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.

It could be a relatively quick way to cool the planet. But it could also unleash untold dangers.

Many worry that solar geoengineering could have unintended consequences, shattering regional weather patterns and damaging everything from agriculture to local economies. And the first steps could be done quietly, by a rogue actor or another nation operating without any regulations or controls.

So the United States is building a system that would allow it to determine if and when others may be trying to tamper with the Earth's thermostat.

"It's some of the most important stratospheric science going on in the world today," David W. Fahey, director of the National Oceanic and Atmospheric Administration's Chemical Sciences Laboratory, which is building the network of balloon sentries, said on a recent afternoon in his office in Boulder.

Both NOAA and NASA have satellites that can detect large quantities of aerosols in the atmosphere but they can't pick up smaller amounts. That's where the balloons come in. Each one carries a six-pound contraption, about the size of a lunchbox, filled with wires and tubes. The device measures tiny airborne particles, or aerosols. A jump could indicate the presence of an unusual amount of aerosols in the stratosphere, possibly to deflect some of the sun's heat back into space.