A looming tragedy of the sand commons.

B etween 1900 and 2010, the global volume of natural resources used in buildings and transport infrastructure increased 23-fold (1). Sand and gravel are the largest portion of these primary material inputs (79% or 28.6 gigatons per year in 2010) and are the most extracted group of materials worldwide, exceeding fossil fuels and biomass (2). In most regions, sand is a common-pool resource, i.e., a resource that is open to all because access can be limited only at high cost. Because of the difficulty in regulating their consumption, common-pool resources are prone to tragedies of the commons as people may selfishly extract them without considering longterm consequences, eventually leading to overexploitation or degradation. Even when sand mining is regulated, it is often subject to rampant illegal extraction and trade (3). As a result, sand scarcity (4) is an emerging issue with major sociopolitical, economic, and environmental implications. Rapid urban expansion is the main driver of increasing sand appropriation, because sand is a key ingredient of concrete, asphalt, glass, and electronics. Urban development is thus putting more and more strain on limited sand deposits, causing conflicts around the world (5). Further strains on sand deposits arise from escalating transformations in the land-sea interface as a result of burgeoning coastal populations, land scarcity, and rising threats from climate change and coastal erosion (5). Even hydraulic fracturing is among the plethora of activities that demand the use of increasing amounts of sand. In the following, we identify linkages between sand extraction and other global sustainability challenges.