An overview of the evolution of different fossil fuel sources, including coal, conventional and unconventional (tight) oil, oil sand, conventional natural gas, unconventional shale gas, and coalbed methane. The chapter provides the data on energy flows and evaluates the global energy consumption for understanding the magnitude of these developments. The chapter also presents the traditional water–energy concept; the amount of water withdrawal and consumption for fossil fuel exploration in the recovery stage (coal mining, oil and gas drilling, mining, hydraulic fracturing, oil enhancement), processing (coal washing, oil refinery), conversion (electricity production), and post-conversion (waste disposal). The chapter examines the water intensity metrics by normalizing the water volume-per-energy unit of electricity, with an emphasis on the distinction between water withdrawals and water consumption, and considering the complete lifetime cycles of water extraction for energy exploration, processing, and generation.

Freshwater biodiversity is threatened by growing human consumption and contamination of fresh water - a globally scarce resource. As human populations increase, the quality and quantity available for freshwater biodiversity declines.The result is a tragedy of the freshwater commons with increasing competition among groups of humans – evident from the hydropolitics of transboundary rivers - and between humans and nature.Humans may even be approaching the planetary boundary for freshwater use.Pollution and contamination are widespread, with emerging threats from microplastics and pharmaceuticals.Dams, drainage-basin disturbance, climate change, alien species, and overexploitation of aquatic animals pose additional threats.Their synergistic effects are evident from a global analysis of rivers: both biodiversity and human water security are at risk in many parts of the world while, in others, investments in infrastructure have enhanced water security although biodiversity remains under threat. Everywhere on Earth where there are substantial human populations, freshwater biodiversity is threatened.In many of these places, human water security is at risk also.