This is going to sound really stupid or as a joke, but... Ever since I was a little kid, I have been confused about the following:

  1. Humans need air/oxygen to breathe in order to live.
  2. Plants/trees/bushes/flowers produce this oxygen, and are thus crucial for humans to survive.
  3. Plants/trees/bushes/flowers need "carbon dioxide" to survive, just like humans need oxygen.
  4. Plants/trees/bushes/flowers MAY also need some oxygen/air? (I assume so.)
  5. Cars (traditional ones) output "carbon dioxide"... yet they are... not good for nature?

I don't understand how this fits together. It seems as if nature should absolutely thrive in the middle of highways and nearby, if they get so much nice carbon dioxide from the cars! Yet cars are instinctively seen (also by me) as "evil nature-polluting machines of death". Car exhaust seems to be the opposite of what a plant or tree wants, yet it's full of carbon dioxide, so... why?

Can somebody explain this once and for all?


7 Answers 7


There are at least two separate answers to your question. First, with respect to plants needing CO2, they have evolved to deal with the limited amounts of CO2 normally in the atmosphere. That's really all they need, or "want": adding more doesn't really benefit them. Think of it this way: you need water to live, right? And drink a certain amount of it every day. But drinking large amounts of water can kill you https://en.wikipedia.org/wiki/Water_intoxication and holding you under water for a few minutes certainly will. (As in the comment above, and even better example would be oxygen: we need it to live, but too much can cause all sorts of problems: https://en.wikipedia.org/wiki/Oxygen_toxicity ) Likewise, plants need nitrates, but applying too much can kill them. So extra CO2 in the air is of little or no benefit to plants.

The second answer has nothing to do with plants. It's that atmospheric CO2 traps solar heat, causing the Earth to be warmer than it otherwise would. Without the extra CO2 from fossil fuels, Earth maintained a comfortable temperature, one that we'd all (and I mean not just humans, but all life) had gotten used to. Add more CO2, and more solar heat is trapped, making the Earth uncomfortably warm, to the point that many plants & animals can't live where they are, and generally can't migrate. So you wind up with a lot of dead plants and animals.

Of course there's more going on than this, things that are feedbacks from and side effects of the increased CO2, such as ocean acidification. If you really want to know more, here's a good starting place: https://history.aip.org/climate/index.htm

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    $\begingroup$ Where is your source for extra CO2 being of no benefit? Here's NASA saying the opposite: climate.nasa.gov/news/2436/co2-is-making-earth-greenerfor-now $\endgroup$ Commented Feb 23, 2020 at 18:40
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    $\begingroup$ @Wayne Conrad: Because there is abundant practical evidence - artificial (or organic) fertilizers, irrigation, &c - that plant growth in natural conditions is seldom limited by CO2. Even when those other factors aren't limiting growth, predicting long-term response isn't a simple matter, e.g. science.sciencemag.org/content/360/6386/317 or newscientist.com/article/… $\endgroup$
    – jamesqf
    Commented Feb 24, 2020 at 19:57
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    $\begingroup$ @jamesqf It would be helpful if you included that supplementing information into your answer, and maybe add some appropriate caveats as theforestecologist suggests. $\endgroup$
    – Bryan Krause
    Commented Feb 25, 2020 at 16:08
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Bryan Krause
    Commented Feb 25, 2020 at 19:40
  • $\begingroup$ @Wayne Conrad What about some "extending", not limiting factor: and does that mean more plants, not just larger ones? $\endgroup$ Commented Jan 8, 2021 at 15:44

Exhaust gasses from combustion contain many compounds in addition to CO2.

Some of them under the right circumstances can be of limited benefit to some plants, but others such as ozone are damaging to all forms of life.

Other compounds in vehicle exhaust that are known to cause damage to plants are oxides of sulfur and nitrogen — these react with water to form acids and contribute to "acid rain" that lowers soil pH thereby interfering with nutrient uptake and may also directly damage plants1.

Finally, as noted in other answers increasing levels of atmospheric CO2 due in part to vehicle exhaust contributes to climate change that has a global consequences for plants — this however seems beyond the scope of the original question which appears to be about local effects on trees growing "in the middle of highways and nearby".

Note: For example, increased CO2 can be growth promoting at least in the short term.


1: Oswalt, C. M., & Clatterbuck, W. K. (2005). Impacts of Air Pollution on the Urban Forest. University of Tennessee, Agricultural Extension Service.


Firstly, I don't think it's entirely clear whether the entire premise of the question is correct. I couldn't find any evidence to suggest that plants don't thrive on highways and areas with lots of cars. If anyone has any data on this, I'd be interested in reading it.

Some of the answers here seem to be a bit light on evidence. Let's look at some data on this.

This report shows that there can be around 1.5-2x increase (from the baseline level of around 410ppm to between 600-800ppm) of CO2 levels in areas surrounding highways. This value varies somewhat depending on whether it is peak time or not, but it seems like the median amount is around 600ppm. I'm aware this is only a single study, so it's perhaps not best to generalise too much, but it should give us a broad idea of how much CO2 increase to expect in areas surrounding highways.

In contrast to what jamesqf said, CO2 enrichment does increase the growth rate of plants, under certain conditions. This has been known for quite a while. Mauney et al (1987) state:

Comprehensive reviews of the plant science literature indicate that a 300 part per million (ppm) increase in atmospheric carbon dioxide (CO2) concentration generally increases plant growth by approximately 30%.

