Table 1. The planetary boundaries (Rockström et al. 2009), their control variables, and the scale at which they are mainly observed.

Issue Control variable Scale
Climate change CO2 concentration and radiative forcing Global
Ocean acidification Surface ocean CO32- saturation state
Stratospheric ozone depletion
 		 Ozone concentration
 
Perturbed biogeochemical flows Human-induced phosphorus inflow to ocean;
fixation of atmospheric N2 to bioavailable forms		 Regional
Atmospheric aerosol loading Not quantified (regional particulate concentration proposed)
Global freshwater use Annual volume of water used in human activities
Land system change Percent of Earth’s land converted to cropland
 
Biodiversity loss Global species extinction rate Local
Chemical pollution Not quantified (concentrations and effects considered)