Also known as solar radiation management, this technique involves injecting tiny particles into the sky that would reflect sunlight back into space. The idea comes from volcanoes, with scientists pointing to an eruption 200 years ago that caused an unusual cold snap, triggering unseasonal summertime frosts. This remains a controversial area of research and has not yet been attempted at scale. The top climate scientist at NOAA received $4 million from Congress—as well as their permission—to develop this technique and another technique using sea salt particles in case the U.S. and other nations fail to reduce global greenhouse gas emissions.