"In terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people," the researchers wrote. "The greatest monetary values are derived in areas with the greatest population density (e.g. Manhattan)." (Brooklyn trees are less concerned with monetary values and more with effortless authenticity.)

When the emerald ash borer began ravaging thousands of trees in the American Midwest, as Lindsay Abrams noted in The Atlantic last year, rates of human death from cardiovascular and respiratory illness increased. One study monitored disease rates in 15 states from 1990 to 2007, where the borer was associated with 6,113 human deaths from illness of the respiratory system and 15,080 deaths from heart disease. And those medical outcomes don't even include the known psychological merits of tree proximity. In one famous 1970s study of patients recently liberated of their gall bladders in a Pennsylvania hospital, those whose rooms had a view of trees recovered more quickly than those looking out at another building. Medical technology is far from developing any device that can help people recover from purposeful incisions to the abdomen just by looking at it.

(Kara Gordon/The Atlantic)

Another medical study found that women recently diagnosed with breast cancer were better able to focus their attention if they spent two hours a week in natural environments, ostensibly because of stress mitigation.

Nowak, just as incapable of fully disconnecting as anyone, replied to me shortly after his automated response. "The takeaway is that trees have a huge impact on pollution," he told me by phone, "and when populations increase, trees have a greater impact based on being close to where people live."

"We need to start having this discussion," Nowak said, referring to factors more quantifiable than did Thoreau, "about the impact of trees on human health."

Before designing urban landscapes simply to optimize air-pollution removal, Nowak said, we'd do well to also consider other benefits of trees related to energy conservation, like changes in air temperature, water, and wildlife. "There are a whole bunch of other things to consider. We're talking billions of dollars a year [in benefit] from urban forests. It adds up, if you look at the whole picture."

Air pollution now kills around seven million people every year globally, according to the World Health Organization. Factoring in the other costs of air pollution—not just to human health, but building and material damage and crop losses—Nowak's current study put the total annual value of pollution removal by U.S. trees at $86 billion.

Public-health researchers are to the point of suggesting people who live in high-pollution areas eat broccoli as a form of "chemoprevention" because it causes us to excrete benzene through our urine. The health argument for investing in trees, particularly of the urban variety, is even more staid. If you've ever scoffed at someone for calling a tree "majestic" or dismissed Johnny Appleseed as "just some crazy guy," maybe it's time to do some soul searching. And where's this anger coming from? Not enough trees, probably.