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Scientists Investigate Drying out the Stratosphere to Reduce Warming

color diagram of 2011 ATREXX flight paths
One of the ATTREX flight tracks flown in the fall of 2011. | Leonhard Pfister/ NASA

In recent years, scientists have been exploring strategies to offset global warming by manipulating the climate system. Now, a new study published in Science Advances examines the feasibility of a new approach to reduce warming: removing water vapor — a greenhouse gas — from the stratosphere.

Drawing on observations from an airborne campaign led by NASA, researchers provide an early-stage assessment of intentional stratospheric dehydration by investigating the practical details and potential effectiveness of injecting ice-nucleating particles (INP) into the stratosphere.

Although the approach would likely have some cooling effect, they say, it wouldn't be nearly enough to counteract more significant warming from carbon dioxide (CO2) emissions. INP injection would have to considered among a portfolio of other climate interventions to determine whether there are any alternatives to reducing CO2 emissions that would meaningfully reduce climate impacts.

It also outlines several technical barriers to consider before such a strategy could be employed, including where, what, and how particles could be injected into the top layer of the tropopause. The tropopause is the boundary between the troposphere (Earth's lowest atmospheric layer) and the stratosphere. Researchers are focused on the tropopause at tropical latitudes because this is where the sun's heating causes air to rise into the stratosphere.

"We need more measurements of how water is distributed in the [tropical tropopause layer] to better understand its sources and sinks in the region," said Joshua "Shuka" Schwarz, a research physicist at the National Oceanic and Atmospheric Administration (NOAA) Chemical Sciences Laboratory and first author.

Probing the Stratosphere

Water vapor is an important greenhouse gas in Earth's climate system. It helps keep the planet warm, but unlike other greenhouse gases, it doesn't accumulate in the atmosphere because it can transform into snow, ice and rain. In this way, water vapor can't drive climate change like human-emitted CO 2 does.

Nonetheless, the United Nations Intergovernmental Panel on Climate Change (IPCC) 6th report suggests that water vapor levels will rise in the atmosphere as the climate warms, because warmer air can hold more of it — a positive climate feedback that is expected to exacerbate warming. This has led researchers like Schwarz to question whether finding ways to reduce atmospheric water vapor could address these secondary effects. One way would be to turn it into ice where the air is coldest, directly below the stratosphere — basically, freeze-dry it.

"At the highest altitudes before entering the stratosphere, the main process removing water vapor is the formation of ice that falls to lower altitudes," Schwarz explained. "Once ice crystals form, they can grow large enough to fall out of the air, thereby removing some of the water from [air packets]."

Schwarz was motivated to investigate this strategy by NASA's 2011-2015 Airborne Tropical TRopopause EXperiment (ATTREX), where researchers used four Global Hawk high-altitude drones to measure the chemical and physical characteristics of the top of the tropical tropopause.

"ATTREX provided unprecedented high-resolution measurements of water vapor and temperature at the position of the airplane in the tropics, in an altitude region that 'normal' aircraft can't access," Schwarz said. "Previously, the analysis of that data was focused on using it to learn about water vapor and clouds in this region, which is very important for stratospheric water. For this paper, however, we changed the focus from understanding those water controls to considering whether we could tweak them for climate benefit."

Modeling Atmospheric Injections

Using ATTREX observations, Schwarz and colleagues outlined a conceptual framework aiming to put numbers to the trategy. How much INP material would be needed? How many planes? How often?

To do this, they started by simulating the injection of ice-nucleating particles using a one-dimensional microphysical model, which traced a path of particles that rose into the stratosphere in a region of the atmosphere in the western Pacific where air masses can naturally "freeze-dry."

"Ice crystals will only form and grow in air that is supersaturated with water vapor — meaning in air with more water than it is 'comfortable' to hold at the specific temperature the air is at," Schwarz explained. Using the model, the researchers demonstrated how INP injection could deplete water vapor. To meaningfully influence water vapor levels, the researchers estimated that they'd need to target a region about the size of Australia, in a 1,000-meter-thick band of air at 17 kilometers altitude.

"We focused on the bismuth triiodide [as a potential INP material] because it has previously been considered for cloud cirrus thinning," Schwarz said, referring to another proposed climate intervention strategy. This material's very small (~10-nanometer) particles could mean that as little as two kilograms of it would need to be injected each week for moderate cooling effects.

Several high-altitude aircraft could do the job, the study noted, but they aren't the only option. "It's possible to imagine all sorts of ways that such tiny amounts could be carried to the [tropical tropopause layer]," Schwarz said. "Could we imagine small, long-duration blimps having a role? [Or] a small fleet of solar-powered aircraft cruising in the region that would only need replenishing once a year?"

However, these conceptual analyses don't address the many technical barriers to making this strategy a reality, such as the need for improved knowledge and forecasts of water vapor in the tropical tropopause. It couldn't serve as an alternative to cutting back CO2 emissions and would need to be used in conjunction with other proposed climate intervention strategies — such as CO2 removal and solar radiation management — to have any meaningful effect.

These strategies have often been referred to as "geoengineering," but Schwarz avoided using the term because of its implication that such strategies are highly refined and dependable.

"In the case of intentionally manipulating climate, we don't want people to misunderstand the level of understanding — or the confidence — that we have right now," Schwarz said. "'Intervention' better reflects our current state: considering doing something, but uncertain about the ultimate impacts."