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The free troposphere is the location of important weather and climate processes. Here, horizontal and vertical transport of energy, mass, and momentum take place, and it holds greenhouse gases, water vapor, and clouds. The free troposphere therefore plays an important role in global climate feedback processes. Mountains provide important ecosystem services for a large lowland population. Mountain ecosystems may react particularly strongly to climatic changes. This is because mountains intersect important environmental and geoecological boundaries such as the snow line and the tree line. In a changing climate, these boundaries may shift. Climate change thus affects mountain glaciers, water resources, and mountain ecosystems. Climates of mountains and of the free troposphere have attracted scientists of the enlightenment and have been studied scientifically at least since the 18th century. High-altitude observatories were installed in the late 19th century, and upper-air measurements were started soon afterwards. However, even in the early 21st century, the climate observing systems do not well cover mountain regions and specifically mountain peaks. The temperature of the free troposphere is dominated by horizontal and vertical transport of sensible and latent heat, condensation and release of latent heat, and radiation to space. Mountain peaks sometimes reach into the free troposphere, but at the same time also share characteristics of surface climate. They are strongly influenced by radiative processes of the surrounding surface, while during the day they are often within the atmospheric boundary layer. With respect to climate change, temperature trends are amplified in the tropical upper-troposphere relative to the surface due to latent heat release, while in the Arctic the surface warms faster than the free atmosphere due to strong inversions and due to feedback processes operating at the surface. Mountain peaks may see both types of amplification. Several processes have been suggested to cause an elevation dependent warming, the most important of which arguably is the snow-albedo feedback. Elevation dependent warming is also seen in model studies and in observations, although detecting this signal in observations turns out rather difficult outside the tropics due to high variability and sometimes low-data quality. The observed climatic changes are expected to continue into the future.

Article

Harald Pauli and Stephan R.P. Halloy

High mountains (i.e., mountains that reach above the climatic treeline) are regions where many interests converge. Their treeless alpine landscapes and ecosystems are key areas for biodiversity, they act as water sources and reservoirs, and they are cultural and religious icons. Yet, mountain environments are threatened by global stressors such as land use impacts and anthropogenic climate change, including associated species redistributions and invasions. High mountains are warming faster than lower elevations. The number of frost days is declining, glaciers are retreating, and snow is remaining for shorter periods, while CO2 partial pressure is increasing. All of these factors affect the way in which ecosystems prosper or degrade. Thanks to the compression of thermal belts and to topographic ruggedness that favors habitat heterogeneity, mountains have a high diversity of biotic communities and species richness at the landscape level. In tropical to temperature regions, high mountains are biogeographically much like islands. With small habitat areas, species tend to be distributed patchily, with populations evolving independently from those on other isolated summits. Although high mountain areas strongly differ in size, geological age, bedrock, glacial history, solar radiation, precipitation patterns, wind exposure, length of growing season, and biotic features, they are all governed by low-temperature conditions. Combined with their distribution over all climate zones on Earth, mountain habitats and their biota, therefore, represent an excellent natural indicator system for tracing the ecological impacts of global climate change. As temperatures rise, plants and animals migrate upward (and poleward). Plant and animal populations on small, isolated mountains have nowhere to go if climates warm and push them upslope. On the other hand, habitat heterogeneity may buffer against biodiversity losses by providing a multitude of potential refugia for species which become increasingly maladapted to their present habitats. Global-scale approaches to monitor climate and biotic change in high mountains as well as modeling and experimental studies are helping explain the nature of these changes. Such studies have found that species from lower elevations are colonizing habitats on mountain summits at an accelerating pace, with five times faster rates than half a century ago. Further, repeated in situ surveys in permanent plots showed a widespread transformation of alpine plant community assemblages toward more warmth-demanding and/or less cold-adapted species. Concurrently to widespread increases in overall species richness, high-elevation plant species have declined in abundance and frequency. Strongly cold-adapted plant species may directly suffer from warmer and longer growing seasons through weak abilities to adjust respiration rates to warmer conditions. Combined effects of warming and decreasing water availability will amplify detrimental effects of climatic stresses on alpine biota. Many of the dwarf and slow-growing species, however, will be affected when taller and faster-growing species from lower elevations invade and prosper with warming in alpine environments and, thus, threaten to outcompete locally established species. Warming conditions will also encourage land use changes and upward movement of agriculture, while loss of snow is a loss to ski fields and scenic tourism.