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From Quarks to Quasars — Cooling Earth could take a fleet of Boeing 777s and millions of tons of sulfur dioxide. A modeling study suggests modified commercial cargo planes could lower Earth’s average temperature by about 1.1°F (0.6°C) by releasing roughly 13 million tons (12 million metric tons) of sulfur dioxide into the upper atmosphere every year. The idea is called stratospheric aerosol injection, a form of solar geoengineering. Sulfur dioxide would react in the atmosphere to form tiny sulfate particles. These particles would reflect a small fraction of incoming sunlight back into space, temporarily cooling the surface. We know this mechanism can work because volcanoes do it naturally. The 1991 eruption of Mount Pinatubo released a vast amount of sulfur dioxide and temporarily lowered global temperatures. Most geoengineering proposals involve releasing particles roughly 12 miles (20 kilometers) above tropical or subtropical regions. Existing cargo planes cannot carry heavy loads that high, so deployment would require specially designed aircraft. Researchers tested a different approach. Using the U.K. Earth System Model, they ran 41 simulations that varied the altitude, latitude, and season of injection. Their most practical scenario released sulfur dioxide about 8 miles (13 kilometers) above regions near 60° north and south during each hemisphere’s spring and summer. That is approximately the latitude of Anchorage and Oslo in the north, and south of the tip of South America in the Southern Hemisphere. This works because the boundary of the stratosphere is lower near the poles. Modified aircraft such as the Boeing 777F could potentially reach it while carrying large payloads. But the easier approach would be highly inefficient. Particles released at this altitude would remain aloft for only a few months, compared with several years at higher altitudes. The strategy achieved only 35% of the cooling efficiency of conventional high-altitude injection, meaning about three times more sulfur dioxide would be required. That could increase acid rain and human exposure to falling particles. Cooling would also be concentrated more heavily near the poles, providing less relief to tropical regions that are especially vulnerable to extreme heat. This was a computer simulation, not a field experiment or deployment proposal. Researchers also warn that suddenly stopping injections after years of use could allow temperatures to rebound dangerously fast. The technique would not remove greenhouse gases, stop ocean acidification, or replace emissions cuts. It could only temporarily conceal part of the warming while introducing new environmental and political risks. Learn more: “Geoengineering technique could cool planet using existing aircraft.” University College London.