Beneath the identifiable risks is not only a gut reaction to the hubris of it all — the idea that humans could set out to regulate the Earth system, perhaps in perpetuity — but also to what it says about where we are today. As the committee’s chairwoman, Marcia McNutt, told The Associated Press: The public should read this report “and say, ‘This is downright scary.’ And they should say, ‘If this is our Hail Mary, what a scary, scary place we are in.’ ”

Even scarier is the fact that, while most geoengineering boosters see these technologies as a means of buying time for the world to get its act together, others promote them as a substitute for cutting emissions. In 2008, Newt Gingrich, the former House speaker, later Republican presidential candidate and an early backer of geoengineering, said: “Instead of penalizing ordinary Americans, we would have an option to address global warming by rewarding scientific invention,” adding: “Bring on the American ingenuity.”

The report, considerably more cautious, describes geoengineering as one element of a “portfolio of responses” to climate change and examines the prospects of two approaches — removing carbon dioxide from the atmosphere, and enveloping the planet in a layer of sulfate particles to reduce the amount of solar radiation reaching the Earth’s surface.