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spheric circulation to stratospheric heating by the aerosols (Shindell et al. 2001; Stenchikov et al. 1998). At the same time, the summer monsoons in India and East Asia are weakened by a smaller temperature gradient between the Indian Ocean and the Asian continent and reduced evaporative flux from the Indian Ocean (Boos and Kuang 2010; Manabe and Terpstra 1974; Oman et al. 2006). Furthermore, an increase in available photochemical surfaces provided by the aerosols catalyzes ozone loss (Kinnison et al. 1994).

High-latitude eruptions, such as that of Katmai in 1912, have somewhat different climate effects. Winter warming does not occur, and weakening of the Indian summer monsoon is more prominent (Oman et al. 2006). The aerosols have a shorter atmospheric lifetime (~8 months) than in tropical eruptions (~12 months) since both the aerosols’ travel from the tropics to the poles and midlatitude storm tracks (where they are removed) account for much of the lifetime of stratospheric aerosols injected into the tropics.

The time of year of the eruption also plays a critical role in determining climate impacts. Aerosols injected in the winter at high latitudes will have reduced radiative effects because of less sunlight and will also be removed from the stratosphere more quickly, in part due to large-scale deposition (Kravitz and Robock 2011).

SIMULATIONS

Climate engineering with stratospheric sulfate aerosols has been studied repeatedly with climate models. Simulations in which globally averaged temperature is returned to a reference state show that the tropics are slightly overcooled and that high latitudes, particularly the Arctic, are warmer than in the reference case (Govindasamy and Caldeira 2000; Kravitz et al. 2013). As distinct from the impacts of large tropical volcanic eruptions, Northern Hemisphere continents do not show winter warming patterns for climate engineering with stratospheric sulfate aerosols (Robock et al. 2008), a method that cools the surface more than the rest of the troposphere, stabilizing the lower atmosphere and weakening the hydrologic cycle (Bala et al. 2008). Studies have not yet revealed whether summer monsoon weakening is a robust feature of climate model response to this method of climate engineering.

Simulated climate effects depend on the method of climate engineering, namely stratospheric sulfate aerosols that are similar to the aerosols from the Mount Pinatubo eruption. Such aerosols have particular compositions (approximately 75% sulfuric acid and 25% water) and sizes (effective radius of ~0.5 μm) (Rasch et al. 2008). They are also assumed to be injected above the equator and distributed through an altitude of 16–25 km. If any of these parameters changes, the radiative and climate effects will likely change as well.



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