The middle and upper stratosphere — the second lowest layer in the atmosphere — is cooling dramatically because of rising CO2 levels, Yale Environment 360 said, citing a study published last month in the journal PNAS by pioneering climate modeler Ben Santer. The study used new satellite data to build on another recent study that illustrated how the mesosphere and lower thermosphere — the layers above the stratosphere — cooled by 1.7°C. The research estimated that by the end of the century, these zones would cool by 7.5°C, two to three times faster than the average warming expected at ground level.

How does the upper atmosphere get colder? The heat that we emit does not all stay in the troposphere — the lowest layer of the atmosphere, Yale Environment explains. When the heat spreads upwards, it escapes into space, which, combined with the trapping of heat in the lower atmosphere, results in a rapid cooling of the upper layers. The cooling of the upper air also causes it to contract. One study found that the depth of the stratosphere has diminished by about 1% since 1980, and another found that the mesosphere and lower thermosphere contracted by almost 1.3 km between 2002 and 2019.

Why this matters: This contraction means the upper atmosphere is becoming less dense, which in turn reduces drag on satellites and other objects in low orbit, causing space junk — bits of equipment of various sorts left behind in orbit — to stick around longer and increases the risk of collisions with satellites.