The Impact of Future Climate Changes on Stratospheric Aerosols

Abstract:

According to baseball player Yogi Berra, “It’s tough to make predictions, especially about the future”. A wide range of futures for stratospheric aerosols is possible in this century. Civilization could be devastated by nuclear winter following an injection of more than 100 Tg of elemental carbon, which heated the stratosphere by 100C, destroyed most of the ozone layer, and created temperatures lower than the last ice age at the surface. Alternatively, or maybe in addition, unchecked greenhouse gas emissions may lead to a warming world in which widespread coastal flooding has begun destroying cities, temperatures are so warm that parts of the globe are no longer habitable because people can’t dissipate body heat, and unchecked population growth has pushed large areas past sustainable limits. If so, Geoengineering may be used, most likely with sulfate aerosols. Yearly injections of a significant fraction of the 10Tg of S emitted by Pinatubo in 1991 might be needed by the end of the century. I will analyze the various environmental changes impacting stratospheric aerosols that are likely by the end of the century in a “rational” world. Carbon dioxide is likely to continue to increase so the stratosphere will cool slightly due to carbon dioxide emission to space, and have a different vertical mixing time. Cooling may alter stratospheric water and the aerosols. It is probable that emissions from coal-fired power plants, particularly in India and China, will have declined significantly, so the flux of SO₂ and of tropospheric sulfates to the stratosphere will be less. COS emissions may change slightly due to changes in vegetation, and industrial emissions. Organic compounds now contribute significantly to the lower stratospheric background. We do not understand the origins of these aerosols, but they were probably trending upward in the past century. N₂O emissions are likely to vary leading to changes in the abundance of HNO₃, which in turn will change the properties of PSCs and tropical sulfates with HNO₃ in them. Of course, a wide variety of volcanic eruptions will occur by the end of the century. There is a very small, but not zero, chance of a mega eruption, which could devastate civilization since we are unprepared for large global crop losses lasting even one season.