Clouds of self-levitating soot might help halt global warming

A new geoengineering method relying on clouds of soot mixed with sun-reflecting particles that propel itself into the stratosphere has been proposed by researchers as a possible solution to curb progressing climate warming if emission reductions fail. 

Inspired by naturally-occurring effects observed in the aftermath of powerful volcanic eruptions, many scientists have theorized that dispersing tiny aerosol particles at high altitudes could help halt climate change. Following major eruptions, large quantities of gas and ash spewed by some volcanoes high into the atmosphere can temporarily cool down the planet. For example, the 1991 eruption of Mount Pinatubo in the Philippines caused a dip of one degree Fahrenheit (0.6 degrees Celsius) in average global temperatures, which was measurable over a period of 15 months.

However, mimicking the explosive powers of Earth's supervolcanoes is not easy. The volcanic cloud generated by Mount Pinatubo contained 15 million tons of sulfur dioxide and reached up to the altitude of 21 miles (34 kilometers). For months, the sulfur dioxide particles remained suspended in the stratosphere, the second layer of the atmosphere that extends between the altitudes of 9 to 30 miles (15 to 50 km). The ability of these particles to reflect solar radiation resulted in somewhat less sunlight reaching the lower levels of the atmosphere, which then experienced an overall cooling.

Solar-powered soot

Authors of a new study, published in the journal Science Advances on Friday (May 14), argue that hauling such massive quantities of sun-reflecting aerosol to such high altitudes is beyond the capabilities of any existing aircraft, and developing new technologies to do as much would be too costly and time-consuming.

To have the desired cooling effect, sunlight-reflecting material would have to be dispersed above the so-called tropopause, the boundary between the troposphere (the lowest layer of the atmosphere affected by dynamic weather patterns) and the much quieter stratosphere. 

Overtime, gravity pulls the particles to the troposphere anyway. Once there, the particles are quickly washed away by rain or dispersed by wind, Karen Rosenlof, a climate scientist at the Chemical Sciences Laboratory of the National Oceanic and Atmospheric Administration (NOAA) and corresponding author of the new paper, told Space.com in an email.

So, in order to make the volcano-inspired climate-cooling method more readily replicable, the authors of the new paper propose using fleets of existing aircraft to dump the sunlight- reflecting sulfur dioxide at the altitude of 6.2 to 7.5 miles (10 to 12 kilometers) and letting the aerosol rise upwards into the stratosphere completely on its own. 

To enable this self-lofting effect, black carbon (which is essentially soot) would have to be added to the sulfur dioxide, the paper states. The black carbon particles would absorb solar radiation, heating the surrounding air in the process. The heated air would subsequently flow upwards, carrying the sunlight-reflecting aerosol with it.

Inspired by wildfires

The researchers were inspired by studies of the 2017 wildfires in Canada, which generated plumes of smoke rich in soot that were later detected by satellites and weather balloons at altitudes of up to 12 miles (20 km). It was the presence of soot that helped the smoke reach such high altitudes, according to the recent paper. 

The team used the Community Earth System Model (CESM) run by the U.S. National Center for Atmospheric Research to simulate how a similar cloud created by humans might behave. 

The scenario envisioned that two Teragram (1,1 million tons) of sunlight-reflecting sulfur dioxide mixed with 11,000 tons of black carbon would be released by a fleet of aircraft over a ten-day period in a 60-mile (100 km) stretch of air in the tropics. 

The researchers explained that the aircraft would have to target a compact area at the altitude of about 6.2 to 7.5 miles (10 to 12 kilometers) to create a cloud so dense that the particles would heat and rise up into the stratosphere as intended. 

"To do this, we estimate needing 335 tanker aircraft flying six two-hour flights daily," Rosenlof said. "This is a large undertaking, requiring both preparation of geoengineering material, determining how to best disperse the light absorbing material, and dedicated use of at least eight runways."

About 20,000 flights would be needed over a ten-day period to achieve this, with the operation having to be repeated on a yearly basis. 

Option of last resort

In modeling the technique, the team predicts that 80% of the black carbon would rise into the stratosphere together with the sun-reflecting sulfur dioxide. 

The authors, however, stress that geoengineering methods should only be seen as a temporary measure and an option of last resort to buy humanity time if all else fails.

"To address the cause of global warming, reductions in carbon dioxide emissions are required," Rosenlof said. "If the technology can be worked out in an energy efficient manner, carbon dioxide removal [would be required] as well. If emission reductions are not achieved sufficiently fast, then the Paris temperature limits may be exceeded for years to decades. In this case, climate intervention with some form of geoengineering could be needed to avoid exceeding temperature limits."

The researchers also said that there would need to be careful consideration of the possible side effects of putting so much material into the atmosphere. The simulation described in the paper, for example, showed that the presence of the heat absorbing carbon would warm the stratosphere by 1.8 degree F (1 degree C). Rosenlof  said that this warming would have no effect on the weather on earth. She, however, admitted that researchers are concerned about the possible loss of ozone in the stratosphere that the presence of polluting particles might trigger. 

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