Imagine the following scenario: it is the year 2035. One large country, dealing with major issues of global warming, decides to take extreme action. The government begins secret deployment of a geoengineering system for pumping large amounts of reflective particles into the air, a technique designed to mimic the cooling effect of a volcanic eruption, only on a much larger scale and over a much longer time horizon. Although such behavior has been discouraged by the international community, research has continued, largely behind closed doors and without real regulation. Now that the climate situation has become more dire, the country has decided that it can no longer afford to wait; they see geoengineering as their only option.
Preparing the United States for security and governance in a geoengineering future (brookings.edu)