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Trees are hot. By that, I mean that trees are cool. More specifically, trees are cooling. Planting trees is in vogue, with tree-planting projects involving billions and even trillions of trees sprouting up across the planet.

Science supports such initiatives, but a seed of doubt belongs with every sapling. Green thumbs are no substitute for emissions red lines. Shade is good, but it must not overshadow the hard work of cutting greenhouse gasses.

Planting trees is good for the environment and good for the species that work, play, reproduce or otherwise take up space in it. The first and most immediate benefit — and the focus of much of the fascination — is their ability to remove particulate matter and other forms of air pollution.

Much of the current vogue stems from their carbon dioxide scrubbing capability: A mature tree absorbs about 22 kg of carbon dioxide from the air per year. Scientists reckon that there is room for an extra 0.9 billion hectares of canopy cover in the world, which would store 205 gigatons of carbon, meaning that “global tree restoration is one of the most effective carbon drawdown solutions to date.”

Those numbers have inspired a number of tree-planting projects, the most ambitious of which is the World Economic Forum’s 1 Trillion Trees initiative (1t.org). Launched last year, it aims to plant 1 trillion trees by 2030, which would, the WEF forecasts, remove two-thirds of carbon emissions created by human behavior and reduce carbon in the atmosphere by 25%.

The EU got in the game in 2018. The Green New Deal aims to expand the EU’s sequestration of carbon to 310 million tons from a current target of 265 million tons, and a key piece of that is planting 3 billion new trees by 2030.

At first glance, that is a lot of lumber. The EU is estimated to have planted almost 300 million additional trees between 2010 and 2015, so this goal would roughly double its existing rate of forest expansion. The EU last week published its new forest strategy, which explains how it is going to pull off that feat. According to EU number crunchers, the plan would potentially remove 4 million tons of carbon dioxide from the atmosphere in 2030 and 15 million tons by 2050.

That’s just a start — in every sense of the word. According to EU calculations, this would yield 2 to 3 million additional hectares of tree cover, but that’s only about 2% of the 10 million hectares of forests lost to the world annually. Worse, 3 billion sounds like a big number but it is a tiny fraction of 1 trillion — 0.3 % to be exact. It’s a seed in a (very big) bucket.

Most projects focus on large plantings — forest-size growths. Scientists are increasingly attentive to smaller efforts and touting their benefits. So-called “miniforests,” or small pieces of land — a tennis court can be enough — with their much greater density than regular forests can absorb 30 times more carbon than a traditional woodland. They rely on a planting style pioneered by Japanese botanist Akira Miyawaki, and scientists are excited by the prospects. These miniforests can be placed anywhere — when conditions are right — even in cities.

As the Tokyo summer swelter intensifies, the appeal of urban trees makes more sense to me. Today, a little more than half the world’s population lives in cities; that figure is expected to reach 66% by 2050, more than doubling the 30% of urbanites in 1950. In Japan, more than 92% of the population lives in cities. Trees can help transform that experience.

Whenever I climb a few floors above street level, the Tokyo cityscape looks very green. But looks can be deceiving and Tokyo is not as it seems. The Japanese capital has a low percentage of park space (6.2%), especially when compared with other big cities such as Moscow (54%), Singapore (47%), Sydney (46%), Washington (24.08%) and New York City (21.33%).

But population density inside the Yamanote Line, a train service which circles the city center, isn’t that high. Tokyo doesn’t have the urban canyons of cities like New York City. That allows more space and light for trees, which is why the city looks so green when seen from up high. And green is good.

An analysis by Jessica Turner-Skoff and Nicole Cavender of the Morton Arboretum in Illinois offers an impressive list of (documented) benefits from trees, ranging from the emotional to the environmental to the economic. They note that the presence of trees is “strongly linked to reduced negative thoughts, reduced symptoms of depression, better reported moods and increased life satisfaction.”

Being able to see trees helps patients recover in hospitals, reduces diastolic blood pressure and cuts stress. People who live in tree-lined streets feel healthier and have fewer cardio-metabolic conditions. There is evidence of improved conditions of people with a neuro-degenerative disease. Planting and taking care of trees can improve physical and mental health.

Trees reduce noise and they are positively correlated with a reduction of crime. Trees increase safety for cars and pedestrians by making it easier to judge distances and introducing barriers. (Most studies conceded that the effects are hard to quantify and that multiple factors contribute to the outcomes but they still give trees some credit.)

Trees lower street temperatures. In Hong Kong, large trees with short trunks and dense canopies reduced average daytime mean radiant temperatures by up to 5.1C at the pedestrian level. They store water, reducing runoff and urban flooding. In his study of urban forests in Tokyo, researcher Satoshi Hirabayashi found that trees reduce flooding that costs the city billions of yen each year, and enhance outdoor comfort, which could prevent heat stroke.

And trees make economic sense — in most cases. In the United States, a mature tree raises the average value of a house by 20%. The annual benefit of a tree is between $21 to $159, depending on the cost of care for the tree. In Lisbon, every dollar invested in urban tree management produced a benefit of $4.48 per resident; in New York City and Indianapolis, the numbers are $5.6 and $6.09, respectively.

Interestingly, a study of Kyoto, the hottest city in Japan (we’re talking temperature, not culture), concluded that “the value of annual benefits generated by street trees in Kyoto city did not outweigh tree-related expenditure.” That’s not to say the trees didn’t generate benefits. Rather, the most common trees in the city had a smaller canopy and Kyoto’s maintenance procedures were among the most expensive in the world.

While much rests on the particulars of a study, the evidence seems to support Polish research that concluded “the presence of trees alleviates almost every stress factor generated by the urban environment.”

Some climate scientists worry about this newfound fondness for foliage, arguing that it can undercut other efforts to cut emissions — planting trees is no substitute for the hard work of reducing greenhouse gasses — or undermine biodiversity if done wrong. (They also note that climate change makes it harder for trees to make positive contributions when they too are threatened by its impacts.)

I remain a fan. So, in these COVID-19-infected times, when you can’t hug someone who really matters, go hug a tree. Or better still, plant one.

Brad Glosserman is deputy director of and visiting professor at the Center for Rule-Making Strategies at Tama University as well as senior adviser (nonresident) at Pacific Forum. He is the author of “Peak Japan: The End of Great Ambitions” (Georgetown University Press, 2019).

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