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This century will be remembered as the urban century, and our generation will be witness to the greatest migration in human history. By 2050, there will be 2.4 billion more people in cities, a rate of urban growth that is the equivalent of building a city the size of London every 7 weeks. But what, if anything, is the role for nature in this urban century? Does an urban Homo sapiens even need nature anymore?

In a recent article in Sustainable Earth, my coauthors and I reviewed three different academic disciplines to answer these questions. Taken together, we believe that trends in these three disciplines suggest that the urban century needs nature to succeed.

Urban economics has explored why cities are booming so much. The major theme of that research is that proximity, the increased potential for interaction that comes from living at higher population density, has its benefits. If humans are the social animal, as Aristotle said, that loves interaction, then cities are in a sense quintessentially human, the perfect setting for interaction. Numerous economic studies show that the increase proximity in cities leads to increases in productivity, innovation, and creativity.

If cities are quintessentially human, they are also shockingly unnatural

Yet if cities are quintessentially human, they are also shockingly unnatural. Living in cities is correlated with an urban psychological penalty, manifest in higher rates of stress, psychosis, and depression. While there are a number of causes of this association, which we review in our article, one of the causes seems to be the increased interaction and stimulation that occurs in cities.

But a growing body of scientific evidence shows that parks and other natural features in cities can remove much of this psychological penalty, with one study in England finding 50% less depression and 43% less stress in neighborhoods with greater forest cover.

While the scientific evidence of nature’s mental health benefits is increasingly clear, cities globally remain mostly grey and dreary: we estimate in our article that while close to half of humanity lives at densities where the urban psychological penalty applies, only 13% of urban dwellers have enough nature around them to have a mental health benefit.

We conclude by arguing that nature in cities can be a way to have our cake and eat it too, to have all the benefits from urban proximity while have a lower urban psychological penalty. We believe that for the urban century to succeed, urban planners will have to incorporate nature into the very fabric of our cities.