Climate Primer #10: The Ninth (and Final?) Planetary Boundary – Atmospheric Aerosol Loading

To summarize, climate crisis is the defining issue of the century. Buildup of anthropogenic greenhouse gases in Earth’s atmosphere is driving global heating, while a convergence of global crises threatens to rupture key planetary boundaries (beyond which organized human life on Earth will be threatened) including: stratospheric ozone depletion, biodiversity loss, toxic substance contamination, climate change (the defining sub-category of climate crisis), ocean acidification, freshwater consumption, land system change, and disruption of the nitrogen and phosphorous cycles.

Today’s post centers aerosols. During the COVID-19 pandemic (still a thing), aerosols have been much more in the news than usual; however, while their relevance to SARS CoV-2 transmission has been front of mind of late, aerosols (which are, according to Wikipedia, “suspension[s] of fine solid particles or liquid droplets in air or [other] gases”) also play an important, if sometimes ambiguous, role relative to climate change. Examples of aerosols include dust, soot (sometimes also called “black carbon” in climate circles), smoke, water droplets, pollen, and ash, to name just a few. As with the anthropogenic nitrogen and phosphorous loading of the biosphere, the problem with human-caused atmospheric aerosol loading is not the presence of aerosols, per se, but the volume and character of the aerosols being released by human activity. Tomorrow, I’m going to briefly explore a few planetary threats that do not fit into the Stockholm Resilience Centre’s framework (and, like asteroid strikes, do not necessarily follow a planetary-boundaries logic), but for now, I give you the ninth and final planetary boundary as described by the SRC:

An atmospheric aerosol planetary boundary was proposed primarily because of the influence of aerosols on Earth’s climate system. Through their interaction with water vapour, aerosols play a critically important role in the hydrological cycle affecting cloud formation and global-scale and regional patterns of atmospheric circulation, such as the monsoon systems in tropical regions. They also have a direct effect on climate, by changing how much solar radiation is reflected or absorbed in the atmosphere. Humans change the aerosol loading by emitting atmospheric pollution (many pollutant gases condense into droplets and particles), and also through land-use change that increases the release of dust and smoke into the air. Shifts in climate regimes and monsoon systems have already been seen in highly polluted environments, giving a quantifiable regional measure for an aerosol boundary. A further reason for an aerosol boundary is that aerosols have adverse effects on many living organisms. Inhaling highly polluted air causes roughly 800,000 people to die prematurely each year. The toxicological and ecological effects of aerosols may thus relate to other Earth system thresholds. However, the behaviour of aerosols in the atmosphere is extremely complex, depending on their chemical composition and their geographical location and height in the atmosphere. While many relationships between aerosols, climate and ecosystems are well established, many causal links are yet to be determined.

Because the dynamics of global capitalism under neoliberalism have led to a shift of many heavy and polluting industries to countries of the Global South (many of which, like India, are in the Northern Hemisphere), residents of the United States and other rich countries who do not live in frontline communities are, today, often oblivious of the burden of air pollution or see it as a problem of “dirty,” “Third World” countries. Of course, we have profound problems with air, water, and soil contamination in the United States as well, but for residents (especially of cities) of India, China, and other countries to which much of the global manufacturing base has migrated, the crisis of air quality is existential. Again, in atmospheric aerosol loading, there is an instance where something that is harmful even at the local or regional level can, when amplified to a certain scale, become a global crisis – though, ironically, one proposed “solution” to global heating is stratospheric aerosol injection, which will be discussed in depth in a future post. Critics of this form of geoengineering via solar radiation management point out, as does the SRC above, that atmospheric aerosols can disrupt key weather patterns, such as the Asian monsoons upon which billions of people rely for either livelihoods, sustenance, or both.

No bonus piece today. Thanks for reading!