Elevation Dependant Warming in Canada’s Saint Elias Mountains
The atmosphere in many of the high-elevation mountains on Earth is warming faster than the global average, a phenomenon referred to as Elevation Dependent Warming (EDW).[1] There is growing evidence that EDW is a global phenomenon, supported by both observations and climate modeling studies, and it appears that many of the highest mountain ranges are warming more rapidly than the surrounding lowlands.
Figure 1. Mount Logan, Yukon (in the background, seen here from the north). The mountain’s upper regions have warmed ~1.6 times faster than the global average since the 1970s. Photo: Zac Robinson
Mountains are distributed unequally around the globe, with the majority of the highest mountain ranges in the middle latitudes. Until recently it was unknown if EDW was occurring at high latitudes (> 60 N) because there is insufficient monitoring in these remote areas. This gap in our understanding includes the vast Saint Elias Mountains of the Yukon Territory, including Mount Logan (5,959 metres), Canada’s highest peak (Figure 1).
Top: Figure 2. The St. Elias Mountains in southwest Yukon. The numbered red dots are meteorological stations and the colours indicate 500 m elevation bands, from sea level to the top of Mt. Logan (5959 m), and into the Kluane Lake (Lhù’ààn Mân) region east of Kluane National Park.
To understand how the atmosphere changes the higher you go up a mountain, and the further away you are from the Equator, it is necessary to examine the composition of the troposphere, the lowest layer of the atmosphere. The troposphere contains approximately three-quarters of the atmosphere’s mass, and the vast majority of this is water vapour. The troposphere is approximately seventeen kilometres thick at the Equator, but only about nine kilometres thick at the poles. This height difference is determined by both the centripetal forces caused by the Earth’s spin and differences in air temperature.
Mountain air gets “thinner” the higher you go. This really means that the air is less dense, and that the mass of gasses and water vapour pressing down on the surface is reduced compared to air at sea level. Water vapour decreases with air pressure, but unlike oxygen and other atmospheric gasses, water vapour decreases exponentially with temperature. So, by 3,000 metres, it is cold enough that most water vapour has been removed from the air. From the perspective of the Earth’s climate, water vapour is important in regulating temperature because it is a powerful greenhouse gas. At altitudes above 3,000 metres, most of the aerosols in the atmosphere, such as sea salts and black carbon, have settled out. And most of the sunlight-driven convective clouds are found below the summits of high mountain peaks.
We designed an experiment to discover if EDW was present in the highest mountain range in Canada.[2] Several sources of observational data were integrated to construct a detailed understanding of the factors that influence temperature in the Saint Elias Mountains (Figure 2), including meteorological (weather) station measurements to validate downscaled air temperature data. We also investigated the possible drivers of EDW by comparing reanalysis temperature trends with vertical profiles of temperature and water vapour pressure from radiosonde balloons, long term climate information from high-elevation ice cores extracted from the St. Elias Icefield, and satellite measurements of high-elevation albedo (or colour of the snow).
Right: Figure 3. The spatial distribution of surface warming rates (°C per year) between 1979 and 2016 in the southwest Yukon. Warming rates above 3000m (red shades) are statistically significant at the p < 0.05 level.
For the years 1979 to 2016, we found a warming rate of 0.028°C per annum between 5,500 and 6,000 metres and this warming was approximately 1.5 times greater than the warming rate at 2,000 to 2,500 meters (Figure 3). The warming rate at the highest elevations was approximately 1.5°C per annum times greater than the global average warming rate over a similar time period (1970-2015). Warming trends in the St. Elias region appears to be driven by recent warming of the free troposphere. The satellite data showed no evidence for an enhanced snow albedo feedback; declining trends in sulfate aerosols deposited in high elevation ice cores suggested only a modest increase in radiative forcing at these elevations; however, water vapour at the summit of Mount Logan suggests that a long-wave radiation vapour feedback is contributing to the observed EDW. Therefore, the most compelling explanation for EDW in the Saint Elias is the increase in tropospheric water vapour. Warming rates at high elevation are extremely sensitive to small changes in water vapour when the absolute concentration is extremely low, which is the case in the Saint Elias Mountains at high elevation.
So, the thin air in the Saint Elias is now warmer and moister than it has been in the recent past. And although the air at the summit of Mount Logan still reaches -50°C even in summer, it is warming more rapidly than the lower slopes.
Author Bio
Scott Williamson is a Post-Doctoral Fellow at the University of Northern British Columbia. He completed degrees in physics, glaciology, and ecology at the University of Alberta.
References
[1]Pepin, N., et al. Elevation-dependent warming in mountain regions of the world. Nature Climate Change 5, 424-430 (2015).
[2]Williamson, S.N., et al. Evidence for elevation-dependent warming in the St. Elias Mountains, Yukon, Canada. Journal of Climate 33, 3253-3269 (2020).