Greenhouse gases warm the air layer close to the surface, but they cool and contract the upper atmospheric layers, resembling a sky that is gradually collapsing towards the Earth’s surface.
The Earth’s atmosphere consists of several layers. The region where weather phenomena that affect humans occur is the troposphere, a thick layer of air that ranges from 8 to 15 km. The troposphere is so dense that it accounts for 80% of the mass but occupies only a small portion of the atmospheric volume.
Above the troposphere are increasingly rarefied air layers, including the stratosphere, which is about 50 km thick, followed by the mesosphere, which is about 80 km thick, and finally the thermosphere, which exceeds 640 km in thickness.
In addition to the troposphere, climate change is also occurring in higher layers of the atmosphere. “The current increase in CO2 is occurring throughout the entire atmosphere,” says Martin Mlynczak, an atmospheric physicist at NASA’s Langley Research Center, in an interview with Wired.
CO2 Heats Surface Air and Cools Upper Atmosphere
Humans emit over 40 billion tons of CO2 annually. This greenhouse gas has the property of absorbing and re-emitting solar radiation. In the troposphere, due to the dense molecular density of the air, emitted CO2 radiation warms surrounding molecules, increasing the overall temperature.
The troposphere, the layer of air closest to the surface, occupies only a small portion of the Earth’s atmospheric volume. (Photo: Shutterstock).
In higher atmospheric layers, due to the rarefied air, CO2 radiation does not interact with other molecules but escapes into space. Because its ability to absorb solar radiation is lower than that of CO2, air molecules lose energy to the greenhouse gas, which also creates a heat-retaining effect in the troposphere. As a result, the upper atmospheric layers are cooling rapidly.
Recent satellite data has confirmed that from 2002 to 2019, the mesosphere and thermosphere have cooled by 1.7 degrees Celsius. Mlynczak estimates that by the end of this century, when CO2 concentrations are expected to double, temperatures in the upper atmospheric layers will drop by about 7.5 degrees Celsius. This rate of heat loss is 2-3 times faster than the rate of warming in the troposphere.
The cooling air also means the upper atmospheric layers are contracting, making the sky literally seem like it is collapsing towards the Earth’s surface. The thickness of the stratosphere has decreased by about 1% or 400 meters since 1980, according to Petr Pisoft, an atmospheric physicist at Charles University (Czech Republic). Above the stratosphere, Mlynczak found that the mesosphere and the lower region of the thermosphere have contracted by about 1,300 meters from 2002 to 2019.
Increased CO2 concentrations in the atmosphere pose risks of extreme weather phenomena. (Photo: NASA).
A cooler atmosphere negatively affects the ozone layer in the stratosphere, which protects humans from harmful solar radiation that can cause skin cancer. The destruction of the ozone layer is exacerbated by stratospheric clouds at the poles, which only form at very low temperatures. A cooler stratosphere means these clouds are easier to form.
“The Sky is Falling” Creates Ozone Holes and Leads to Extreme Weather
The ozone hole in Antarctica is recovering as CFCs gradually disappear following the 1987 Montreal Protocol. Meanwhile, in the Arctic, the cooling atmosphere is worsening ozone layer depletion, according to Peter Von der Gathen, a researcher at the Alfred Wegener Institute (Germany). It remains unclear why the ozone layer in the Arctic is more severely affected than in Antarctica by the effects of a cooling atmosphere.
In the spring of 2020, the Arctic first experienced an ozone hole. Peter von der Gathen believes that the cause is the concentration of CO2. In a paper published in Nature Communications, he warns that the continued cooling of the atmosphere will hinder the goal of fully healing the ozone layer by mid-century.
Jet streams (in red) cause various weather phenomena on the Earth’s surface, influenced by the upper atmospheric layers. (Photo: NASA).
Areas below ozone holes in Antarctica were previously uninhabited, but areas beneath the ozone holes forming in the Arctic are populated. Depending on the future location of these holes, von der Gathen predicts that the affected regions could include Central and Western Europe, areas with high population density.
Atmospheric physicists are also concerned that the cooling effect on the atmosphere could alter air movement in the upper atmosphere, impacting ground-level weather and climate. The westerly winds in the stratosphere periodically reverse, leading to significant temperature fluctuations, and the stratosphere can heat up to 50 degrees Celsius within a few days. The temperature increase forces the air to sink, affecting the Atlantic jet stream at the top of the troposphere.
Jet streams, which control weather systems across the Northern Hemisphere, begin to move and can cause various types of severe weather, from summer droughts to prolonged cold spells in North America, Europe, and some regions of Asia.
Most simulation results so far are consistent that the stratosphere is sensitive to CO2 concentrations, says Mark Baldwin, a climate scientist studying the stratosphere at the University of Exeter (UK). However, some models predict there will be more sudden warming events, while others predict fewer.
“If we understand the upper atmospheric layers better, long-term weather forecasts and climate change predictions will improve,” Baldwin states.
