Atmospheric Aerosol Loading
Safe Operating Space
The rise in airborne particles from human activities or natural sources influences the climate by altering temperature and precipitation patterns. Currently the interhemispheric difference in atmospheric aerosol loading is within the Safe Operating Space.
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Importance
The Boundary for atmospheric aerosols has not been breached, but some regions are still experiencing increases in aerosol concentrations. The levels of atmospheric aerosols have increased since industrialization as a result of the burning of fossil fuels and other forms of air pollution. Adopting clean energy will not only prevent the Boundary being breached but will also limit negative health effects for people and animals exposed to aerosols.
Impacts
The interhemispheric difference in aerosol loading leads to an asymmetric radiative forcing, causing relative cooling in the Northern Hemisphere and a southward shift in tropical precipitation. This interhemispheric difference in AOD affects monsoon dynamics, with higher Northern Hemisphere AOD weakening monsoon precipitation.
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Studies, including IPCC AR6, indicate that human-caused Northern Hemisphere aerosols have contributed to decreased global land monsoon precipitation from the 1950s to the 1980s, underscoring the sensitivity of tropical precipitation to aerosol distribution and particle size. Regionaly, higher AOD values in monsoon regions likely lead to significantly lower rainfall, ultimately affecting biosphere integrity.
Control Variables
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Interhemispheric difference in Aerosol Optical Depth (AOD)
AOD measures how many aerosols (small particles suspended in the air) block the transmission of light by absorbing and scattering it. The interhemispheric difference refers to the variation between the Northern and Southern Hemispheres in terms of this specific parameter or variable. Globally, this control variable measures the interhemispheric difference in aerosol concentrations. Regionally, AOD correlates with PM2.5 concentration, which is important for justice considerations regarding human health. However, this regional correlation is not yet fully integrated into the Planetary Boundaries framework. The difference in aerosol optical depth between the Northern and Southern Hemispheres has been decreasing from 2006 to 2023, indicating that we are moving further into the Safe Operating Space.
Global Map of Recent Change in Aerosol Loading
Although AOD is decreasing globally, the varied patterns — with some regions seeing increases — indicate a complex mix of local factors, such as industrial emissions, deforestation, and climate change-driven events like wildfires.
Key Drivers
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The main anthropogenic cause of changes in AOD (i.e., the direct physical-chemical driver of transgression for the global control variable) is human activities such as fossil fuel and biomass burning. A key factor influencing the global control variable, particularly the difference between the Northern and Southern Hemispheres, is that aerosol emissions historically followed divergent trends in the Northern and Southern Hemispheres. This disparity is largely explained by the greater land area and higher population density in the Northern Hemisphere, leading to higher aerosol emissions compared to the Southern Hemisphere.
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However, the impacts, formation processes, and sources of aerosols extend beyond the current control variable selection. For instance, while the global Planetary Boundary remains within its Safe Operating Space, regional AOD levels can exceed safe thresholds, potentially altering precipitation patterns and affecting human health. More research is needed to understand past natural conditions, consolidate observations and modeling, and grasp both local causes and effects as well as global-scale consequences.
Connected Tipping Points
This Planetary Boundary is linked to the following tipping points:
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The collapse of the Greenland and West Antarctic Ice Sheet
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Continued retreat of glaciers
Read more about other Boundaries
Planetary Boundary process inside Earth's safe operating space
Planetary Boundary process outside Earth's safe operating space
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