Planetary Boundaries

The boundaries that should not be crossed for us to keep the Earth in its currently desirable state (Holocene)

Some of these boundaries show evidence of planetary scale tipping points and some do not. Those that don’t show evidence of tipping points, however, are helpful in regulating and preventing tipping points of other boundaries.

There are strong interactions among the different boundaries. Transgressing one boundary may cause cascading effects on others

“Our model results demonstrate that one of the most powerful means that humanity has at its disposal to combat climate change is respecting the land system change boundary. Bringing total global forest cover back to the levels of the late 20th century would provide a substantial cumulative sink for atmospheric CO2 in 2100. This reforestation seems unlikely, however, given the current focus on biomass as a replacement for fossil fuels and the creation of negative CO2 emissions via bioenergy with carbon capture and storage. Both activities are already serving to increase pressure on Earth’s remaining forest area.” (Richardson et al., 2023, p. 11)

CORE BOUNDARIES

Operate and regulate the entire Earth system, and they are the endpoint depending upon how the other boundaries operate.

1. climate change (TRANSGRESSED) CORE BOUNDARY
   • Climate can get into glacial or extremely hot periods
   • Control variable: GHG Concentration - 350-450 ppm
2. Biosphere Integrity (used to be Biodiversity loss) (TRANSGRESSED) CORE BOUNDARY
   • Biodiversity is key to regulating the Earth’s functions and provision of Ecosystem Services
     • Pollinating bees and agriculture
     • Plant carbon sequestration
   • Functional - measured by human appropriation of Net Primary Production (NPP) (HANPP), focuses on the ability of the biosphere as a whole to provide functional feedbacks in Earth system.
   • Genetic -
3. Chemical Pollution -Novel Substances- (TRANSGRESSED)
4. Ocean acidification
   • Ocean can be in an oxygen rich state or oxygen free
   • Variable: carbonate ion concentration in surface seawater
5. Ozone Depletion
   • Necessary to protect biosphere from harmful radiations
6. Land Use Change (TRANSGRESSED)
   • 40% of land is used for agriculture and urban areas
     • Affect biodiversity and ecosystem services provision
   • Control variable: forest cover remaining compared to the potential area of forest in the Holocene
7. Biogeochemical loading: Global Nitrogen and Phosphorus Cycle (TRANSGRESSED)
   1. phosphorus in fertilizers cause freshwater eutrophication and ocean deadzones
   2. nitrogen causes air pollution, acid rains, warming and overstimulation of growth of aquatic plants and algae
8. Freshwater change (used to be Global freshwater use) (TRANSGRESSED)
   • 25% of large rivers no longer reach the ocean because we’re taking out so much water
     • More water will be needed to feed a growing population
     • All biodiversity depends on water

   Wehereuse streamflowasaproxy torepresent blue water(surfa ceand groundwater) and root-zone soil moisturetorepresent green water(plantavailable water) (46–48). Controlvariables aredefined asthe percentage ofannual global ice-freeland areawith streamflow/rootzone soil moisturedevia tions from preindustrial variability (46, 48). Paper - Earth beyond six of nine planetary boundaries by Richardson et al 2023

9. Aerosol Loading
   1. affects moonson rainfall

How Are Boundaries Measured?

• For each boundary we identify one or more control variable (e.g. GHG concentration)
• Then we look at what science says is the range of safety for that variable
• At which level will the variable cause a regime shift or start affecting other boundaries?
  • There’s high uncertainty on those
  • The lowest bound of the range is considered (for precaution)

Boundaries Co-evolution

• Planetary boundaries co-evolve with Humans and technology
  • The ozone depletion boundary co-evolved from humans’ emission of ozone-depleting substances

Resources:

• Course - Planetary Boundaries