Dust. Volcanic ash. Belching smokestacks. Wildfire plumes that send particles skyward. These are some of the many phenomena captured in the planetary boundary for atmospheric aerosol loading. The boundary includes aerosols produced by both natural and human sources. Aerosols are defined as the tiny solid and liquid particles suspended in the atmosphere and exhibit significant diversity in their composition, distribution, and in their duration in the atmosphere. All of this diversity creates significant uncertainties about their total impact. Calculating these effects is further complicated by whether aerosols are liquid or solid. These and other factors affect how the planetary boundaries framework tallies the impact of atmospheric aerosol loading on the Earth system. The measurement for aerosol loading uses what is known as aerosol optical depth, or AOD, which is a calculation of how aerosols in the atmosphere interact with clouds and ice crystals to variously absorb or scatter radiation from the Sun. By scattering radiation aerosols can cool the climate, an effect that partially offsets global warming driven by greenhouse gas emissions (Bellouin et al. Reference Bellouin, Quaas and Gryspeerdt2020).

The measure of AOD is an attempt to provide an ‘integrated measure of the overall reduction in sunlight reaching Earth’s surface caused by all absorption and scattering in the vertical air column’ (Richardson et al. Reference Richardson, Steffen and Lucht2023: 7). In other words, a maximum value for AOD would mean that no radiation reaches the Earth’s surface; the radiation, in strict terms, would be extinct. In the 2009 planetary boundaries report the value for aerosol loading remained to be determined. By 2023, it was set in a way that recognized that aerosol loading had an asymmetric impact on the planet, such as by cooling one area but also potentially shifting precipitation patterns elsewhere. Interestingly, accounting for these differences means that the planetary boundary for AOD is a measure of differences between hemispheres and is set at 0.1 and based on annual averages across hemispheres. These interhemispheric impacts, combined with the many kinds of matter that comprise the category of ‘aerosols’, create significant uncertainties about aerosol interactions with the climate and especially with global and regional water cycles. For instance, aerosols are known to affect the water cycle, and especially the monsoon rains upon which large populations of humans and non-humans rely across South and Southeast Asia (Li et al. Reference Li, Lau and Ramanathan2016). Aerosol loading in the atmosphere can affect the timing and intensity of monsoons, and aerosols in one hemisphere can affect precipitations patterns in the other (Krishnamohan & Bala Reference Krishnamohan and Bala2022; Roose et al. Reference Roose, Bala and Krishnamohan2023).

Currently, atmospheric aerosol loading is one of the few planetary boundaries that remain in a ‘safe space’ for humanity. This is despite well-known effects on human health of particulates at the bottom of the atmosphere where humans and other species breathe aerosols in and out. Extreme air pollution in urban centres like Delhi, Beijing, London, or Riyadh has significant effects on human health, local economies, and regional ecosystems. These are recognized in the planetary boundaries framework, but they are not the driving concern when it comes to planetary resilience.

The two chapters in this part consider atmospheric aerosol loading from the perspectives of international law and environmental geography, respectively. The first, by Leslie-Anne Duvic-Paoli, troubles the distinction between the air being breathed daily and atmospheric loading of aerosols. In Duvic-Paoli’s view, the tools of international law are critical yet underequipped to address the global governance challenge aerosols present. Her concerns then turn to the ethical considerations of fairness and equity in response to what international law can and cannot (yet) do. The next chapter, by Jeremy Schmidt, examines the ethics of purposefully loading the atmosphere with aerosols in attempts to cool the climate – often referred to as solar geoengineering or solar radiation management. After explaining some of the polarizing debates over these proposals, he examines the ethics at work in a growing global agenda that does not necessarily seek to manage solar radiation but instead to experiment with technologies that could do so. Together, the two chapters offer insights into how ethical issues on aerosol loading pivot not only on how calculations are made with respect to the Earth system, but also on the instruments and techniques of governing aerosols, and how the impacts of having more or less of them loaded into the atmosphere are known.