But the amount of land space taken up by cities is actually relatively small compared with the number of people they shelter: satellite image composites show that urban sites cover only 2.8 per cent of the Earth’s land; accordingly the UN estimates that about 3.3 billion people occupy an area less than half the size of Australia. (pdf)
This gainsays conventional environmental wisdom in several ways. Clearly, the best strategy to curb future population growth is to speed up the ‘demographic transition’ in developing countries – and this transition towards women having fewer babies is inextricably linked both with increasing levels of prosperity and with urbanisation. Therefore rising rates of economic growth and the expansion of cities are good news for the environment because they will restrain the future growth in human population. Moreover, although the idea of getting close to the land in small-scale communities has a deep cultural resonance in some schools of environmentalist thought, in reality this is probably the worst thing that anyone can do.
All around the world, rural depopulation is leading to forest regrowth in abandoned areas – from the vast tracts of secondary broadleaf woodland in America’s New England states to tropical forests in Puerto Rico, the Dominican Republic and many other areas. In Costa Rica, abandoned cattle pasture is nurturing a flourishing young forest that in turn now supports a stable population of jaguars and other threatened fauna. A recent scientific paper looking at Latin America lists ‘similar patterns of ecosystem recovery following rural-urban migration’ in Patagonia, northwest Argentina, Ecuador, Mexico, Honduras and the montane deserts and Andean tundra ecosystems of Bolivia, Argentina and Peru. Even in rich countries, proposals for ‘rewilding’ – which I strongly support – only stand a chance of success in areas where rural populations have collapsed and formerly subsidised unproductive farms can be shut down to allow them to revert to nature.
This suggests that rural depopulation should not necessarily be opposed with ‘sustainable development’ schemes aimed at improving rural life to stop people migrating to cities. Equally, instead of encouraging low-tech traditional farming methods it may be preferable to focus on improving high-yield mechanised agriculture on the most fertile farmland to feed the new urban residents, whilst allowing mountainsides and other marginal lands to revert to forest. This is already happening by default in Latin America and elsewhere: in Vietnam, forest area has been increasing since the 1990s after small-scale, unproductive agriculture was made uncompetitive by more intensive, larger-scale farming in the more open market economy.
As always, one should not oversimplify. Cities themselves consume resources, including food, timber, water and energy, harvested over vastly wider areas than the land that they physically occupy, and this greater footprint needs to be considered in any overall assessment to get a true picture. When peasants move to the cities, their land might just as easily be turned over to large-scale cattle ranching or plantations as allowed to revert to forest. Studies have suggested that this is particularly the case in Amazonia, where most deforestation is carried out by ranchers, so population density is less clearly linked with the fate of the forest. Moreover, even after people move out, the recovery of forests cannot always be left to chance – it needs active management and ecologically friendly government policies. Whether secondary forest can help avoid large-scale species extinctions also depends on the extent to which animals and plants accustomed to old-growth forests can successfully recolonise new areas.