According to a study carried out by researchers at the University of Lausanne (Switzerland), the results of which were published on Thursday (2) in the journal Science, vegetation has increased on almost 80% of the surface of the Alps above 1,700 meters, where usually only there is snow and mountain. Antoine Guisan, a biologist and co-author of the study, does not hide his concern in an interview with RFI.
RFI: What new scientific fact does your study bring to what we already knew about the effects of climate change on mountains?
Antoine Guisan: As far as we know, this is the first study to show a clear decrease in permanent snow at high altitudes, and such a significant green in return. We focus on areas above 1,700 meters along the Alps. In these areas, vegetation has increased by 77% of the surface in 40 years. This is spectacular, we weren’t expecting it. At the same time, the permanent snow area has decreased by almost 10%, which is a lot.
The reason why snow decreased in less area than vegetation increased could be that only the presence or absence of snow could be measured by satellite, while a more gradual change was measured by plant biomass.
However, while the images used show a shrinking snow cover, they say nothing about its depth, which may also have decreased and may therefore disappear very quickly if global warming continues.
The increase in Arctic green space has been studied since the early 1980s. It is now known that the entire alpine arc is also affected by this change, and in a massive way.
RFI: How did you arrive at this result?
AG: We use NASA satellite imagery. In the past, we’ve run tests in small areas and had such strong responses that we thought it would be interesting to see what was happening at the scale of the entirety of the Alps.
The other contribution of this study is that we work with a much more precise image resolution than the only previous study also carried out in the entire alpine arc: each examined pixel represents 30 meters by 30 meters of relief, in the entire alpine arc, and this in several hundred images from 1984 to 2021, that is, almost 40 years. At these very high altitudes, the relief is so uneven that it is important to be able to work with the highest possible resolution. The result, as I said, is that 77% of these pixels show a clear sign of increasing green area.
RFI: What are the reasons for this increase in vegetation at high altitudes?
AG: Climate change, because the Alps have warmed by 2°C since the industrial age. This is huge, as it is almost double the world average. This warming leads to an increase in plant species at high altitudes and therefore also to an increase in the forest boundary. And one of the precursors of this increase in forest cover is the fog from the pastures just above the tree line, which are transformed into swamps with rhododendrons, blueberries and junipers, for example. A vegetation index [o NDVI] makes it possible to very accurately determine the increase in this biomass from space.
RFI: But isn’t that good news greener in the mountains?
AG: This increase in green in the mountains could theoretically be positive if it allowed more carbon to be captured by increasing vegetation. But this effect should be relatively insignificant on the global climate.
There are, however, threats. The first, biological, is that this is happening to the detriment of the alpine biodiversity at these altitudes: the flora, but also the fauna, such as butterflies and other insects linked to host plants, and even soil microorganisms.
Plants occupying the higher altitudes up to the summits will be overtaken by species from lower altitudes, which could not withstand excessive frost or cold weather. The latter, which are more vigorous, will replace the former, which are more fragile, such as silenium, dwarf willows, gentiles and even daisies, which grow very slowly at these altitudes, sometimes by just a few millimeters per year.
There are already clear migrations at altitude. This could lead to major extinctions of alpine species, some of which are unique to the Alps. If they disappear, it will be forever, as extinctions on a global scale are irreversible.
RFI: What other consequences will these changes bring?
AG: The increase in vegetation combined with the reduction in snow is expected to increase global warming. When the snow disappears, there is less reflection of the sun’s rays. Vegetation absorbs the radiation, warming the mountains, which melts snow and glaciers. This creates a cyclical effect.
This heating also melts the permafrost, the permanently frozen ground. Permafrost is the cement of mountains. When it thaws, everything collapses, creating landslides, which we are increasingly seeing happening.
Another threat is the drying up of the water. In summer, snow (and glaciers) partially melt and feed torrents and hydroelectric dams. They are also natural drinking water towers for people and livestock, even at low altitudes. In this sense, our study of snow decline in the Alps is quite worrying.
RFI: Is this change reversible?
AG: Yes, at least from the vegetation point of view. If we can stabilize the warming, the vegetation should quickly react accordingly. This certainty is more questionable for snow and glaciers, but this is outside my field of expertise, and I prefer to leave that to my cryologist colleagues.
