Caldeira and physicist David Keith of the University of Calgary in Canada have been in charge of deciding how to dispense the money. The pair have been informal energy and climate advisers to Gates for several years, and they say they remain independent. "This is philanthropic money and when it arrives [to Calgary] Gates does not control it," says Keith.

Recipients of the funding include Armand Neukermans, an inventor based in Silicon Valley who is working with colleagues to design spray systems for the marine clouds, and students and scientists working for Keith and Caldeira. Funding has also helped support scientific meetings in geoengineering in Cambridge, Massachusetts, and Edinburgh, Scotland, and aeronautics research related to altering the stratosphere.

There are other grantees, Keith says, but he declined to identify them or say why. "This is like a little private funding agency," he says, though he says they plan to release more information.

Gates has shown interest in geoengineering research before. He is an investor in Intellectual Ventures, a Seattle, Washington–area firm that pursues inventions and has applied for patents on techniques to geoengineer the stratosphere. Along with officials from that organization, Gates applied for a patent in 2008 to sap hurricanes of their strength by mixing surface and deep ocean water.

What's his ultimate goal? Gates "views geoengineering as a way to buy time but it's not a solution to the problem" of climate change, says spokesperson John Pinette. “Bill views this as an important avenue for research—among many others, including new forms of clean energy.” (Pinette works for BCG3, a think-tank type firm Gates started last year which has no apparent role thus far in supporting geoengineering.) “Scientific and technological advances are making it possible to solve big, complicated problems like never before,” writes Gates on the Web site of the Gates Foundation, which is also not involved in the geoengineering work.