantities of satellite archives." In particular, we used pairs of so-called 'stereoscopic' images taken by the ASTER satellite, which has been in orbit since 2000 and is still active today, and thanks to which we were able to generate time series of 3D maps ", explains the glaciologist. By comparing these maps, scientists can then calculate changes in volume for any glacier on the Earth's surface, at both individual and regional scales.

A question of debris

Fanny Brun began by applying this approach to the high mountains of Asia, and with her colleagues produced the first map of all glacier mass changes in this part of the world, from 2000 to 2016. This work gradually led him to address another issue, one that is the subject of much debate in the world of glaciology. Some glaciers are partly covered in debris, most often rocks, but what impact does this " detrital cover " have on the mass balance of glaciers ?" We want to know whether rocky debris protects the ice or, on the contrary, accelerates its melting," explains the researcher. However, if the debate continues, it's because there are arguments in favor of both answers. " On the one hand, thin debris tends to accelerate melting through its albedo effect ; as rocky elements are darker than snow and ice, they absorb more energy from solar radiation and therefore also transmit more of it to the glacier. On the other hand, when the debris is more than a few centimetres thick (some can be 1 or even 2 metres thick), it becomes insulating and plays a protective role, decoupling the ice from the atmosphere.

" It's complicated to answer this question when a geodetic mass balance is carried out using satellite data, as these do not allow us to quantify ablation and accumulation separately ", notes Fanny Brun. The researcher and her colleagues then worked on an Asia-wide scale, comparing glaciers in different categories, studying in particular the predictors of their mass balance, such as their altitude, region and detrital cover." We realized that the presence or absence of detrital cover does not have a homogeneous influence on a glacier. Indeed, while debris reduces glacier melt, it also allows them to spread to lower altitudes than white glaciers (without debris), and therefore to warmer environments, where they ultimately become more sensitive to climate change. "

From space to terrain

In addition to breathing new life into the satellite approach to glacier studies, Fanny Brun also spends much of her working time in the field. Her travels have taken her to the rooftops of the world, as high as 6 400 meters in the Nepalese mountains - expeditions that require a certain amount of preparation.   The missions begin with an approach trek, as the glaciers can only be reached on foot, and this walking phase allows you to acclimatize to high-mountain conditions," she recounts. After that, it's a matter of logistics ; we place the instruments in the right place, prepare the wooden stakes by cutting bamboo, etc. ". The teams set up their tents on the bangs of the glacier before going up each day to take their measurements, then breaking camp for the next glacier.

 " It's as much a scientific adventure as a human one, each time in breathtaking surroundings and in a caring atmosphere," continues Fanny Brun. Leaving for long expedition weeks allows you to get to know your colleagues better, which is very enriching, as long as you can manage the absence of intimacy for a while. " The adventure is not without risk, however. Indeed, expeditions can be tough, as scientists work in rather cold and uncomfortable environments, at altitudes that require a certain level of physical fitness. " Everyone acclimatizes differently, and it's important that we all listen to our own symptoms as well as those of others ", stresses the glaciologist.

Witnesses to global warming

Fanny Brun's work gives her a front-row seat to the impact of global warming on glaciers. Her work has helped to show that they are undergoing a global loss of mass, with a drastic acceleration over the last twenty years. On a global scale, glaciers thinned by 4 meters between 2000 and 2010, then by 6 meters between 2010 and 2020, an increase in the rate of thinning of 50 % from one decade to the next. In some regions with little ice cover, such as the Alps, a 30 % loss in glacier volume has been observed over the last thirty years." These values make us realize that mass losses will continue, even if the climate were to stabilize below the target of the Paris Agreements, which aim to limit warming to 1.5°C ", deplores Fanny Brun.

And yet, as glaciers melt, they raise sea levels ; indeed, they are the main contributors to this phenomenon, if we exclude the thermal expansion process whereby, as water temperature rises, the water already present in seawater takes up more space. In fact, Fanny Brun's work has shown that, between 2003 and 2018, melting glaciers caused sea levels to rise by around 0.75 millimetres per year, i.e. 0.20 and 0.42 millimetres per year respectively more than the melting of the Greenland and Antarctic ice caps. However, glaciers store relatively little water - around 40 centimetres of water equivalent, compared with 7 metres for Greenland and several tens of metres for Antarctica." Whatever the climate scenario, glaciers are a highly reactive reservoir ; they react quickly to warming ", explains the researcher, who cites as an example people who knew the Chamonix glacier in the 1990s, when it descended much further down the valley." These are particularly tangible and visible signs of climate change. They make us aware of a terrible consequence of the crisis we are living through : the generation that preceded us saw landscapes that have disappeared forever, and today we are seeing landscapes that the next generation will never see... "

A remarkable start to her career

Armed with the climatic sensitivity inherent in her subject of study, Fanny Brun wants to go further. Her laboratory is already part of a collective that has pledged since 2019 to reduce its greenhouse gas emissions to align with the goal of limiting the rise in temperatures due to global warming to 1.5°C."  It's a crucial step, one that involves questioning the meaning of our profession and how it can be aligned with respect for this climate trajectory ", she believes. This commitment is reflected in the breadth of her work : author of over forty publications to date, she is also scientific editor for the Journal of Glaciology, and devotes much of her time to training young researchers, both trainees and PhD students. She is also very active in the international community, the International Association of Cryospheric Sciences having appointed her to co-chair the Regional Assessments of Glacier Mass Change working group since 2019.

In 2024, Fanny Brun is the winner of the Collège de France Award for Young Researchers, whose theme this year is " water on our planet ". This distinction recognizes the excellence of her career, the innovation she has demonstrated throughout her research, and her pursuit of multidisciplinary solutions to problems that are part of a highly topical theme. To mark this brilliant and promising start to her career, which has already left its mark on glaciology, Fanny Brun is invited to the Collège de France, on December 12 2024 at 6 pm , where she will speak in public with journalist Natacha Triou about her experience and passion.

Article by William Rowe-Pirra