The impact of global warming is particularly marked in the Alps. Like the Arctic, the mountain range is becoming greener. In an article published in the journal Science, researchers from the University of Lausanne (UNIL) and the University of Basel (UNIBAS) have shown, using satellite data, that the productivity of vegetation above the tree line has increased in almost 80% of the Alps. The snow cover at high altitudes has decreased, albeit only moderately.
The melting of the glaciers has become a popular symbol of climate change in the Alps. However, the reduction of the snow cover, although already visible from space, is not nearly as dramatic. The most marked change is a pronounced and widespread increase in vegetation at high altitudes in the Alps. This is the conclusion reached by a research team led by Professor Sabine Rumpf from UNIBAS and Professors Antoine Guisan and Grégoire Mariéthoz from UNIL.
In collaboration with research groups based in the Netherlands and Finland, the scientists examined changes in snow cover and vegetation using high-resolution satellite data collected from 1984 to 2021. Over this period, plant biomass has increased above the tree line in more than 77% of the Alps. This phenomenon of "greening" due to climate change is already well documented in the Arctic and is beginning to be identified in the mountains as well.
Increase in plant biomass in three quarters of the Alps
"The scale of change is absolutely massive in the Alps", says Sabine Rumpf, the first author of the study and, since February 2022, assistant professor at UNIBAS. The Alps are becoming greener because the vegetation is colonising new areas and becoming denser and higher overall.
Previous studies have focused mainly on the impact of global warming on Alpine biodiversity and on changes in the distribution of plant species. Until now, however, no one had conducted such a comprehensive analysis of the evolution of plant productivity in the Alps. The authors show that the increase in plant biomass is primarily attributable to changes in the precipitation regime and the lengthening of the plant growing season as a result of rising temperatures. "Alpine plants are adapted to harsh conditions, but they are not very competitive", explains Sabine Rumpf. As environmental conditions change, these highly specialised species lose their advantage and are overtaken by competition: "The unique biodiversity of the Alps is therefore under considerable pressure."
Slight reduction in snow cover
In contrast to the vegetation, the extent of snow cover above the tree line has changed only slightly since 1984. The experts excluded regions below 1700 metres, glaciers and forests from their analysis: they found that the snow cover decreased significantly in almost 10% of the remaining regions. This may seem moderate, but the scientists stress that it is nevertheless a worrying trend.
"Previous analyses of satellite data had not identified such a trend", says Antoine Guisan, professor at UNIL and one of the authors who co-led the study. Perhaps this is due to the insufficient resolution of the satellite images or the fact that the periods considered were too short.
The scale of change is absolutely massive in the Alps.
Sabine Rumpf, assistant professor, UNIBAS
"For years, local ground measurements have shown a decrease in the depth of the snow cover at low altitudes", adds Grégoire Mariéthoz, professor at UNIL and the second author to have co-directed the study. As a result of this reduction, some regions are now largely devoid of snow. Using satellite data, it is possible to distinguish the presence or absence of snow, but this data does not provide information on the depth of the snow cover.
With global warming, the Alps will become less and less white and more and more green, entering a vicious circle: "Greener mountains mean less reflection of sunlight, which will further increase warming and mechanically reduce the snow cover and its reflectivity", explains Sabine Rumpf. Warming also increases the melting of glaciers and the thawing of permafrost, which increases the risk of landslides, rockslides and mudslides. In addition, Sabine Rumpf emphasises the important role that snow and ice in the Alps play in the supply of drinking water, as well as in recreation and tourism.
This article is based on a press release issued by UNIL and UNIBAS on 2 June 2022 at 20:00.
Original publication: Sabine Rumpf et al. From white to green: Snow cover loss and increased vegetation productivity in the European Alps,Science (2022), doi: 10.1126/science.abn6697
Photo: View of the Swiss Alps from Pischahorn to the Plattenhörner (©Sabine Rumpf)
About the experts:
Sabine Rumpf, Head of the Ecology Research Group, Department of Environmental Sciences, UNIBAS
Antoine Guisan, Head of the Spatial Ecology Group, Department of Ecology and Evolution, Faculty of Biology and Medicine and Institute of Land Surface Dynamics, Faculty of Geosciences and Environment, UNIL
Grégoire Mariéthoz, Head of the Geostatistical Algorithms & Image Analysis research group and director of the Institute of Land Surface Dynamics, Faculty of Geosciences and Environment, UNIL
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