"Although hurricanes could probably be controlled by stratospheric sulphate-injection geoengineering, the amounts needed by 2100 would be large and so likely very damaging, e.g. effects on stratospheric ozone, or if sulphate injection was done only in the north, then there would probably be a negative impact on African drought and monsoon," John Moore of Beijing Normal University told environmentalresearchweb. "So it would require a lot of political agreement to make this worthwhile, and removes the need for agreement on the mitigation of greenhouse gases."

Moore and colleagues from China, Finland, Denmark, the UK, Germany, the US, Australia and Japan used eight Earth-system model simulations of climate from the Geoengineering Model Intercomparison Project (GeoMIP) under two geoengineering scenarios – one that balances warming under the RCP4.5 scenario by increasing the amount of sulphate injection into the stratosphere from 5 Tg of SO2 per year in 2020 to 10 Tg per year by 2069, and one that uses a fixed injection rate of 5 Tg per year from 2020 to 2069, roughly equivalent to one Pinatubo eruption every four years. The researchers projected changes in storm frequency under warming with a temperature-dependent generalized extreme-value statistical model calibrated with historical storm surges and temperature measurements from the southeastern US dating back to 1923.

Previous work has shown that the tide-gauge record of storm surges is an excellent predictor of hurricane frequency and strength, according to Moore, because the surges travel much more widely than the extreme winds, and floods are the main cause of damage from hurricanes. The frequency and magnitude of storm surges also varies with temperature.

"Hurricanes are too small to resolve by the 200 km Earth-system models," added Moore. "So we need a proxy for the size and frequency of storms – this turns out to be temperatures. Interestingly, it's not just simple global temperature that's the best. The temperatures in several different parts of the world, especially the tropical Atlantic and the tropical Pacific, have opposing impacts on hurricanes."

Under the geoengineering scenarios, temperatures in the tropical Atlantic, where hurricanes that hit the US east coast form, rose more slowly than under the RCP4.5 scenario, the team found. According to the generalized extreme-value model, this would reduce both hurricane frequency and intensity, with geoengineering that balances RCP4.5 keeping Hurricane Katrina-level storms at frequencies similar to today. (Other estimates of tropical-storm intensity show smaller trends with temperature.) The fixed-injection-rate geoengineering scenario, meanwhile, produced a less dramatic effect on storm frequency. But the study indicates that it would reduce damage to coastal infrastructure, halving expected flood levels for 50-year surges in Atlantic City by reducing sea-level rise.

"We show that although it may be expected that geoengineering would be especially effective against hurricanes, since geoengineering preferentially cools the tropics – and that's where hurricanes form – actually its effectiveness is more complex because of the complicated teleconnections that help determine hurricane size," said Moore. "Reduction of city flooding under geoengineering is mostly because global sea-level rise is less due to the cooling of the ice sheets."

Now, the researchers plan to use the Earth-system model results to try a less statistical and more process-driven global hurricane prognosis for the 21st century. "The GeoMIP data are huge and the potential analyses have really scratched the surface," said Moore. "This is one of the first results from the new Chinese geoengineering programme that will run five years and which I lead."

The team intends to carry out further GeoMIP simulations using different geoengineering techniques. "We can use those results to consider other impacts that are very important, such as storms, droughts [and] agricultural productivity," said Moore.

Moore and colleagues reported their results in PNAS.

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