MIT Scientists Quantify Stratospheric Air Flow for the First Time

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For the first time, scientists have quantified the speed at which air circulates through the stratosphere, the atmospheric layer that shields Earth from harmful ultraviolet radiation. A team from MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS) reports that a parcel of air takes roughly 1.5 years to travel through this layer before descending back toward the surface. The findings, published in the journal Nature Geoscience, also estimate that the stratosphere draws up about 7 billion kilograms of air per second from the tropics, lifting it to heights of up to 20 kilometers.

Previous attempts to measure this circulation, known as the stratospheric cycle, relied on tracking water vapor rising from equatorial regions. That approach, however, offered only a narrow view, according to lead author Marianna Linz. "Others have looked at this region of the equator where they think most of the stuff is coming up, and they've tried to characterize this using water vapor," Linz said in a press release. "But that's just looking at this narrow region, and it's difficult to infer what the rest of the circulation looks like."

To overcome that limitation, Linz and her colleagues adopted a global strategy, using two chemical compounds as tracers to follow air movement throughout the stratosphere. "The thinking is that what goes up must come down," she explained. By compiling measurements of these chemicals taken between 2007 and 2011, the team calculated the transit time and the volume of air involved in the cycle.

Why the Lower Stratosphere Matters

The researchers found that their calculations aligned most closely at the 20-kilometer altitude mark, which represents the lower boundary of the stratosphere. This is a critical zone, as it is the entry point for chemicals that influence ozone formation and the accumulation of greenhouse gases. "The most important thing to know in terms of impacts on climate change and ozone is what this circulation strength is like at this lower altitude, because that's what is supplying chemicals to the stratosphere," said Alan Plumb, professor emeritus at EAPS, in the MIT press release.

However, the study's precision diminishes at higher altitudes. "We have this data and can say what the strength is at this level, but because we don't have the data higher up, we can't say nearly as much," Linz noted. "So we really do need better observations in the upper stratosphere."

Implications for Climate and Ozone Predictions

The new measurements carry significant implications for climate modeling. If climate models miscalculate the strength of stratospheric circulation, they would also produce inaccurate ozone predictions, which in turn would affect projections of global warming trends. This comes as global temperatures continue to set records, with July experiencing unprecedented heat and extreme weather events such as Hurricane Harvey.

The study underscores the interconnectedness of atmospheric layers and the importance of precise data in forecasting environmental changes. As Linz emphasized, accurate circulation data at lower altitudes is essential for understanding the supply of chemicals that drive ozone dynamics and greenhouse gas behavior.