Editing IPCC:AR6/WGII/Cross-Chapter-Paper-5 (section)

=== FAQ CCP5.3 | Does climate change pose a risk to mountain species and ecosystems, and will this affect people? ===

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''Treeline position, bioclimatic zones and species ranges move up in elevation as the climate warms, increasing the risk of extinction for species isolated on mountain tops as a result of exceeding their physiological limits, loss of habitat or competition from colonising species. Additionally, climate change may alter the quality and quantity of food and natural products on which the livelihood of many mountain communities depends.''

Mountain regions cover about a quarter of the Earth’s land surface, are scattered around the globe and may support a wide range of climates within short horizontal distances. Mountains have experienced above-average warming, and this trend is expected to continue. Mountains provide a variety of goods for people, are home to many Indigenous Peoples and are attractive for tourism and recreational activities. Mountain regions support many different ecosystems, and some are very species rich. Mountain regions can be vast and diverse, and climate change and its impacts on ecosystems vary greatly from location to location.

With increasing average global temperatures, the climatic conditions under which plants and animals can thrive are shifting to higher elevations. The movement of some plant taxa towards mountain tops has been observed in recent decades. However, for species restricted to the highest elevations, there is nowhere to move to, meaning they are increasingly at risk of extinction. Climatic conditions may exceed the physiological limits for species and habitats may become unsuitable for others. There is also a risk from competition with colonising native species and invading non-native species, spreading to higher elevations, and some species cannot move quickly enough to keep pace with changes in the climate. The most vulnerable species are those that reproduce and disperse slowly and those that are isolated on mountain tops, including endemic species, which may face global extinction. In other cases, species will be lost from some parts of their current range. Mountains can, however, allow other species to survive in areas where they otherwise would not because of small-scale variations in climate with elevation or different aspects of slopes.

Changes in snow cover and snow duration are related to changes in temperature and precipitation and are also critical for plants and animals. In particular, glacier retreat and changing snow patterns affect both streamflow dynamics (including extremes) and soil moisture conditions and can cause moisture shortages during the growing season. A change in snow patterns can critically affect animal movements in mountains. Other processes creating stresses on mountain ecosystems are direct human impacts, such as the influence of grazing, tourism, air pollution and nitrogen deposition on alpine vegetation. In some cases, these impacts can be so large on the goods and services provided by alpine ecosystems that they can overshadow the effects of climate change or exacerbate its effects.

In many mountain regions, multiple sources of evidence point to tree expansions into treeless areas above (and in some cases below) the forest belt. This may increase forest productivity at the upper treeline. Treelines have moved up in the last 30–100 years in many mountain regions, including, for example, the Andes, Urals and Altai. At the same time, since the 1990s, treeline responses in different parts of the Himalaya have been highly variable, in some places advancing upslope, in others demonstrating little change and in yet others moving downwards. This can be explained by site-specific complex interactions of the positive effects of warming on tree growth, drought stress, change in snow precipitation, land use change, especially grazing, and other factors. Treelines are affected by land use and management around the globe, and changing land use practices can supersede climate change effects in some mountain regions. An upward shift in the elevation of bioclimatic zones, decreases in the area of the highest elevation zones and an expansion of the lower zones can be expected by mid-century, for example in regions such as the Himalaya.

In some regions, the livelihoods of many local mountain communities depend on access to firewood, pastures, edible plants and mushrooms, and medicinal and aromatic plants. Climate change can alter the quality and quantity of these ecosystem services; however, the degree and direction of change are context specific. The appeal and feasibility of mountains for tourism and recreational activities are also affected by climate change.

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