Climate change: The Alps are greening up
Satellite images show: In the Alps, snow is retreating and vegetation is spreading. This has serious consequences for the ecosystems there.
Climate change is making the Alps greener - a change that can even be seen from space. That's according to researchers led by Sabine Rumpf of the University of Basel. They have shown that over the past four decades, the snow cover in the Alps has shrunk and more plants are growing there instead. This has far-reaching consequences for humans and nature, the research group writes in the journal Science.
Rumpf and her team analyzed 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. The resulting images reveal changes in snow cover and vegetation. Photosynthetically active plants reflect about six times as much in the near infrared as in visible red light. If the corresponding reflectance values are calculated, the result is a measure of the plant mass in the respective area - called the vegetation index. Whether snow is present or not can be determined from the measured values in the short-wave infrared and in the visible green light.
There is an area-wide color change
The "Landsat" images of the Alpine region show: On 77 percent of those areas that lie above the tree line, the vegetation has become denser over the past 40 years. At the same time, snow cover dwindled on 10 percent of the areas there. "The Alps are changing from white to green," the scientists say. Earlier studies by other teams had already shown that the snow layers in the Alps were becoming thinner and thinner: Readings from 800 weather stations show an average decline of 8.4 percent per decade.
Similar developments can be observed all over the world. The leaf area index, the ratio of leaf to ground surface, has increased globally in recent decades. The reason: humanity burns fossil fuels en masse and releases about 36 billion metric tons of carbon dioxide (CO2) annually, driving up atmospheric levels and leading to warming of air layers near the ground. Plants absorb carbon dioxide from the atmosphere and photosynthesize it into biomass. With increased supply of the substance, more biomass is produced and plant growth is more vigorous. In addition, as average temperatures rise, the growing season lengthens in many places.
However, carbon dioxide fertilization is becoming less effective over time: since the 1980s, it has declined by around 40 percent. This is because plants need not only carbon dioxide to grow, but also water and nutrients such as nitrogen and phosphorus compounds. In many regions of our planet, there is a shortage of nutrients or water, or both at the same time; drylands alone account for two-fifths of the world's land area. Despite increasing levels of CO2 in the air, the vegetation there eventually fails to thrive more vigorously because it lacks the other substances it needs.
In the Alps, greening will have noticeable consequences, write Rumpf and her team. More vegetation and less snow lead to a reducedalbedo ofthe earth's surface, which heats up more in sunlight. This causes temperatures to rise further, resulting in increased thawing of permafrost and glaciers and more avalanches and landslides. Increasing melting of snow and ice at high altitudes also threatens the freshwater supply of the local population, because glaciers and snowfields store water that valley dwellers need in times of low precipitation.
Moreover, mountain ecosystems are coming under pressure. "Alpine plants are adapted to harsh conditions but are not very competitive," Rumpf says. If environmental conditions change, she says, these specialized species lose their niche advantage and are displaced by other plants. "The unique biodiversity of the Alps is under considerable pressure."
Spectrum of Science
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