Latest / The Climate Translation / The Chemistry of Shade

Transcript

The Chemistry of Shade · The Climate Translation

0:00
If you've ever walked across the city on a hot summer afternoon, you know how much
0:12
a difference a single tree can make.
0:15
Step out from an open sidewalk and into the shade, and the temperature seems to change
0:20
almost instantly.
0:22
And in cities where concrete and asphalt trap heat, trees can be one of the simplest tools
0:28
for making a neighborhood more livable.
0:31
But trees are doing more than casting shade.
0:34
They're also releasing chemicals into the air.
0:37
And when some of those compounds mix with pollution and strong sunlight and heat, the chemistry
0:43
can change, sometimes making summer air unhealthy to breathe.
0:49
New research is showing that when it comes to urban forests, the type of tree we plant
0:54
may matter more than we once realized.
0:58
Welcome back to the podcast, I'm Dr. Mac, and this is the Climate Translation.
1:09
When we look at a tree, most of us think about what it takes out of the atmosphere, namely
1:14
carbon dioxide.
1:16
Many of us learned about photosynthesis in school.
1:20
Plants take in carbon dioxide, use sunlight to build sugars, and they release oxygen back
1:26
into the air.
1:28
But the exchange can go in different directions.
1:31
Trees are constantly releasing other substances into the atmosphere as well.
1:36
Water vapor is one of them.
1:38
Trees pull water from the soil, move it through their roots and trunks, and eventually release
1:43
it through tiny openings in their leaves.
1:47
But they also release hundreds of different organic chemicals.
1:52
It is called many of these biogenic volatile organic compounds, or BVOCs.
2:01
The name sounds intimidating, but you can break it down pretty easily.
2:06
Biogenic means it came from a living thing.
2:09
Organic tells us that the molecule contains carbon, and the volatile means that the substance
2:15
can easily evaporate and enter the air.
2:19
You'd probably experience some of these compounds without even realizing it.
2:23
Think about that unique smell of a pine tree, or the scent that's released when you
2:28
crush a leaf between your fingers.
2:31
Those smells come from volatile chemicals produced by the plant.
2:36
One of the most important BVOCs for our story is isoprene.
2:42
You probably won't smell it when you're walking beneath a tree, but plants around
2:46
the world release enormous amounts of it into the atmosphere every year.
2:52
Isoprene appears to help plants deal with environmental stress, particularly heat.
3:00
The leaf, sitting in a direct summer sun, can become increasingly warmer than the air
3:06
around it.
3:07
At high temperatures, the structures inside the plant cells that carry out photosynthesis
3:13
can become stressed.
3:15
Research has shown that plants capable of producing isoprene are better protected against short
3:22
periods of intense heat, in a sense it's part of the plant's defense system.
3:29
And while a lot of plants produce that chemical, not every tree produces the same amount,
3:35
some species release very little isoprene, others release a lot.
3:42
Oaks are well known isoprene emitters, so are poplars, willows, and eucalyptus trees.
3:49
Other species produce different compounds, including chemicals called monoterpenes.
3:55
So while all of these trees have the potential to provide a shade from the sun, chemically,
4:00
they behave very differently from one another.
4:03
And that difference is at the center of new research that was just published in August
4:08
of 2026.
4:10
A team, led by Xianjun He, and Bin Yuan, at Jinan University in China, working with
4:19
researchers, including an atmospheric scientist, Thomas Karl at the University of Innsbruck
4:25
in Austria, studied emissions in Beijing.
4:29
It's a city that offered them an interesting natural laboratory.
4:34
The capital of China has millions of trees, including large numbers of willows and poplars.
4:40
The researchers wanted to understand exactly what those trees were putting into the atmosphere,
4:46
but what they discovered wasn't simply that the trees were releasing chemicals.
4:51
They also found out how important those chemicals could become once they entered the air above
4:58
a hot, polluted city.
5:01
You've probably heard of ozone.
5:03
It's a chemical in the stratosphere, far above our heads, that absorbs and protects
5:08
us from the harmful UV radiation from the sun.
5:12
But ozone can also be found at the surface.
5:15
In fact, it's one of the primary components of what we call smog.
5:21
It all starts with nitrogen oxide, sometimes called NOx.
5:26
It comes mostly from human activity.
5:29
It's a byproduct of combustion engines as well as power plants and industrial facilities,
5:36
and can occur wherever fuel burns at high temperatures.
5:40
In a large city, there can be a lot of NOx in the air.
5:44
And then you have to consider volatile organic compounds.
5:49
Some are released by trees, but others come from gasoline, paints, solvents, factories,
5:56
and countless other human sources.
5:59
Then you add in the final ingredient, sunlight, and chemistry begins.
