Ever-higher temperatures are melting the ice sheets faster than projected. Sea level is rising. International efforts to reduce greenhouse gases are taking longer than expected. It's a nightmare scenario that could soon demand an emergency response. What to do? One idea gaining traction is to seed marine clouds with salt water or other particles, increasing their potential to reflect solar rays, cooling the earth. It's part of a nascent and controversial branch of science known as "sunlight reflection methods," or SRM. "We think SRM could buy time for other (carbon-reduction) measures to be put in place," said Philip J. Rasch,...