From The New York Times:
In a guarded compound at the foot of the Rockies, government scientists are working on a new kind of global alarm system: One that can detect if another country, or maybe just an adventurous billionaire, tries to dim the sun.
Every few weeks, researchers in Boulder, Colo., release a balloon that rises 17 miles into the sky. Similar balloons are launched with less frequency from sites in Alaska, Hawaii and New Zealand; Reunion Island, near the coast of Africa; and even Antarctica. They make up the building blocks of a system that would alert American scientists to geoengineering. As the planet continues to heat up, the idea of intentionally trying to block solar radiation — sometimes called solar radiation modification, solar geoengineering, or climate intervention — is gaining attention. Governments, universities, investors and even environmentalists are pouring millions of dollars into research and modeling of geoengineering systems.