Current emission scenarios envisage the potential draw-down of around 810 Gt CO2 from the atmosphere by 2100. This would require some 110×1012 kWh of reversible work, and the energy needed to drive the CO2 capture processes would approach, and may even exceed, the energy gained in burning the fossil fuel in the first place. Negative Emission Technologies (NETs) would be needed at huge scale, motivating a search for novel, highly energy-efficient approaches. Separation processes such as CO2 capture involve dissipation of energy in diffusion, the losses being greater when higher transfer rates are utilised to reduce equipment size. A process for generating power for air capture by humidifying dry air is analysed using an equivalent reversible process, which yields minimum utility requirements and maximum (reversible) efficiency. Comprehensive socio-economic and environmental assessments of NETs are needed. The Process Analysis Method provides a triple-bottom line sustainability assessment, but results are dependent on uncertain external factors such as the introduction of flue gas-CCS and the growth rate of renewables. The challenge for NETs becomes easier the faster the transition to a low-carbon economy occurs.