Atmosphere

Two major planetary boundaries (Rockström et al., 2009) occur within the Atmospheric biome of the planet: Ozone depletion and aerosol loading. 

Stratospheric Ozone Depletion

Stratospheric ozone depletion occurs when human-made gases, particularly halocarbons from industrial activities, are released into the atmosphere. The stratospheric ozone layer is Earth's "sunscreen" – protecting living things from too much ultraviolet radiation from the sun. The gases like halocarbons cause long-term damage to Earth's ozone layer. 

Safe Operating Boundary

The safe level for ozone is set at 276 Dobson units (DU). This boundary allows up to a 5% reduction from the preindustrial level of 290 DU. Measurements are taken by latitude.

Current Status

The Montreal Protocol, adopted in 1987, has helped reduce ozone depletion (Nair et al., 2015; Pazmiño et al., 2018). In 2020 the global average ozone level was estimated at 284 DU (Rockström et al., 2009). This level indicates that ozone depletion is within the safe operating space. However, ozone depletion still exceeds safe levels over Antarctica and southern high latitudes during 3 month period of the Austral spring. In summary, while global efforts have improved the situation, some regions, such as Southern Australia and New Zealand and South America, still experience significant seasonal ozone depletion.

Atmospheric aerosol loading - Aerosols and Their Impact on Earth

Overview

Aerosols are tiny particles in the air that come from both natural sources (like desert dust and wildfire soot) and human activities (like industrial emissions). They impact climate, weather, and ecosystems, making them an important factor in planetary boundaries.

Aerosols have significant impacts on the Earth's systems. Anthropogenic aerosols have increased, affecting natural aerosols like desert dust and soot. The Sahara Desert is a major dust source. Changes in monsoon patterns have dried up regions like the Sahara, altering ecosystems and displacing human settlements. Quantifying aerosol impacts is complex due to various sources, compositions, and impacts. Aerosol optical depth (AOD) measures sunlight reduction due to aerosols. Higher AOD in monsoon regions decreases rainfall, affecting ecosystems.

AOD values are elevated in regions like southern Asia and East China, potentially surpassing defined limits. Differences in AOD between northern and southern hemispheres influence monsoon systems and precipitation patterns. Northern hemisphere aerosols contribute to cooling and impact tropical precipitation. Interhemispheric AOD differences impact monsoon rainfall and water availability.

Human-caused aerosols in the northern hemisphere affect monsoon dynamics. The interhemispheric AOD difference is a key control variable for aerosol loading. Present-day differences in AOD suggest more northern hemisphere aerosols, with projected decreasing asymmetry. The current planetary AOD boundary is 0.1 with uncertainty between 0.1 to 0.25. Aerosol-cloud interactions further complicate effects.

Understanding aerosol impacts is crucial for refining boundaries. Aerosols affect regional climate, precipitation, and broader ecosystems. The contribution of aerosol-cloud interactions to radiation balance is uncertain. Anthropogenic aerosols impact air quality and ecosystems. Studying biogenic aerosols is essential for a comprehensive understanding of aerosol effects on Earth's systems. 

Point form summary

Increase in Aerosols

  • Human Activity: Aerosols from human activities have increased (Carslaw et al., 2017). 

  • Natural Aerosols: It’s hard to measure changes in natural aerosols (Bellouin et al., 2020) since the preindustrial era, but evidence shows dust deposition has doubled since 1750 (Hooper & Marx, 2018).

  • Sahara: Currently, the Sahara is the largest source of dust. Historically, it was a lush area with lakes and wetlands (Knippertz & Todd, 2012).

Measuring Aerosol Impact 

  • Aerosol Optical Depth (AOD): This measures how much aerosols block sunlight. Higher AOD means more sunlight is blocked.

  • Regional Boundaries: In southern Asia, AOD values over 0.25 can reduce rainfall. Current AOD is about 0.3 to 0.35, exceeding safe limits. In East China, AOD is 0.4.

Global Impact

  • Global Mean AOD: The average global AOD is 0.14, but it varies widely.

  • Monsoon Effects: Differences in AOD between the northern and southern hemispheres affect monsoons, shifting rainfall patterns.

Proposed Control Variable

  • Interhemispheric AOD Difference: The current difference between hemispheres is about 0.076, up from 0.03 in the preindustrial era. A proposed safe boundary is 0.1.

Additional Effects

  • Climate and Precipitation: Aerosols impact rainfall and regional climates.

  • Uncertainties: There are uncertainties in how aerosol-cloud interactions contribute to climate effects.

Other Impacts: Aerosols also affect air quality and ecosystems

References:

Bellouin, N., Quaas, J., Gryspeerdt, E., Kinne, S., Stier, P., Watson-Parris, D., . . . Stevens, B. (2020). Bounding Global Aerosol Radiative Forcing of Climate Change. Reviews of Geophysics, 58(1), e2019RG000660. doi:https://doi.org/10.1029/2019RG000660

Carslaw, K. S., Gordon, H., Hamilton, D. S., Johnson, J. S., Regayre, L. A., Yoshioka, M., & Pringle, K. J. (2017). Aerosols in the Pre-industrial Atmosphere. Curr Clim Change Rep, 3(1), 1-15. doi:10.1007/s40641-017-0061-2

Hooper, J., & Marx, S. (2018). A global doubling of dust emissions during the Anthropocene? Global and Planetary Change, 169, 70-91. doi:https://doi.org/10.1016/j.gloplacha.2018.07.003

Knippertz, P., & Todd, M. (2012). Mineral dust aerosols over the Sahara: Meteorological controls on emission and transport and implications for modeling. Reviews of Geophysics, 50. 

Nair, P. J., Froidevaux, L., Kuttippurath, J., Zawodny, J. M., Russell III, J. M., Steinbrecht, W., . . . Anderson, J. (2015). Subtropical and midlatitude ozone trends in the stratosphere: Implications for recovery. Journal of Geophysical Research: Atmospheres, 120(14), 7247-7257. doi:https://doi.org/10.1002/2014JD022371

Pazmiño, A., Godin-Beekmann, S., Hauchecorne, A., Claud, C., Khaykin, S., Goutail, F., . . . Quel, E. (2018). Multiple symptoms of total ozone recovery inside the Antarctic vortex during austral spring. Atmos. Chem. Phys., 18(10), 7557-7572. doi:10.5194/acp-18-7557-2018

Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S., Lambin, E. F., . . . Foley, J. A. (2009). A safe operating space for humanity. Nature, 461(7263), 472-475. doi:10.1038/461472a