Our Human Boundaries – Part III

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It's finally time to consider Safe and Just Earth System Boundaries proposals to reclaim and reset Earth Systems Boundaries. First up, the three most significant (at least in my view): climate, water, and nutrients. The authors of the paper in Nature challenged us to read their proposals and then discuss them. In the spirit of that challenge, I will let them do the talking.


“We find that global warming beyond 1.0 °C above pre-industrial levels, which has already been exceeded, carries a moderate likelihood of triggering tipping elements.”

As a reminder, tipping elements are abrupt changes, “large and difficult to reverse ...” It is a process on the planetary scale that we cannot foresee clearly; it is a known unknown as many shifts have occurred in the Holocene.

  • Above 1.5 °C or 2.0 °C warming, the likelihood of triggering tipping points increases to high or very high, respectively … stabilizing at or below a safe ESB of 1.5 °C warming avoids the most severe climate impacts on humans and other species.” This includes the “melting of the Greenland ice sheet and  loss of tropical coral reef systems.”
  • Rising temperature and sea levels will simultaneously make it more difficult to throw off excess body heat, a health need, and displace significant portions of the population.
  • “We conclude that if exposure of tens of millions of people to significant harm is to be avoided, the just (NSH) boundary should be set at or below 1.0 °C. Since returning within this boundary may not be achievable in the foreseeable future, adaptations and compensations to reduce sensitivity to harm and vulnerability will be necessary.”

Justice Considerations

“The Earth Commission’s justice analysis shows that people and ecosystems are already experiencing significant harm at the current 1.2°C of warming. … By setting the safe and just Earth System Boundary for climate at 1°C, the Commission is amplifying the injustice inherent in current world targets and the urgency to act to limit warming to as close to 1.0°C as possible.” [emphasis added]


The experts consider “blue” or freshwater, which more directly impacts human life as surface and groundwater. They mention in passing “green” water is – “critical for maintaining the atmospheric water cycle, which regulates seasonal precipitation levels; can support a significant proportion of global agricultural production with less impact on aquatic ecosystems than blue water use.” Of course, we exert little, if any, control over the atmospheric water cycle, so its adjustment to support global agricultural production is a bit wistful. [1]

Surface water is what we see in streams, rivers, and lakes; groundwater has entered the soil collecting in aquifers and ground tables. There is an interdependence between the two freshwater sources.

  • “Flow alterations of surface water “leads to reduced water quality, natural habitat and biodiversity losses… [resulting from] damming of rivers, the direct extraction of water from rivers and/or lakes, and other water resource developments.” – our human uses
  • “Much of the water we use for drinking, sanitation and agriculture comes from groundwater. Life would be impossible without it.” Over-extraction impacts surface water flow, but more importantly “puts agricultural production at great risk and can lead to irreversible land subsidence [the settling or sinking of land].”
  • The safe boundary for surface water is set at a 20% reduction in historical flow as “empirical studies showing that flow alterations within 20% support native fish species and flow alteration beyond this level strongly affects biodiversity and ecosystem structure and function.”
  • “Unsafe levels of groundwater extraction occur when drawdown exceeds replenishment rates … [we propose] groundwater considered safe if drawdown is less than recharge. …The groundwater extraction that may safely occur within this boundary naturally varies across the planet and, where possible, should be defined based on local-scale monitoring …”

Justice Considerations

“We align our just ESBs [boundaries] for water with the safe ESBs while noting that adhering to the boundaries would considerably restrict current use and will require policies to ensure distributive justice.”    

  • “At a minimum, water needs to be safe for consumption and irrigation, meaning that acceptable standards for faecal coliforms and salinity must be met.”
  • Minimizing harm varies globally - “harm associated with poor water sanitation and hygiene conditions disproportionately impacts the health of young children in low-income countries, particularly in Sub-Saharan Africa and South Asia.”
  • “…the allocation of allowed alterations between communities, sectors or nations sharing the water body, whether directly or indirectly via virtual water. … is particularly challenging where the safe [boundaries] require drastic reductions in water use.”


“Elevated N and P concentrations cause harm through the consequences of eutrophication … the health impacts of air pollution from ammonia-derived aerosols … [and] drinking surface or groundwater with elevated nitrate concentrations.”

The report considers two specific nutrients, Nitrogen (N) and Phosphorus (P), agricultural nutrients recognizing that plants are the basis of all relevant food chains. Fertilizer has supplanted the human activity of agriculture as a disrupter of the Earth.

  • Fertilizer enhances crop yield; our current carrying capacity to feed ourselves requires fertilizer.
  • The harm of fertilizer is in eutrophication – the natural runoff of excess N and P “sets off a chain reaction in the ecosystem, starting with an overabundance of algae and plants.” The algal bloom reduces available oxygen for sea life, perturbing fish and mollusk habitats. Loss of mollusks compounds the issue because their filter-feeding normally reduces N and P. The excess algae and plant matter decompose, producing carbon dioxide, which lowers the pH, acidifying the water and slowing the growth of sea life.
  • “Huge amounts of nutrients are lost and wasted between field and plate” – Surplus fertilizer is that which is not “used and removed by the crops… that drives environmental degradation.”
  • “The surplus can be reduced to as close to zero as possible through increased fertilizer use efficiency.”
  • While excess fertilizer causes harm, insufficient fertilizer results in hunger – there is a problem of distribution.

Justice considerations

  • “The just nutrient boundaries are based on the point at which widespread harm occurs to people… primarily driven by environmental degradation.
  • The global Just boundary for Phosphorus aligns with its Safe limit because human harm here occurs due to eutrophication.
  • For Nitrogen, the global Just boundary is … slightly more stringent than Safe because in a few places Nitrogen over-use pollutes drinking groundwater before environmental degradation occurs.
  • Additional justice considerations include lack of access to N and P fertilizers … extraction of phosphate rock, which is a limited resource currently underpinning food production but exposes poor and marginalized communities to mining waste, destroyed land, and human rights abuses.
  • Synthetic nitrogen fertilizers also require climate-polluting natural gas and lots of energy to make, but could be replaced by “green ammonia.”

[1] “Green water is defined as the percentage of ice-free land area that in any month has root-zone soil moisture levels outside the 95th percentile of the local baseline variability. The boundary value is set at 10%, corresponding to the median departure level from mid-Holocene conditions.”

Here are the other parts of the article:

Part I – Earth System Boundaries – the science

Part II – Earth System Boundaries, the methodologies of “Justice”

Part IV – Earth System Boundaries, Aerosols, Biosphere and some final thoughts

Source: Safe and Just Earth System Boundaries Nature DOI: 10.1038/s41586-023-06083-8