The dominant direct physical processes responsible for desertification are water erosion, wind erosion and salinization. Other threats that degrade the soil include loss of biodiversity, loss of soil organic matter, fire, changing water resources, soil compaction, soil sealing and contamination. Soil management inevitably combines human and physical effects. Climate, which is the most important driver of the physical systems, is now being rapidly modified by human action, and at a scale which is much coarser than any local remedial action. In a model of near-subsistence systems, productivity is limited by climate and available labour, with some options for additional inputs through improved seed, fertilizer or tillage equipment. Optimum solutions in a particular environment depend on both climate and access to markets. Agricultural surpluses, if any, allow investment in infrastructure – some of it directly supporting agriculture through irrigation and market systems, some less directly useful through, for example, warfare or pyramid building. Today some traditional drivers of desertification, based on subsistence agriculture and grazing, may have become less relevant, as land, particularly in the global South, is developed for intensive irrigated farming, and populations move into mega-cities. The dominant drivers may become soil sealing around cities and transfers of urban and irrigation water. In semi-arid areas this will lead to competition for the best land – for urban expansion and agricultural land with irrigation potential. Desertification then becomes an issue increasingly focussed on abandoned marginal land, maintaining biodiversity, managing regional water resources and controlling erosion in the face of global climate change. Display omitted •Desertification is discussed in the context of a mixed subsistence/cash economy.•A model estimates labour surplus as a function of climate, fertiliser and population.•The model is applied for both arable and pastoral economies.•Labour surplus provides for re-investment in the land or improving infrastructure.•Can arid lands be sustainably managed in an urbanising world?