Stratospheric Aerosol Injection
The field of geoengineering was actually inspired by a 1991 volcano at Mount Pinatubo in the Philippines. As a result of the eruption, an estimated 20 million tons (18.1 million metric tons) of sulfur were scattered into the atmosphere, and global temperatures dropped about a half degree Celsius over the next year. Why? The clouds of sulfur blocked some of the sun's rays, keeping temperatures down. This might not sound like much, but it's more than half what the planet has warmed naturally over the previous 130 years.
Now, some of the more radical geoscientists are talking about replicating the Mount Pinatubo's effects by spraying sulfur into the atmosphere, hoping to bring down the planet's temperature yet again.
In 2010, David Keith, director of University of Calgary's Energy and Environmental Systems Group told NPR that injecting sulfur dioxide via jet is easy and inexpensive. "It takes so little material to alter the whole planet's climate," he told NPR. "The costs of doing it are just absurdly cheap."
But it also has significant disadvantages that science doesn't fully understand. For example, one possibility is that stratospheric aerosol injection could impact agriculture in parts of Africa and Asia. Plus, sulfur is known to create air pollution that's potentially fatal, and it can be destructive to the ozone layer.