In 2009, the
were introduced. They consist of 9 global control variables, and corresponding
thresholds which, if crossed, could generate unacceptable environmental change
. The idea has been very successful, but also controversial. This paper picks up the debate with regard to the boundary on
global freshwater use
: it argues that such a boundary is based on mere speculation, and that any exercise of assigning actual numbers is arbitrary, premature and misleading.