6:07
Strong sunlight provides energy that breaks apart and rearranges molecules in the atmosphere.
6:13
When nitrogen oxides and volatile organic compounds are present together, they can enter
6:20
into a complicated series of reactions.
6:23
The exact chemical processes are not necessary to understand the result.
6:30
But think of it as a recipe being followed.
6:34
First you've got oxygen in the atmosphere.
6:36
Then you add in NOx as the next group of ingredients.
6:41
Volatile organic compounds are added, and then sunlight supplies the energy needed to start
6:46
mixing everything together.
6:49
One of the products that can emerge from that chemistry is ozone.
6:54
And while ozone high in the air is beneficial, ozone at the ground is not.
7:01
It's the same molecule, three oxygen atoms joined together.
7:05
But in the lower atmosphere where we live and breathe, ozone is a pollutant.
7:11
It can irritate your lungs.
7:13
It can make breathing more difficult, and it can worsen conditions like asthma.
7:18
And unlike smoke coming from a wildfire or exhaust coming from a tailpipe, ozone isn't
7:24
released directly from a source.
7:27
There isn't an ozone pipe sticking out the back of your car.
7:31
Instead, ozone is forming through those chemical reactions in the atmosphere after other pollutants
7:37
have already been released.
7:41
Meteorologists and air quality scientists thus call it a secondary pollutant.
7:46
And it's one reason ozone can be difficult to understand.
7:50
You can walk outside on a perfectly clear summer afternoon.
7:54
You don't see smoke, and there may not be any obvious haze or smell.
7:59
But the chemistry above your head is very active.
8:03
And the weather matters too.
8:05
Ozone tends to be a problem on hot summer days because sunlight drives the reactions that
8:11
produce it.
8:13
If winds are light, pollutants can also remain over the same area for longer periods of
8:19
time, giving time for those chemical reactions to occur.
8:24
This is why many cities issue ozone alerts during stretches of hot summer weather.
8:30
I remember announcing many ozone action days during my time as a TV meteorologist in
8:37
Corpus Christi, Texas.
8:39
So that brings us back to the trees in this picture.
8:43
A tree releasing isoprene is providing one of the ingredients that can take part in that
8:49
chemistry.
8:50
It's a reactive chemical that can be part of the soup that makes up smog.
8:56
And researchers wanted to know how much this chemical could influence ozone chemistry
9:02
when trees are planted inside an atmosphere that already contains all the other ingredients
9:08
needed to make it.
9:10
And what they found is that rising temperatures due to climate change may impact this influence
9:17
far more than they initially thought.
9:21
In the study in China, the researchers wanted to move beyond simply estimating what trees
9:26
might be releasing into the atmosphere.
9:29
They wanted to measure it directly.
9:32
So they installed instruments on Beijing's meteorological tower, including equipment on
9:37
a platform about 330 feet above the ground.
9:42
From there, they could measure the movement of volatile organic compounds from the surface
9:48
and the atmosphere above it.
9:50
And when they analyzed the results, they found something that looked strange.
9:55
The compounds coming from vegetation were only about a tenth of the total volatile emissions
10:01
they measured.
10:03
Most of the volatiles were associated with human activities.
10:08
But the researchers weren't interested about how much chemical was present.
10:12
They wanted to know about chemical reactivity.
10:16
In simple terms, reactivity tells us how quickly a chemical is likely to participate in the
10:22
reactions taking place in the atmosphere.
10:25
A high reactivity means it's much more likely to produce a product.
10:30
A low reactivity is the opposite.
10:33
And by that measure, the trees suddenly became more important.
10:38
Now they found that those volatile biogenic compounds accounted for nearly half of the total
10:45
chemical reactivity that was observed and most of that came from isoprene.
10:51
In fact, isoprene alone accounted for more than 90% of the reactivity from plant-based
10:58
volatiles.
11:00
But the part of the study that really caught my attention was what was happening as temperatures
11:05
have increased.
11:07
When temperatures rose from 68 degrees Fahrenheit to 95 degrees, the chemical reactivity
11:14
of those volatile compounds that came from vegetation increased by 7 to 8 times.
11:22
The reactivity of volatiles from human activities increased by only about 40%.
11:28
So as the day became hotter, the balance shifted.
11:32
At cooler temperatures, vegetation only accounted for about 21% of the total reactivity going
11:38
on.
11:39
At hotter temperatures, that climbed to 74%.
11:44
The chemistry had changed because the trees were responding to the heat.
11:50
And on days when ozone production was especially sensitive to the amount of volatiles in the
11:56
air, the researchers found that the peak ozone concentration increased with temperature
12:04
at nearly the same rate as that volatile reactivity.
12:08
In other words, as it became hotter, plants were producing more reactive chemicals and
12:15
releasing that into an air that already contained the ingredients needed for smog.
