That's what this "research letter" asks, hosted at a Harvard University server. What a strange question, right? Why do they want to inject substances into the atmosphere without the public learning of it?

https://geoengineering.environment.harvard.edu/files/smith_2018_environ._res._lett._13_124001_0.pdf

"...how much would it cost, and could it be done in secret?"

"one possible concern that may underlie this discomfort is what one might call a deployment program’s presumed ‘secret driver’ properties: the prospect that deploying SAI even at scale could remain undetected"....

"We conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands of flights annually by airliner-sized aircraft operating from an international array of bases."

Whatever they are doing in secret, God knows, and shall reward every man according to his works.

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Keywords used in PDF:

• solar geoengineering
• albedo modification
• solar radiation management
• high-altitude aircraft

Others may call this: cloud-seeding or chemtrails.