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Editors’ Vox is a blog from AGU’s Publications Department.
Mountain regions cover approximately a quarter of the Earth’s land surface, although the exact percentage depends on criteria used to define them. The snow and rain that falls in mountains eventually flows downstream to provide water for millions. China and India, two of the most populous countries on the planet, depend on water supply from Tibet Plateau and Himalaya. In arid areas, such as the western USA and the west, mountain ranges provide critical water supply. It is becoming increasingly clear that climate change and global warming often have an impact on high-mountain areas. Understanding why this happens is crucial.
A NEW articlePublished in Geophysics ReviewsThe latest developments in climate change research in mountain regions are presented. We asked some authors to explain how mountain areas have responded to climate change, and what is still unknown.
What is it? “Elevation-dependent Warming”?
Elevation-dependent heating (EDW), is a phenomenon in which the rate of warming depends on the elevation (or height above the sea-level). This means that higher elevations may experience faster or slower warming than those located below. When moving up a mountain, you may notice that the vegetation and landscape tend to change with height – you may start with farmland and areas where people live, then forests thinning to alpine meadows, and finally snow and ice.
EDW can occur because these zones can respond at different rates to warming. Melting snow and Ice can enhance warming by decreasing reflectivity. This happens when reflective snow is replaced beneath. This can only happen if there is snow at all, and it might only be at a certain height. This depends on whether the mountain is located near the poles or the tropics or somewhere in-between. Temperature is not the only variable that determines climate. Recently, the concept was expanded to include other variables such as precipitation. This concept is called “elevation-dependent climate change” (EDCC).
What are the main changes in mountain climates that have occurred over the past century
Although there are many variations in time and space, the average temperature of mountains has been between 25 to 50 percent higher than the global mean since 1950, when extensive records were started. This is a similar phenomenon as Arctic amplification, but at an even lower level.
There is also evidence that mountain precipitation is not as high as it was in the past due to rising air from mountain slopes. Even though a warmer world is predicted to have a faster hydrological cycle, which will lead to more evaporation and more intense precipitation episodes, this change seems to be more evident in lowland areas than it is in the mountains. Even though precipitation is increasing in many mountain areas, it is not increasing at the rate expected given a warmer climate.
These changes in temperature, precipitation (there’s also a change between snow and rain) have all been bad for snow, glaciers, and ice. Nearly all mountain glaciers are receding around this time. This has happened in many places over the past 20-30 year. Many species and their habitats have also seen an uphill migration of climate zone, which has led to an increase in the number of them moving upslope. This could eventually lead a mass extinction event at the tops of isolated peaks, where there is no mountain to move up.
These changes can be seen globally, or are they different by region?
There is a lot of variation between areas because not all mountains have a similar climate. Mountains can be found everywhere from the tropics (e.g. The high mountains of East Africa like Kilimanjaro, Mt. Kenya and Rwenzori, as well as the tropical Andes), via the big chains of mid-latitudes. Alps, Himalayas, and Arctic (e.g. Denali in Alaska).
The enhancement of warming by melting or retreating snow (so-called snow-ice albedo feedback), is less relevant in tropical mountains, as there is so much snow. For example, the Kilimanjaro glaciers will disappear in the coming century. The Andes’ climate change is different from the east to the west. The Amazon basin moisture changes have an influence on the east while the Pacific Ocean has an impact on the west. Because of the barriers between mountains such as the Rockies, Andes, and the Pacific Ocean, it is common for each side to experience different climate change impacts.
How do mountain locations compare to the lower areas in the vicinity?
This is a very important question but the definition of “lower areas nearby” is not simple or obvious. There are low areas on either side of high mountain barriers. You will get different results depending on which side your mountain changes are compared to. On average, high mountains are warmer than their immediate environment. Nine of the eleven studies that examined this question using paired temperature stations found this to be true.
There is also the question about what makes up the lowland regions of the planet and whether the oceans should be included in that category. This is a significant difference in any comparison. It is also true that land reacts faster to climate warming than the ocean, which responds slower. On a global level, land is warming faster than the sea. If you include ocean in the “lower areas nearby” category then you are almost bound to get higher rates of mountain warming for solely this reason.
What are some social impacts of changes in snow and/or ice?
Mountains are often said to be the “water towers of the world” since they provide freshwater supplies across the world. In fact, 1.6 billion peopleHigh mountain water supplies are essential for their survival. China and India, the two largest countries in the world, rely on Himalayan resources. Since snowpack melts gradually, especially during the summer at mid-latitudes in mid-latitudes and watering the landscape, it is often a reliable source of water. Bushfires can become more common in mountain forests with less snowpack and in dry climates like the west United States.
The majority of winter tourism in mountains depends on snow and the ski industry will suffer major impacts. Even in areas where skiing is not a major activity, snow and ice can be a significant boost to tourism. This is evident on Kilimanjaro, Tanzania, where tourists climb the mountain partly to see the glaciers, and the gleaming snows at its summit. Finally, when snow is replaced in heavy rain, mountain flash flooding and associated hazards like landslides can become more common in many areas.
What are some of the unresolved issues that require additional research, data or modelling?
While we have a good understanding of temperature and snowpack changes, we are not very knowledgeable about mountain precipitation and how it might change. It is difficult to measure snowfall as a lot of it is thrown sideways, and therefore cannot be measured by a traditional rain gauge. Mountain rainstorms are notoriously localized. Unfortunately, our observation networks are not advanced enough to be able to determine what is happening in many cases. We also have more weather stations lower down on mountain valleys (where people live), than higher up on mountain slopes (where it’s difficult to access and humans can’t survive). Below 5,000 meters, there is a very small number of permanent settlements and no longer than 20-year-old weather stations that can be used to provide reliable climate analysis.
This data void can be very dangerous, especially considering that we know that significant changes are taking place at this height (or higher). Computer models can be used for simulations of many of the effects that we are interested in. However, the critical issue here is the spatial resolution which is often too poor to capture fine-scale details. Because mountains are so complicated – being a wrinkled landscape rather than a flat constant surface- – we need to know what is happening at very small spatial scales, and that means a lot of computer power. This is a limitation at this time.
—Nicholas Pepin ([email protected], 
0000-0001-6200-4937), University of Portsmouth, UK; Carolina Adler (0000-0002-8787-2797), University of Bern, Switzerland; Sven Kotlarksi (0000-0001-9542-6781), MeteoSwiss, Switzerland; and Elisa Palazzi (0000-0003-1683-5267), University of Turin and Institute of Atmospheric Sciences and Climate, Italy
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
