To a non-physicist, it seems for all the world that the shade from trees could be transformational for the issue of a heating climate.  When we’re walking along a baking hot road in the summer mid-day and suddenly reach a shady stretch sheltered by a towering leafy tree, we think, “Man, it’s like night and day – trees are just the OBVIOUS ANSWER!”

So when we ask the physicist, “What volume of air could we say is cooled, and cooled by how much, from the shade of one tree?” He just mutters and shakes his head.  “We couldn’t.”

Non-physicist (with the righteous petulance of a 3-year old): “WHY NOT?!?!? IT’S PHYSICS!”

Physicist (grimacing, head-shaking, visibly withdrawing): “No, I mean yes, it’s physics, but it’s, well, (squints, shudders) it’s biology! mixed up with geology and geography and meteorology, and well – just no.”

“BUT (now ranting) IT’S OBVIOUS the air is SO much cooler when I walk through a shaded spot on a hot tar road.  If all the roads were shaded, it would HAVE to make a difference that would change the whole climate!!!”

At length, after repeated explanations, remonstrations, and research, the non-physicist must conclude that “shade from trees” can’t solve all our problems.  BUT! Trees and vines and vegetation of all sorts do make a huge difference–it’s just not simple or obvious.

To begin with, it turns out the physicist’s main buzz kill, that it only FEELS cooler in the shade because the radiant heat isn’t hitting me, is true.  My perception of much cooler air is mixed with this strong sensation and confuses me into thinking air is 20 degrees cooler when it’s usually really only 2 degrees cooler or less.

That said, studies have apparently shown that peak 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 is cooler at all is due to transpiration by tree leaves:  the sun’s rays get absorbed by the leaves or needles, and the stomata (holes) in the leaf surface emit water, which cools the tree and cools the air by turning to vapor, thus somehow responding to the law of conservation of energy.

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

The radiation that would have hit us hits the tree instead.  It also doesn’t hit the pavement, so the pavement doesn’t absorb and radiate heat back up at us, and this can lower surface temperatures by 20-40 degrees F, according to the EPA.

A tree canopy can intercept as much as 90% of the sun’s radiation that would have hit the pavement and been re-radiated back into our faces for the rest of the day and into the night.**

This works for buildings too.  Apparently, studies have shown that shaded buildings can require 20-30% less air conditioning. Vines on buildings can have this cooling effect too–in one EPA-cited study, vines on a building caused a 36-degree F reduction in a building’s wall temperatures.

As our heating climate poses increasing threats to health, especially in cities throughout the world, the use of plants to cool “urban heat islands” is receiving much attention.  While data are difficult to compile due to the “messiness” of trees, enough scattered studies are underway to allow scientists to confidently make the case for trees, vines, hedges, grasses, and pretty much all vegetation being woven aggressively into the urban landscape to cool and protect the health of urban populations.

There! The non-physicist can claim at least a half-win: the urban planners are on our side.

So, we have here a meager attempt to synthesize information that comes up 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 hold some amount of water, but we just can’t say how much.

In a final rain-on-your-parade gesture, it must be noted: yes, trees can cool us in our daily lives and contain whole ecosystems within their leafy reaches. But another thing they can’t do, even if it SEEMS OBVIOUS they could, is make much difference in our heating climate.

Turns out, no amount or speed of tree planting world-wide can compensate for the rate at which CO2 is pouring into our atmosphere, or significantly decrease the furious pace of global warming.  For details of how afforestation, deforestation, or other climate initiatives might alter our perilous trajectory, visit the free online EnROADS climate simulator at https://en-roads.climateinteractive.org/scenario.html?v=21.7.1.

Meanwhile, do plant a tree or two! It certainly won’t hurt.

*Can Trees Really Cool Our Cities Down?” Roland Enos, The Conversation, Dec. 22, 2015.

**U.S. Environmental Protection Agency. 2008. “Trees and Vegetation.” In: Reducing Urban Heat Islands: Compendium of Strategies. https://www.epa.gov/heat-islands/heat-island-compendium.

Cynthia Stancioff, B.A., M.P.A., is a nature lover with a proclivity for proof-reading and re-writing 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 at [email protected].

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