The mesosphere is positioned above the stratosphere and below the thermosphere. This layer extends roughly from 50 kilometers to 85 kilometers above the Earth’s surface, marking a transition zone between the lower atmosphere and the vacuum of space. Temperatures in the mesosphere plummet with increasing altitude, reaching the coldest point in the entire atmosphere—the mesopause—where temperatures can drop as low as -90°C. Though challenging to study directly, as it is too high for conventional aircraft and too low for orbiting satellites, the mesosphere plays several interconnected roles. This remote layer acts as a protective shield, a dynamic engine for atmospheric circulation, and a sensitive barometer for global atmospheric change.
Shielding Earth From Space Debris
The mesosphere acts as Earth’s primary defense system against incoming space debris. Meteoroids, which are small pieces of rock and dust from space, enter the atmosphere at extremely high velocities. At these speeds, the thin atmosphere of the mesosphere creates intense friction.
As a meteoroid slams into the mesosphere, typically between 80 and 85 kilometers in altitude, this friction rapidly heats the meteoroid’s surface. This causes it to burn up, vaporize, and disintegrate completely before it can reach the surface below. This process is what we observe from the ground as “shooting stars” or meteors. Without this natural atmospheric shield, Earth would be constantly bombarded by space objects, resulting in widespread surface impacts. The remnants of these vaporized meteors leave behind a persistent layer of metallic dust.
The Role in Global Atmospheric Circulation
The mesosphere serves as a link in the global atmospheric system, connecting low-altitude weather systems with the upper reaches of space. This process, known as atmospheric coupling, involves the large-scale transfer of energy and momentum upward from the lower atmosphere into the upper layers. The mesosphere is a dynamic region where atmospheric waves play a dominant role in this transfer.
Waves generated in the troposphere and stratosphere, such as gravity waves and atmospheric tides, propagate upward. Gravity waves, for example, are created by air flowing over mountains or by large weather systems. As these waves ascend into the increasingly thinner air of the mesosphere, their amplitude grows significantly.
When these waves reach a critical point, they “break,” dissipating their energy and momentum into the mesosphere. This breaking process drives the large-scale global circulation patterns that push air from the summer pole toward the winter pole. The energy and momentum released by these breaking waves influence the thermal structure and wind patterns in the mesosphere and the layer above it, the thermosphere.
Indicators of Climate and Atmospheric Change
The mesosphere functions as a barometer for long-term climate and atmospheric changes occurring closer to Earth’s surface. Scientists monitor a phenomenon called Noctilucent Clouds (NLCs), also known as Polar Mesospheric Clouds (PMCs), which are the highest clouds in Earth’s atmosphere, forming near the mesopause at altitudes of around 80 to 85 kilometers. These clouds are composed of tiny ice crystals that form only when the mesosphere is at its coldest and sufficient water vapor is present.
Observations show that NLCs have become more frequent, brighter, and visible at lower latitudes in recent decades. This change is linked to two primary factors driven by human activity. First, the increase in greenhouse gases like carbon dioxide heats the lower atmosphere, but paradoxically, it causes the mesosphere to cool by radiating heat into space. This cooling makes it easier for ice crystals to form.
Second, rising levels of methane, another greenhouse gas, are transported to the mesosphere where it oxidizes and produces water vapor. The combination of a colder environment and more water vapor significantly increases the frequency and visibility of NLCs. The increasing visibility of these clouds serves as a measurable, long-term indicator that human-caused changes are altering the middle layers of the atmosphere.