
Preface.
In 2009, Johan Rockström proposed that there were nine planetary boundaries we must not cross. In 2023, Richardson et al found that 6 of the 9 boundaries had been transgressed: Climate change CO2 and radiative forcing, Biosphere integrity, land system change, freshwater change, biogeochemical flows, and novel entities (synthetic chemicals, pollutants, endocrine disruptors, microplastics, nuclear waste, etc). Of the remaining three still within the safe operating space, ocean acidification is very near to crossing the boundary. Only atmospheric aerosol loading and stratospheric ozone depletion are still within it.
Now Kaushal (2023) has proposed that there may be a 10th: Salt. Human activities are making Earth’s air, soil and freshwater saltier, which could pose an “existential threat” if current trends continue.
The study’s authors called for the creation of a “planetary boundary for safe and sustainable salt use” in much the same way that carbon dioxide levels are associated with a planetary boundary to limit climate change.


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