My daily commute here at Cambridge is glorious. Depending on the day of the week, I either walk through verdant cow pastures or wind my way through medieval cobblestone streets. By the time I get home, I’m always in a better mood.
Time in green, natural spaces relaxes and uplifts me. In August, my return to an urban setting was jarring. I spent the summer at an abbey in the foothills of Colorado; compared the peace of that vast natural landscape, Denver felt chaotic, noisy, and dirty.
Our need for green
The human need for green space isn’t news, of course. We have long known that time in “nature”¹ is good for the human mind and body.² Those who spend time in woodlands, gardens, and parks are less stressed and have lower heart rates. Furthermore, they are less likely to suffer from depression and anxiety. Interestingly, engaging with green spaces seems to improve attention and enhance memory functions.
From an evolutionary perspective, it makes sense that green space enhances human life in these ways. After all, the vast majority of our evolutionary history had occurred before the birth of the first city.
Beyond the human person, green spaces have a host of other social and ecological benefits. They facilitate socializing and exercising, improve air quality and reduce temperatures, provide animal habitats, and reduce climate change.
Our need for green space is an urgent problem in the modern world, when more than half of the global population lives in urban areas. Green space is particularly hard to come by in under-resourced neighborhoods and communities of color, and is therefore linked to health disparities.³
Your gut also likes green
Green space can now add one more thing to its list of health benefits: the gut. The microbes that live in the gut are called the microbiome. These bacteria are essential players in the immune system, nervous system, and metabolism. A recent review in Science presents evidence that, as individuals and populations move to urban spaces, their microbiomes change — showing less diversity, and changes in the balance of bacteria. A number of things could trigger these changes: antibiotics and other medicines, processed food, a highly sanitary environment (think: hand sanitizer).
Regardless of its origin, the “industrialized microbiome” is vulnerable. It’s a combination of bacteria that has never been experienced by human beings. This is likely a factor in the rise of inflammation and chronic disease, and may not be reversible.
The “industrialized microbiome” is just one example of the collateral damage of urbanization. By 2050, the global population is expected to near 10 billion people, most of whom will be living in cities. Many of the changes wrought by such a dramatic change in human life are unforeseeable.
Back to green
I’m not proposing an end to urban centers, or an fantasy of returning to rural life. But I do think that we — individually and collectively — have to seriously examine the quality of our urban spaces. Where do you spend most of your time? Can you alter your routine to engage more green spaces? Are you an advocate for the importance of green spaces in your community?
More broadly, the scientific community is increasingly aware of the interconnectedness, the “one health,” of our world and all that lives within it. Social and political changes, facilitated by innovation in urban planning and design, can foster healthier lives. And not just for those who can afford to surround themselves with green spaces — for the vulnerable and marginalized as well. Though it’s countercultural, and costly, it’s necessary. In the words of Wendell Berry: “Whether we and our politicians know it or not, Nature is party to all our deals and decisions, and she has more votes, a longer memory, and a sterner sense of justice than we do.”
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 Full disclosure: I hate calling green space “nature” because that implies that human beings and their work are separate from the rest of creation.  See this study on stress, this study on physical health, this study on mental health, these two on psychological well-being, and this one on cognitive function.  Read this paper for more.