Scientists have proposed an audacious novel geoengineering technique: intentionally dehydrating the stratosphere.
A study published in Science Advances involves the ambitious and contentious idea of seeding the upper atmosphere with particles to prevent water vapor from entering in the stratosphere.
Water vapor is important because it’s the most abundant greenhouse gas on Earth. The greenhouse effect occurs when gases in the atmosphere trap heat from the sun, keeping the planet livable. Water vapor is made up of complex molecules that absorb heat radiated from the Earth’s surface and re-radiate it back to the planet.
Water vapor is constantly cycling through the atmosphere, evaporating from the Earth’s surface, condensing into clouds, being blown by the wind, and then falling back to the Earth as rain or snow.
Researchers, led by Shuka Schwarz of the National Oceanic and Atmospheric Administration (NOAA), argue that water vapor in the stratosphere plays a critical role in trapping heat from the Earth’s surface.
Science.org reported:
By targeting rising, moist air and seeding it with cloud-forming particles right before it crosses into the stratosphere, geoengineers could cool the world with an intervention far more delicate than other schemes. Drying the stratosphere might take as little as 2 kilograms of material a week, says Shuka Schwarz, the study’s lead author and a research physicist at the Chemical Sciences Lab of the National Oceanic and Atmospheric Administration (NOAA). “That’s an amount of material that helps open the mind to imagine a whole bunch of possibilities.”
“Intentional stratospheric dehydration,” as it’s called, could only cool the climate moderately, offsetting roughly 1.4% of the warming caused by increased carbon dioxide over the past few hundred years. But for geoengineers who have talked about cooling the planet by loading the stratosphere with thousands of tons of reflective particles, “it’s clearly a new idea,” says Ulrike Lohmann, an atmospheric physicist at ETH Zürich. “This is something that could work.”
The scheme relies on a key fact: Only a few places in the world are hot enough to generate the powerful updrafts needed to lift air into the stratosphere, which begins between 9 and 17 kilometers above the surface, depending on latitude. The most important of these portals is found above the western equatorial Pacific Ocean, in a region roughly the size of Australia.
Along its upward journey, much of the water condenses into clouds and rains out of the air. But in the past decade, NASA used a high altitude, jet-powered drone to study the cold layers just below the stratosphere and found plenty of air masses moist enough to form clouds, but lacking in particles that would allow the moisture to condense into ice crystals and ultimately rain. “It’s a question of chance, whether they get to this coldest spot on their journey and there’s enough cloud nuclei left to do anything,” Schwarz says. The NASA studies also found that this moisture was concentrated: Just 1% of the air parcels explored accounted for half of the water that could end up in the stratosphere.
In a simple model, the team simulated injecting bismuth triiodide, a nontoxic compound that has been used in lab studies of ice nucleation, into the 1% areas most ripe for water harvesting. In an optimistic scenario, just 2 kilograms a week of seeds 10 nanometers in diameter would be enough to convert those moist air parcels into clouds, they found. Such an amount could be sprayed by balloons or drones, with no airplane needed.
The prospect of manipulating the stratosphere in such a manner is not without its critics.
Critics argue that the potential risks associated with stratospheric manipulation could outweigh the moderate climate cooling benefits that Schwarz’s study suggests.
Experts like Daniel Cziczo, an atmospheric chemist at Purdue University, caution against the unforeseen impacts such as the inadvertent formation of cirrus clouds which could exacerbate warming instead of mitigating it.
“You’re basically exploring a technique that could have a warming effect and not a cooling effect,” Cziczo wrote.
Mark Schoeberl, a respected atmospheric scientist, echoes the need for caution and comprehensive analysis before adopting such techniques, noting the importance of understanding the full range of potential impacts and the actual cooling effect that might be achieved.
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