Study finds stratosphere temperatures 12 to 15 times greater than natural
The middle and upper stratosphere extend from 25 kilometres to 50 km above the Earth’s surface. Photo: iStock The Earth’s upper atmosphere, called stratosphere, has been dramatically cooling in response to human-induced climate change since 1986, according to a recent study. This is in complete contrast to the effects on the lowermost part of the atmosphere, the troposphere.
Temperatures in the stratosphere were 12 to 15 times greater than what would have occurred naturally, without human influence, the study published in journal PNAS stated.
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“This is the clearest evidence of a human fingerprint on the climate system I’ve seen in 30 years of atmospheric research,” 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, said in a press statement.
Previous studies, based on computer modelling, created a simulation by increasing levels of atmospheric carbon dioxide levels from 150 to 300 to 600 parts per million (ppm) and saw increased warming of the troposphere and increased cooling of the stratosphere. Subsequent studies based on computer models also confirmed this prediction.
Researchers from the University of California San Diego evaluated whether the cooling in the middle and upper stratosphere (extending from 25 kilometres to 50 km above the Earth’s surface) can be attributed to human activities.
The team used data from satellites and computer models in their analysis and found the greenhouse gases released from human activities led to a mean cooling of about 1.8 to 2.2 degrees Celsius in the middle and upper stratosphere globally from 1986-2022, even as the troposphere warmed up.
Without human activities, natural variations would have caused global-mean stratospheric temperature changes no larger than about 0.15 degrees Celsius over the same period.
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Natural variations include changes in solar activity, volcanic activities and climate patterns such as El Nino and La Nina. “Each human and natural factor leaves an identifiable fingerprint in the atmosphere,” Santer explained.
“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,” he added.
The opposite impacts of warming troposphere and cooling stratosphere across all latitudes are a unique fingerprint of greenhouse gases, the researchers explained.
If solar activities were driving the temperature rise in the troposphere, then the upper stratosphere would have also warmed up. But this is not the case.
Previously, another study based on satellite data showed the summer mesosphere (extending 50 to 85 km above Earth’s surface, sitting above the stratosphere) over Earth’s poles is also cooling.
The air in the stratosphere and mesosphere is thin and carbon dioxide molecules are not in close proximity. So they do not have a chance to bump into each other and transfer Earth’s heat.
“Down near Earth’s surface, the atmosphere is thick,” James Russell, an atmospheric scientist at Hampton University in Virginia, said in a statement.
“Carbon dioxide traps heat just like a quilt traps your body heat and keeps you warm.” In the lower atmosphere, there are plenty of molecules in close proximity, and they easily trap and transfer Earth's heat between each other, maintaining that quilt-like warmth,” he added.
More heat escapes into space from the Earth’s upper layers of the atmosphere due to increasing greenhouse gases like carbon dioxide.
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