12:22
The researchers then compared Beijing with more than 20 other cities, and Beijing stood
12:27
out.
12:28
It wasn't simply because the city had a lot of trees.
12:32
It was because of what trees were growing there.
12:36
About 35% of Beijing's urban trees are species known to release isoprene, including large
12:44
numbers of weeping willows and Chinese white poplars.
12:49
Across the cities the researchers examined isoprene emissions varied by more than tenfold,
12:56
depending largely on the mix of tree species.
13:00
And that might be the most important finding of this study.
13:04
As cities are getting hotter due to man-made climate change, the question is no longer simply
13:10
how many trees we plant, it's the types of trees we choose.
13:16
So after hearing all this, you might reasonably ask, should we stop planting trees in cities?
13:23
And of course the answer is, absolutely not, and the researchers behind this Beijing
13:27
study were very clear on that.
13:31
Thomas Karl, from the University of Ennsbrook, he emphasized that their findings are not
13:36
an argument for removing mature trees or reducing urban green space.
13:42
Trees provide too many benefits, they shade our streets and our buildings.
13:46
They cool the air through evaporation, they capture airborne particles, they absorb
13:52
carbon dioxide, and their roots help slow storm water run off.
13:57
And beyond the atmospheric science, trees simply make cities more comfortable and beautiful
14:02
places to live.
14:05
The lesson from the research is not that we need fewer trees, it's that we need to become
14:09
more thoughtful about what trees we plant.
14:13
For decades, urban forestry decisions have focused on practical questions, will the trees
14:19
survive the local climate, how much water does it need, will the root damage the sidewalks,
14:26
how large will it grow, is it resistant to insects and disease?
14:31
These researchers are suggesting that cities ask another question.
14:36
What does this tree release into the atmosphere?
14:39
That doesn't mean there's one perfect species that every city should plant either.
14:45
The answer will be different depending on the climate, the local vegetation, and the
14:49
pollution that's already present in the air.
14:53
A tree that may work well in Helsinki may not be the best choice for Beijing.
14:58
A species planted in Atlanta may behave different than one growing in Phoenix.
15:04
This is urban planning becoming a lot more like atmospheric science, you have to consider
15:09
the whole system.
15:11
The researchers demonstrated what that can mean in Beijing.
15:15
They estimated that if this city gradually replaced about one-tenth of its trees with
15:21
the highest isoprene emissions, with lower emitting species, during normal tree replacement,
15:28
the total isoprene emissions could fall by almost 30%.
15:33
Thus they suggested that as trees died, became damaged, or had to be replaced, the type
15:39
of tree that took its place should be carefully considered.
15:43
In effect, they suggested that cities use atmospheric chemistry as one more factor in deciding
15:50
what gets planted next.
15:53
And there's another thing to consider, remember the recipe we discussed earlier?
15:57
Isoprene by itself does not create ozone.
16:00
You need nitrogen oxides, which are actually a larger component of the soup of smog.
16:07
And much of that NOx comes from human activity, such as cars, trucks, power plants, and industry.
16:15
That means reducing NOx emissions remains one of the most important ways cities can control
16:22
ground-level ozone and smog.
16:26
Tree selection would complement these efforts, not replace them.
16:30
And this becomes especially important as cities warm due to climate change.
16:35
Hotter weather can increase emissions from vegetation at the same time that strong sunlight
16:41
is driving that ozone chemistry.
16:45
So the urban forests of the future may need to do several jobs at once.
16:50
Yes, provide shade, handle heat, survive drought, and support wildlife, but also interact
16:57
with the atmosphere in ways that don't add to an existing pollution problem.
17:04
If you can tell, the message is more complicated than saying plant more trees.
17:09
Trees are important.
17:11
We need to continue to increase their canopy, especially in highly dense urban environments.
17:17
But just as important is the type of tree we select.
17:21
The question is not whether cities should become greener.
17:24
The question is how we design greener cities that also have cleaner air.
17:31
When we step beneath the tree on a hot summer afternoon, we note their benefits.
17:43
But this research reminds us that the tree is also part of the chemistry of the air around
17:48
it.
17:49
And that chemistry depends on the species, the temperature, the sunlight, and the pollution
17:55
already present in the city.
17:57
Planting more trees can make our cities cooler and more livable, but choosing the right
18:02
trees while continuing to reduce pollution can help make that shade cleaner too.
18:10
I'm Dr. Mac, and this has been the Climate Translation.
18:14
If you have a question about the climate you've been too afraid to ask, or if you have
18:17
a differing opinion, I want to hear from you.
18:20
I can use your viewpoints in future episodes.
18:23
You can reach me at theclimatetranslation at gmail.com.
18:28
I'll see you next time.