To a non-physicist, it seems to everyone that tree shade could be transformational for the issue of a warming climate. When we walk along a scorching road in mid-summer day and suddenly reach a shady stretch sheltered by a towering leafy tree, we think, “Dude, it’s like day and night – them. trees are just the OBVIOUS ANSWER! “

So when we ask the physicist, “How much air could we say is cooled, and how much cooled, in the shade of a tree?” He just mumbles and shakes his head. “We could not.”

No physicist (with the just petulance of a 3 year old child): “WHY NOT?!?!? IT’S PHYSICS!

Physicist (grinning, nodding, visibly withdrawing): “No, I mean yes, it’s physics, but it’s okay, (squinting, shivering) it’s biology!” mixed in with geology and geography and meteorology, well – just no.

“BUT (now delirious) IT IS OBVIOUS that the air is SO much cooler when I walk through a shady spot on a hot asphalt road. If all the roads were shaded it SHOULD make a difference that would change the whole climate !!! ”

Finally, after repeated explanations, admonitions and research, the non-physicist must conclude that “the shade of the trees” cannot solve all of our problems. BUT! Trees, vines, and vegetation of all kinds make a huge difference – it’s just not straightforward or obvious.

For starters, it turns out that the physicist’s main buzz kill is that he only FEELS cooler in the shade because radiant heat doesn’t hit me, that’s right. My perception of much cooler air is mixed with that strong feeling and confuses me into thinking that the air is 20 degrees cooler when it is usually only 2 degrees cooler or less.

That said, studies have apparently shown that maximum temperatures in tree groves can be 9 degrees F cooler than in unshaded areas, and suburbs with shade trees can be 4 to 6 degrees F. cooler than those without trees. *

Most of the reason it’s cooler is due to the transpiration of the leaves of trees: the sun’s rays are absorbed by the leaves or needles, and the stomata (holes) on the surface of the leaves emit l water, which cools the tree and the air by turning into vapor, thus meeting in some way the law of conservation of energy.

A large oak tree, according to the EPA, can emit 40,000 gallons of water per year. I mean, doesn’t that feel like it’s worth something?

Rather, the radiation that would have hit us hit the tree. It also doesn’t hit the pavement, so the pavement doesn’t absorb or radiate heat towards us, which can lower surface temperatures by 20 to 40 degrees F, according to the EPA.

A tree canopy can intercept up to 90% of the solar radiation that would have hit the sidewalk and been reflected back in our faces for the rest of the day and into the night. **

It also works for buildings. Apparently, studies have shown that shaded buildings can require 20-30% less air conditioning. Vines on buildings can also have this cooling effect – in a study cited by the EPA, vines on a building caused a 36-degree F reduction in the temperature of a building’s walls.

As our heating climate poses increasing threats to health, especially in cities around the world, the use of plants to cool “urban heat islands” is receiving a lot of attention. Although the data is difficult to compile due to the “mess” of the trees, enough scattered studies are underway to allow scientists to confidently argue that trees, vines, hedges, grasses and little Almost all vegetation is aggressively woven into the urban landscape to cool and protect the health of urban populations.

The! The non-physicist can claim at least a half-victory: the town planners are on our side.

So here we have a meager attempt at synthesizing information that emerges when one tries to substitute the weight of a visceral and somewhat romantic impression for an energy physics equation. The visceral impression turns out to contain a certain amount of water, but we just can’t say how much.

In a last gesture of rain on your parade, it should be noted: yes, trees can refresh us in our daily life and contain entire ecosystems within their reach. But another thing they can’t do, even though it SEEMS OBVIOUS they could, is make a big difference in our heating climate.

It turns out that no amount or speed of tree planting around the world can offset the rate at which CO2 is pouring into our atmosphere, or significantly reduce the frenetic rate of global warming. For more details on how afforestation, deforestation or other climate initiatives could alter our perilous trajectory, visit the free online EnROADS climate simulator at https: // en-roads.climatinteractive.org/scenario.html? v = 21.7.1.

In the meantime, plant a tree or two! It certainly won’t hurt.

* Can trees really cool our cities? »Roland Enos, La Conversation, December 22, 2015.

** US Environmental Protection Agency. 2008. “Trees and vegetation”. In: Reducing Urban Heat Islands: A Compendium of Strategies. https://www.epa.gov/heat-the islands / the-islands-of-heat-compendium.

Cynthia Stancioff, BA, MPA, is a nature lover with a penchant for proofreading and rewriting things, and enough curiosity to ask her physicist husband, Paul Stancioff, about the facts of the universe and sometimes accept the answers. . She can be reached at [email protected], and he has [email protected].