We would mimick a volcano. If we put 5 billion tons of sulfates a year into the atmosphere 20 kilometers high, and do that for 50 years. These sulfates don't stay suspended forever. Bit by bit they fall out of the air.
In the second model, the amount of sulfates gradually increases to around 10 billion tons per year after 50 years, to match the global economy's steadily-growing production of CO2. Both scenarios are based on a widely-used future projection of climate change in which only modest attempts at curtailing CO2 take place.
The scientists found (in their scenario where sulfate injection is doubled over time) that the incidence of Katrina-level hurricanes could be kept at roughly the rate we see today. And when these hurricanes do hit, their most devastating effect to coastal cities—storm surges—would be mitigated by 50 percent.
How much sulfate exactly is 10 billion tons per year? It is like a 1991 Pinatubo volcanic eruption every two years. It would cost about $10 billion per year.
It would somewhat ameliorate Atlantic hurricane intensity and frequency, but there will be more “Katrina”-level events than during the past 30 years. Geoengineering would likely be reasonably effective at controlling coastal flood risk relative to the coastal flood risk expected under the Representative Concentration Pathway (RCP) 4.5 greenhouse gas warming, in part, due to its impact on global sea level rise, although flood risk would still be significantly larger than at present.
Devastating floods due to Atlantic hurricanes are relatively rare events. However, the frequency of the most intense storms is likely to increase with rises in sea surface temperatures. Geoengineering by stratospheric sulfate aerosol injection cools the tropics relative to the polar regions, including the hurricane Main Development Region in the Atlantic, suggesting that geoengineering may mitigate hurricanes. We examine this hypothesis using eight earth system model simulations of climate under the Geoengineering Model Intercomparison Project (GeoMIP) G3 and G4 schemes that use stratospheric aerosols to reduce the radiative forcing under the Representative Concentration Pathway (RCP) 4.5 scenario. Global mean temperature increases are greatly ameliorated by geoengineering, and tropical temperature increases are at most half of those temperature increases in the RCP4.5. However, sulfate injection would have to double (to nearly 10 teragrams of SO2 per year) between 2020 and 2070 to balance the RCP4.5, approximately the equivalent of a 1991 Pinatubo eruption every 2 years, with consequent implications for stratospheric ozone. We project changes in storm frequencies using a temperature-dependent generalized extreme value statistical model calibrated by historical storm surges and observed temperatures since 1923. The number of storm surge events as big as the one caused by the 2005 Katrina hurricane are reduced by about 50% compared with no geoengineering, but this reduction is only marginally statistically significant. Nevertheless, when sea level rise differences in 2070 between the RCP4.5 and geoengineering are factored into coastal flood risk, we find that expected flood levels are reduced by about 40 cm for 5-year events and about halved for 50-years surges.
SOURCES - PNAS, Popular Mechanics