Stratospheric cooling: The concerning flip side of global warming

UCLA-led research finds human-generated greenhouse gases are responsible for the troubling changes in upper atmosphere

Alison Hewitt | August 21, 2023

Key takeaways

  • A UCLA-led study is the first to search for human-caused climate patterns in the middle and upper stratosphere.
  • The research found that temperature decreases in the stratosphere over the past three decades have been caused by humans, not nature.
  • According to the study, cooling in the middle and upper stratosphere is a consequence of human-caused increases in greenhouse gases, which cause heat to be retained more effectively in the troposphere, the lowest level of the atmosphere.

Human-driven climate change has caused large and concerning temperature decreases in the stratosphere since at least 1986, according to a UCLA-led study published in the Proceedings of the National Academy of Sciences.

That sustained stratospheric cooling, the authors report, is evidence that the warming of Earth’s surface and lower atmosphere is not a natural occurrence.

In particular, the study confirms the effects of human causes on the overall climate: The temperature changes in the stratosphere were 12 to 15 times greater than what could have been caused by nature.

“This is the clearest evidence of a human fingerprint on the climate system I’ve seen in 30 years of atmospheric research,” said Benjamin Santer, the study’s lead author, a climate scientist at the UCLA Joint Institute for Regional Earth System Science and Engineering and the Woods Hole Oceanographic Institution.

The study, by researchers from UCLA and several other universities and institutes worldwide, is the first to search for human-caused climate change patterns in the middle and upper stratosphere. Those atmospheric layers — roughly 15 to 31 miles (about 25 to 50 km) above the Earth’s surface — rest above the elevations where most weather activity occurs. That high up, natural phenomena like El Niño or La Niña have relatively little impact, which makes it easier to isolate the role of human influence, Santer said.

Human-driven climate change has already led to the warming of the Earth’s surface by 1.1 degrees Celsius since the late 1880s. (The Paris Climate Accords have set a target of not exceeding 1.5 degrees Celsius of warming to avoid the most dangerous climate consequences.)

Previous research projected that as carbon dioxide trapped heat in the troposphere, which is the lowest level of the atmosphere, the stratosphere above it would cool down. Recent improvements in satellite data and computer model simulations have enabled researchers to search for that predicted cooling pattern in the middle and upper stratosphere, and to see how it affects efforts to identify human fingerprints on climate.

From 1986 to 2022, temperatures declined in the higher levels of Earth’s atmosphere (starting at top left, blue shades) while increasing in the layers of atmosphere closest to the Earth’s surface (bottom row, red shades).

The new research shows that from 1986 to 2022, the human-produced greenhouse gases that caused warming of the Earth’s surface and the troposphere also led to a mean cooling of about 1.8 to 2.2 degrees Celsius in the middle and upper stratosphere globally. In contrast, global-mean stratospheric temperature changes caused by natural variations were no larger than about 0.15 degrees Celsius over the same period.

“In addition to human influences, natural changes in the sun, volcanic eruptions and fluctuations caused by phenomena like El Niño and La Niña affect Earth’s climate,” Santer said. “Each human and natural factor leaves an identifiable fingerprint in the atmosphere.

“We looked at whether natural factors could plausibly explain the distinctive observed pattern of warming of the troposphere and cooling of the stratosphere. They can’t.”

Santer said the findings are a direct rebuttal to disinformation efforts that have blamed climate change on natural factors.

“Hundreds of researchers worldwide have shown that climate changes on the land, in the ocean and in the atmosphere are not consistent with natural variation,” he said.

When Santer began work on “vertical fingerprinting” — the effort to detect human influence on atmospheric temperature — satellite data and computer simulations didn’t yet reliably measure temperatures more than about 15 miles (25 km) up, which is only partway into the stratosphere.

The lower stratosphere, which includes the ozone layer, has experienced temperature variation from human-caused damage to the ozone layer and its later recovery. Ozone changes made it difficult to decipher the effects of carbon dioxide alone.

Measuring the middle and upper stratosphere makes the human influence five times clearer than if vertical fingerprints stop at the lower stratosphere, Santer said. That finding confirms predictions in a landmark 1967 paper by Nobel Prize–winning physicist Syukuro Manabe and meteororolgist Richard Wetherald that the strongest indications of human-caused carbon dioxide increases would be measurable in the middle and upper stratosphere.

“This work is intellectually gratifying, as I’ve been wanting to probe this part of the atmosphere for 30 years,” Santer said. “But humans are fundamentally changing the thermal structure of Earth’s atmosphere, and there is no joy in recognizing that.”

The study’s other authors are from Colorado State University, Lawrence Livermore National Laboratory, MIT, Nanjing University of Information Science and Technology, the NOAA/NESDIS Center for Satellite Applications and Research, Santa Rosa, California-based Remote Sensing Systems, University of East Anglia and University of Washington.

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