Planet Care

266: SCoPEx


Emergency measures may need to be taken to counteract anthropogenic global warming.


The idea is simple: spray a bunch of particles into the stratosphere, and they will cool the planet by reflecting some of the Sun’s rays back into space.

This high-risk geo-engineering is inspired by historical events. An intense volcanic eruption of Mount Tambora (Indonesia) in 1815 released an incredible amount of volcanic ash, droplets of sulfuric acid, and water into the atmosphere, obscuring the Sun and creating a global cooling event. For exactly this reason, 1816 was known as the “year without summer”.

Equally, when Mount Pinatubo erupted in the Philippines in 1991, it injected an estimated 22 million tons ( 20 million tonnes) of sulfur dioxide into the stratosphere — the atmospheric layer that stretches from about 6 mi to 31 mi (10 to 50 km). above Earth’s surface. The eruption created a haze of sulfate particles that cooled the planet by around 0.5 °C. For about 18 months, Earth’s average temperature returned to what it was before the arrival of the steam engine.

In 2009, while at the University of Calgary in Canada, experimental physicist David Keith founded the company Carbon Engineering, in Squamish, which is working to commercialize technology to remove carbon dioxide from the atmosphere.

After joining Harvard University, Keith used research funding he had received from Microsoft co-founder Bill Gates, to begin planning the experiment. In 2011 he moved to Harvard Univesity where he teamed up with atmospheric chemist James Anderson, who had been investigating a variety of geo-engineering options off and on for more than 25 years.

They were joined by Frank Keutsch to prepare the Stratospheric Controlled Perturbation Experiment (SCoPEx). The experiment is backed by Bill Gates, the Hewlett Foundation, the Alfred P. Sloan Foundation, as well as university grants and donations from other groups and individual contributors.

At the heart of SCoPEx is a scientific balloon, fitted with re-purposed off-the-shelf airboat propellers. The re-purposed propellers serve two functions. First, the propeller wake forms a well mixed volume (roughly half a mile long and 330 ft in diameter (1 km long and 100 m) that serves as an experimental ‘beaker’ to which are added particles of calcium carbonate powder which is expected to absorb less heat than volcanic sulfates and to have less impact on ozone.

Second, the propellers allow repositioning the gondola to different locations within the volume to measure the properties of the perturbed air. The payload can achieve walking speed relative to the surrounding air, generally for about ten minutes at a time. The advantage of the SCoPEx-propelled balloon is that it allows the team to create a small controlled volume of stratospheric air and observe its evolution for over 24 hours. (

In February 2019, Raven Aerostar of South Dakota, specialized in stratospheric (high-altitude) balloons and airships was selected to make the prototype flying machine. The first phase involves two flights 12 mi. (20 km.) above the southwest United States, when small plumes of calcium carbonate, each of around 3.53 oz. (100 g.) will be released. The balloon will then turn around to observe how the particles disperse.

The technical aspects of this experiment are far less important than its political, social, and geopolitical implications. After all, the risks of geo-engineering could not be more serious. If deployed at scale, SRM (Solar radiation management) could disrupt the monsoons in Asia and cause droughts in Africa, affecting the food and water supplies of two billion people.

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