Rockström et al. found that humanity had crossed the safe boundaries for three of the seven defined parameters in 2009 (two boundaries had yet to be defined):
- Climate change, with both CO2 (then 387 ppmv) and change in radiative forcing (1.5 W/m2) exceeding the respective 350 ppmv and 1 W/m2 boundary levels,
- Biodiversity loss, with then extinction rate of >100 species/million species/year far exceeding the boundary of 10 species/million species/year, and
- Biogeochemical cycles, with the then amount of fixed nitrogen per year (121 million metric tons) far exceeding the boundary of 35 million metric tons. Meaning we've really screwed up the natural nitrogen cycle to go with the messed up carbon cycle.
|Figure 1 from Rockström et al. (2009) showing the nine planetary boundaries as well as those which humanity had exceeded.|
|Revised planetary boundaries from Steffen et al. (2015).|
Why is the phosphorus cycle important? Phosphorus, like nitrogen, is a key limiting nutrient to plant growth, both in terrestrial and in aquatic habitats. Excess phosphorus leads to algal and cyanobacteria blooms in freshwater (think of what happened to Toledo, Ohio, last summer when a cyanobacteria bloom contaminated their drinking water) and dead zones in marine habitats. Dead zones have been detected worldwide, from the Chesapeake Bay and the Gulf of Mexico to the Baltic Sea to southern Australia. This creates numerous problems for aquatic animals (after all, every animal needs oxygen) as well as humans.
|Map showing the locations of known dead zones. From NASA GES DISC.|