New research has confirmed that the same gases responsible for climate change are cooling the Earth’s upper atmosphere, raising concerns about effects on orbiting satellites, ozone, and weather—and providing “incontrovertible” evidence of the human influence on global temperatures.
The findings illustrate the “paradox at the heart of our changing climate,” writes Yale Environment 360. “While the blanket of air close to the Earth’s surface is warming, most of the atmosphere above is becoming dramatically colder.”
Divided into four layers, the atmosphere becomes progressively less dense as the distance from Earth increases. The lowest level—where humans live—is the troposphere, extending upwards around 10 kilometres above sea level and containing 80% of the atmosphere’s mass but only a small portion of its volume. The next layer is the stratosphere, which extends about 50 kilometres up, followed by the mesosphere with a height of 85 kilometres, and then the thermosphere, extending between 500 and 1,000 kilometres. Beyond that is space.
The power of greenhouse gases to warm the troposphere is widely acknowledged, but their cooling effect on upper levels—like the stratosphere—is less well known.
“In the thinner air aloft, most of the heat re-emitted by carbon dioxide does not bump into other molecules,” explains Yale e360. “It escapes to space.”
“Combined with the greater trapping of heat at lower levels, the result is a rapid cooling of the surrounding atmosphere.”
According to satellite data, the mesosphere and lower thermosphere cooled by 1.7°C between 2002 and 2019—and this is compounded by rising temperatures lower down.
Climate models have predicted this phenomenon and recognized the different temperature impacts as a “fingerprint” of human influence on climate, but the study published this month in the Proceedings of the National Academy of Sciences is the first to use satellite sensors to quantify it in detail. In doing so, it has confirmed the accuracy of models that identify surface warming as human-made, and increased the detectability of the human “fingerprint” by a factor of five.
“Extending fingerprinting to the upper stratosphere with long temperature records and improved climate models means that it is now virtually impossible for natural causes to explain satellite-measured trends in the thermal structure of the Earth’s atmosphere,” the study states.
By providing “incontrovertible evidence of human effects of the thermal structure of the Earth’s atmosphere,” the research shows that humanity is “fundamentally changing” the atmosphere’s thermal structure, said lead author Ben Santer, adjunct scientist at the Woods Hole Oceanographic Institution. “These results make me very worried.”
The findings also corroborates scientists who have said climate change needs to be regarded as a phenomenon that affects more than just the lowest regions of the atmosphere.
“If we don’t get our models right about what is happening up there, we could get things wrong down below,” said Gary Thomas, an atmospheric physicist at the University of Colorado Boulder.
Carbon dioxide increases are now “manifest throughout the entire perceptible atmosphere,” said Martin Mlynczak, an atmospheric physicist at the NASA Langley Research Center in Hampton, Virginia. They are “driving dramatic changes scientists are just now beginning to grasp.”
The rapid cooling is causing the stratosphere to contract, which in turn makes upper levels less dense. That dropping density has its own effect of reducing the atmosphere’s drag on satellites and other objects in low orbit, which increases the risk of collisions among low-orbit debris.
Upper atmospheric changes could also weaken the ozone layer and reduce the protection it gives humans from harmful solar radiation. And since ozone destruction is most severe in polar stratospheric clouds that form over low-temperature regions, the weakening ozone is likely to negatively affect polar and Arctic regions.
There is also potential for changes in the upper atmosphere to affect weather closer to home. Such changes—driven by extreme wind and temperature fluctuations in the stratosphere—could affect the Atlantic jet stream that drives weather systems across the Northern Hemisphere. Outcomes could include extreme weather events like “persistent intense rains to summer droughts and ‘blocking highs’ that can cause weeks of intense cold winter weather from eastern North America to Europe and parts of Asia,” Yale e360 says.
