  The production of waste as a consequence of human activities is one of the
most fundamental challenges facing our society and global ecological systems.
Waste generation is rapidly increasing, with corresponding shifts in the
structure of our societies where almost all nations are moving from rural
agrarian societies to urban and technological ones. However, the connections
between these radical societal shifts and waste generation have not yet been
described. Here we apply scaling theory to establish a new understanding of
waste in urban systems. We identify universal scaling laws of waste generation
across diverse urban systems worldwide for three forms of waste: wastewater,
municipal solid waste, and greenhouse gasses. We show that wastewater
generation scales superlinearly, municipal solid waste scales linearly, and
greenhouse gasses scales sublinearly with city size. In specific cases
production can be understood in terms of city size coupled with financial and
natural resources. For example, wastewater generation can be understood in
terms of the increased economic activity of larger cities, and the deviations
around the scaling relationship - indicating relative efficiency - depend on
GDP per person and local rainfall. We also show how the temporal evolution of
these scaling relationships reveals a loss of economies of scale and the
general increase in waste production, where sublinear scaling relationships
become linear. Our findings suggest general mechanisms controlling waste
generation across diverse cities and global urban systems. Our approach offers
a systematic approach to uncover these underlying mechanisms that might be key
to reducing waste and pursing a more sustainable future.