Importantly, the range of increase that you see in areas surrounding highways certainly increases the growth rate of plants. Ainsworth & Rogers (2007) state:

Across a range of FACE experiments, with a variety of plant species, growth of plants at elevated CO2 concentrations of 475–600 ppm increases leaf photosynthetic rates by an average of 40%

Furthermore, a recent study showed that the human-caused increase in CO2 levels has actually caused a relative greening of the worlds global vegetated area. So this isn't just an effect of ideal conditions in FACE experiments, it also occurs 'in the wild'.

However, it's not quite as simple as increased CO2 -> increased growth. As we can see from the figure above, the levels of CO2 in the highway vary temporally between peak and non-peak times. Fluctuations in CO2 levels actually reduce the growth rate of plants, for various reasons. White et al (2020) state:

..we calculated that yields increased 65% as much in fluctuating elevated CO2 of FACE as in constant elevated CO2

So it certainly seems like this might have an effect of reducing plant growth on highway areas.

As other people have alluded to, the other thing to remember is that cars produce a high level of other particulate from the exhaust fumes, which certainly have a large negative impact on the growth of plants. Power et al (2011) state:

A wide range of effects were detected, including growth stimulation and inhibition, changes in gas exchange and premature leaf senescence. This was complemented by controlled fumigations with NO, NO(2) and their mixture, as well as a transect study away from a busy inner London road. All evidence suggested that NO(x) was the key phytotoxic component of exhaust emissions, and highlights the potential for detrimental effects of vehicle emissions on urban ecosystems.

Overall, it seems likely that a combination of fluctuating CO2 levels (rather than constantly high levels) and particulate pollution might cause a reduction in plant growth around highways.


Back in the early days of climate science, there were some scientists who considered the possibility that increased CO2 production would lead to increased plant growth, and that this in turn would result in keeping the oxygen-carbon dioxide balance more or less stable. This was referred to as the "Gaia hypothesis" and basically revolved around viewing the entire ecosphere as a "living organism" that would maintain itself.

To an extent, this does happen. Increased CO2, by itself, may result in increased plant growth. So may increased temperature resulting from the increased CO2 trapping the sun's heat (the greenhouse effect).

The problem is that we're producing way too much of it, way too fast. Even if the atmosphere and climate are becoming more favorable for plant growth, plants can only grow so fast - and it's not fast enough to stop carbon buildup from increasing the planet's temperature to levels that may make things difficult for existing forms of life (including humans).

Also, we're destroying plants, both through direct deforestation and indirect desertification, faster than they can grow even with the more favorable conditions, which doesn't help matters.

So you're not wrong - carbon dioxide production may increase plant growth. But it also causes problems, and plants can only mitigate those problems - and they can't mitigate them fast enough.

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    – tyersome
    Commented Feb 23, 2020 at 18:31

Ignoring for a moment the other pollutants emitted from car exhaust, increased CO2 levels are warming the planet, through what is called the "greenhouse effect".

Photosynthesis is a set of reactions where CO2, water and sunlight are turned into chemical energy used for plant life.

However, these reactions only work well in a narrow temperature range.

As plants heat up past a certain point, they can't run some photosynthetic reactions as well, or at all.

As the atmosphere heats past a safe range, plants must divert resources to making so-called "heat shock proteins" or HSPs to protect photosynthetic machinery.

If the atmosphere gets too hot, plants suffer irreversible damage, can no longer generate energy to live, and therefore die.


  • $\begingroup$ You really have a good point, maybe you want to check this paper bsapubs.onlinelibrary.wiley.com/doi/full/10.3732/ajb.95.2.165 It looks like C3 plants could thrive in the near future, although C4 species could have an advantage during years with water stress.But temperature wise, in a high CO2 environment C3 plants perform better $\endgroup$
    – francesco
    Commented Feb 25, 2020 at 9:25

It seems as if nature should absolutely thrive in the middle of highways and nearby, if they get so much nice carbon dioxide from the cars!

In fact, some species of plants are likely to thrive from the excess pollution coming from cars, provided they can tolerate the hotter summers and grow quickly enough to take advantage of the higher CO2 levels. Unfortunately, many of the plants people care about are not going to be the ones that benefit most.

Yet cars are instinctively seen (also by me) as "evil nature-polluting machines of death".

"Nature" colloquially refers to "plants and animals as they are currently". Pollution doesn't destroy nature, but it does change it, which people tend to perceive as bad because we are prepared for the world around us being a certain way. For example, more Co2 is likely to be very good for cockroaches and mosquitoes (which thrive in warmer weather), so some of the plants and animals that currently do well will be exchanged for more bugs. People who have to deal with more roaches and mosquitoes will probably perceive this as bad, even if those insects are still part of nature.

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    $\begingroup$ Excellent point about "change" -- the long time scale perspective suggest "change" and not "destruction" its occurring. However, you need to provide citations or other forms of support for the basics points made in your response. Thanks $\endgroup$ Commented Feb 24, 2020 at 3:57
  • $\begingroup$ 'Destruction' is only the name we give to rapid changes $\endgroup$
    – francesco
    Commented Feb 25, 2020 at 9:31

Anthropogenic Climate Change is why carbon dioxide, from fossil fuel powered transport in combination with other fossil fuel uses is considered problematic - i.e. "bad".

With respect to plant health insufficient carbon dioxide is not a problem; even with some plants getting some benefit from raised carbon dioxide concentrations those benefits are outweighed by the harms and costs expected to accrue from climate consequences of profligate fossil fuel burning.

About the climate consequences the US National Academy of Sciences says -

Climate change is increasingly affecting people’s lives. It is having significant effects on infrastructure, agriculture, fisheries, public health, and the ecosystems that support society. It is also changing the environment in ways that affect the distribution, diversity, and long-term survival of species of plants, animals, and other forms of life on Earth.


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