Climate change is making the Alps greener – a change that can even be seen from space, as researchers led by Sabine Rumpf from the University of Basel report. They have shown that over the past four decades, the snow cover in the Alps has shrunk and that more plants are growing there instead. This has far-reaching consequences for humans and nature, writes the working group in the journal »Science«.
Rumpf and her team evaluated data from the "Landsat" satellites, which have been observing the Earth for 50 years. The satellites are used for remote sensing and regularly image the Earth's surface at different wavelengths. Based on the images that are created, changes in the snow cover and vegetation can be detected. Photosynthetically active plants reflect about six times as strongly in the near infrared as in visible red light. If the corresponding reflection values are calculated, a measure of the plant mass on the respective area vegetation index is obtained. Whether there is snow or not is clear from the measured values in the short-wave infrared and in the visible green light.
There is a nationwide color change
The "Landsat" pictures of the Alpine region show: 77 percent of those areas that are above the tree line, vegetation has become denser in the past 40 years.
At the same time, the snow cover disappeared to 10 percent of the areas there.
"The Alps walk from white to green," the scientists sum up.
Already in earlier examinations of other teams it was noticed that the snow layers in the Alps were getting thinner: the measured values of 800 weather stations show an average decrease of 8.4 percent per decade.
Similar developments can be observed all over the world. The leaf area index, the ratio of leaf to soil surface, has increased globally in recent decades. The reason: mankind burns fossil fuels en masse and releases about 36 billion tons of carbon dioxide (CO2) annually, which drives up the atmospheric content and leads to the warming of near-ground air layers. Plants absorb carbon dioxide from the atmosphere and photosynthetically process it into biomass. With an increased supply of the substance, more biomass is created and plant growth is stronger. In addition, with increasing average temperatures, the growing season is lengthening in many places.
However, carbon dioxide fertilization works worse and worse over time: since the 1980s it has decreased by around 40 percent. Because plants not only need carbon dioxide, but also water and nutrients such as nitrogen and phosphorus compounds. In many regions of our planet there is a lack of nutrients or water or both at the same time; Two fifths of the global land space alone are eliminated in dry areas. Despite increasing CO2 content in the air, the vegetation there no longer thrives there because it lacks the other necessary substances.
In the Alps, greening will have noticeable consequences, write Rumpf and her team. More vegetation and less snow reduce the albedo – the reflectivity of the earth's surface. As a result, it heats up more strongly in sunlight. This causes temperatures to rise further, causing permafrost soils and glaciers to thaw more and lead to more avalanches and landslides. The increasing snow and ice melt in the heights also endangers the fresh water supply of the local population, because glaciers and snowfields store water that the valley inhabitants need during periods of low precipitation.
In addition, the mountain ecosystems come under pressure. "Alpine plants are adapted to rough conditions, but not very competitive," says Rumpf. If the environmental conditions change, according to the researcher, these specialized species lose their niche advantage and are displaced by other plants. "The unique biodiversity of the Alps is under considerable pressure."
