Mesosphere
The mesosphere is a layer of Earth's atmosphere located between approximately 50 kilometers and 100 kilometers in altitude, characterized as the coldest atmospheric layer. Temperatures in this region can plummet to around -125°C during summer, with the warmest temperatures reaching about -5°C just above the stratopause. Notably, the mesosphere plays a role in climate dynamics, particularly through the oxidation of methane, which is a significant natural source of water vapor leading to the formation of noctilucent clouds. These clouds, appearing at altitudes around 82 kilometers, are typically visible only when the sun dips between 6° and 16° below the horizon.
Anthropogenic activities have been linked to increases in methane concentrations, which have grown by 150% since the onset of the industrial age, potentially contributing to the recent observation of noctilucent clouds. Space exploration also influences this atmospheric layer, as rocket exhaust can create artificial noctilucent clouds. Furthermore, the mesosphere is subject to cooling due to rising carbon dioxide levels, which could have implications for satellite operations and global temperature patterns. These dynamics illustrate the intricate relationship between human activity, atmospheric science, and climate change.
Subject Terms
Mesosphere
Definition
The mesosphere is the region of the atmosphere extending from the top of the stratopause (about 50 kilometers high) to the bottom of the mesopause-lower thermosphere boundary (MLT), about 100 kilometers high. It is the coldest region of the atmosphere. The warmest temperatures in the mesosphere are found just above the stratopause, where air temperatures may be as high as -5 °Celsius. Mesospheric temperatures decrease with increasing altitude, with the lowest temperatures, around -125° Celsius, occurring during summer at the mesopause.
Significance for Climate Change
The natural source of water in the mesosphere is oxidation of methane. Atmospheric concentrations of the greenhouse gas (GHG) methane have increased dramatically since the beginning of the and may have influenced the appearance of noctilucent clouds. Water vapor emitted by rockets and the space shuttle has been observed to form noctilucent clouds. Exhaust from one shuttle mission may increase the appearance of noctilucent clouds by as much as 20 percent. Carbon dioxide (CO2) in the mesosphere releases heat to space. Solar proton events have been observed to cool the lower mesosphere by causing photochemical reactions leading to ozone depletion, causing an estimated temperature drop of up to 3° Celsius.
Gravity waves are the dominant form of motion in the mesosphere, with wavelengths of around 10 kilometers. Wind speeds on the order of 100 meters per second may occur in the MLT. Gravity wave is lowest at solar maximum. Direction of atmospheric transport is from the summer hemisphere to the winter hemisphere.
In the mesosphere, clouds can form only at the coldest temperatures, within about thirty days of the summer solstice at latitudes above 50° north and below 50° south. Because they form at 82 kilometers altitude, the thin filamentous noctilucent clouds (sometimes called polar mesospheric clouds, or PMCs) are illuminated only when the Sun is between 6° and 16° below the horizon.
There has been speculation that noctilucent clouds are a recent phenomenon resulting from anthropogenic activity, as they were not reported prior to 1885, when scientists began studying the colorful twilight displays that followed the eruption of Krakatoa. Soon after the discovery of noctilucent clouds, speculations began that the particles composing the clouds were of extraterrestrial origin. In 1962, rockets launched to investigate noctilucent clouds determined that the cloud particles had nuclei of iron or nickel dust of extraterrestrial origin, which were coated by water ice. One of the most surprising aspects of noctilucent clouds is their strong radar reflectivity. After some study, an explanation was advanced in 2008 that molecules of sodium and molecules of iron form a thin metallic film over the tiny ice crystals. This film reflects radar waves in a special, amplified way that makes the clouds more prominent on radar than if they were a cloud of disordered metal dust. Noctilucent clouds are postulated to remove about 80 percent of the sodium and iron from their environment in the mesosphere.
Water is formed in the mesosphere as a result of the oxidation of methane gas, rather than arising from Earth’s surface. Because methane is released as a by-product of incomplete combustion, as well as through coal-mining activity and production from animals, swamps, and rice paddies, its concentration has been rising steadily as a result of anthropogenic activity. The concentration of methane in the atmosphere has increased 150 percent since the industrial age began, around 1750.
Scientists in the 2020s discovered that the mesosphere is cooling because of high carbon dioxide levels. This cooling could affect orbiting satellites as well as the temperature on Earth. The same gases that warm the air closest to Earth cool the mesosphere.
Space exploration also affects the mesosphere. After rockets are launched through the mesosphere, their exhaust trails may form artificial noctilucent clouds. These noctilucent trails can form in the mesosphere in temperate latitudes, but ground observations of them are made only when the Sun is between 6° and 16° below the horizon.
After the launch of the STS-107 shuttle mission in January 2003, it was determined that vaporized iron and water from its exhaust traveled to Antarctica at 110 kilometers altitude in about two days. This exhaust plume went on to form noctilucent clouds in the southern polar summer. Data from this event indicated that a single space shuttle’s exhaust might cause a 10 percent to 20 percent increase in the appearance of seasonal noctilucent clouds.
Bibliography
Arnold, Neil. “Solar Variability, Coupling Between Atmospheric Layers, and Climate Change.” Philosophical Transactions of the Royal Society of London A 360, no. 1801 (December, 2002): 2787-2804. Reprinted in Advances in Astronomy, edited by J. M. T. Thompson. Vol. 1. London: Imperial College Press, 2005.
Bellan, Paul M. “Ice Iron/Sodium Film as Cause for High Noctilucent Cloud Radar Reflectivity.” Journal of Geophysical Research D 113 (2008): 16,215-16,218.
Brasseur, Guy, and Susan Solomon. Aeronomy of the Middle Atmosphere: Chemistry and Physics of the Stratosphere and Mesosphere. 3d rev. ed. Dordrecht: Springer, 2005. eBook Collection (EBSCOhost). Web. 20 Mar. 2015.
Glickman, Todd S., ed. Glossary of Meteorology. 2d ed. Boston: American Meteorological Society, 2000.
Pearce, Fred. "The Upper Atmosphere Is Cooling, Prompting New Climate Concern." Yale Environment 360, 18 May 2023, e360.yale.edu/features/climate-change-upper-atmosphere-cooling. Accessed 10 Dec. 2024.
Proud, Simon R., et al. "The January 2022 Eruption of Hunga Tonga-Hunga Ha'apai Volcano Reached the Mesosphere." Science, vol. 378, no. 6619, 3 Nov. 2022, pp. 554-557, doi.org/10.1126/science.abo4076. Accessed 25 Jan. 2023.
Schroeder, Wilfried, and Karl-Heinrich Wiederkehr. “Johann Kiessling, the Krakatoa Event, and the Development of Atmospheric Optics After 1883.” Notes and Records of the Royal Society of London 54, no. 2 (May, 2000): 249-258.