—David W. Keith, founding faculty director of the Climate Systems Engineering initiative at the University of Chicago, and Wake Smith, a lecturer at the Yale School of Environment and a research fellow at the Harvard Kennedy School.   For half a century, climate researchers have considered the possibility of injecting small particles into the stratosphere to counteract some aspects of climate change. The idea is that by reflecting a small fraction of sunlight back to space, these particles could partially offset the energy imbalance caused by accumulating carbon dioxide, reducing warming as well as extreme storms and many other climate risks. Cooling the planet with this form of solar geoengineering, called stratospheric aerosol injection,  would require a purpose-built fleet of high-altitude aircraft, which could take decades to assemble. This long lead time encourages policymakers to ignore the hard decisions about regulating its deployment. Such complacency is ill-advised. Our analysis suggests a country could conceivably start a subscale solar geoengineering deployment in as little as five years, one that would produce unmistakable changes in the composition of …