how much CO2 do American cars produce?

23 May 2025

TLDR: About $1.520 \cdot 10^{12}$ kg/year. This increases the CO$_2$ in the atmosphere by about $0.048$% per year.

Let’s gather some facts:

• The average American (16 or older) drives about 13,476 miles per year (US DoT).
• There are 265,653,749 Americans aged 16 or older (US 2020 Census).
• Finished motor gasoline releases about 18.73 pounds of CO$_2$ per gallon (US Energy Information Administration).
• New light duty vehicles (those weighing 10,000 pounds or less) get about 26.0 miles per gallon (mpg) as of 2024 (US DoE).
• Freight trucks are much, much worse, at around 5-7 mpg. (US DoE)

Assume that the weighted average car is getting 20 mpg. This includes passenger and freight. Passenger cars are higher and freight vehicles are lower.

Then:

$$\begin{align*} & (265,653,749 \text{ Americans}) \\ &\cdot (13,476 \text{ miles} / (\text{year} \cdot \text{American})) \\ &\cdot (18.73 \text{ pounds of CO$_2$} / \text{gallon of gas}) \\ &\div (20.0 \text{ miles} / \text{gallon}) \\ &= 3.352 * 10^{12} \text{ pounds/year} \\ &= 1.520 * 10^{12} \text{ kg/year} \end{align*}$$

Quick unit analysis to sanity check that equation:

$$\begin{align*} &(\text{people})\cdot(\text{miles/(people$\cdot$year)}) \\ \rightarrow &\text{miles/year} \\ &(\text{miles/year})/(\text{miles/gallon}) \\ \rightarrow &\text{gallon/year} \\ &(\text{gallon/year})\cdot(\text{pounds/gallon}) \\ \rightarrow &\text{pounds / year} \end{align*}$$

Checks out.

The atmosphere weighs about $5.15 \cdot 10^{18}$ kg (Lide, David R. Handbook of Chemistry and Physics. Boca Raton, FL: CRC, 1996: 14–17).

By mole fraction, the atmosphere is about 78.08% $N_2$, 20.95% $O_2$, 0.93% $Ar$, and 0.04% CO$_2$ (wikipedia).

Using the periodic table, one mole of each molecule weighs: $$\begin{align*} N_2 = 14.007*2 &= 28.014 g \\ O_2 = 15.999*2 &= 31.998 g \\ Ar &= 39.95 g \\ CO_2 = 12.011 + 15.999*2 &= 44.009 g \\ \end{align*}$$

The weight of one mole of atmosphere is then:

$$\begin{align*} &0.7808 \cdot 28.014 g\\ + &0.2095 \cdot 31.998 g\\ + &0.0093 \cdot 39.95 g\\ + &0.0004 \cdot 44.009 g\\ = &28.966 g \end{align*}$$

Since the atmosphere is 0.04% CO$_2$, we can compute the fractional weight of CO$_2$ in atmosphere as $44.009 g \cdot 0.0004 / 28.966 g = 0.0006077$. This number tells us what fraction of the mass of the atmosphere is CO$_2$. We established above that this number is $5.15 \cdot 10^{18}$ kg, so the weight of all the CO$_2$ in the atmosphere is therefore $3.129 \cdot 10^{15}$ kg.

We know that Americans emit $1.520 \cdot 10^{12}$ kg/year of CO$_2$. We know that the CO$_2$ in the atmosphere weighs $3.129 \cdot 10^{15} kg$. Therefore, every year, Americans increase the CO$_2$ in the atmosphere by a factor of:

$$(1.520 \cdot 10^{12}) / (3.129 \cdot 10^{15}) = 0.00048$$

or 0.048%.

$\blacksquare$

This guy used CO$_2$ ppm readings + the known mass of the atmosphere to arrive at a figure of 3,208 Gt, matching my 3,129 figure very closely. Wikipedia cites a figure of 3,341 Gt using the same ppm + total mass technique. So we’re all within a pretty tight range of each other.

That Wikipedia article also claims that we’ve only increased the CO$_2$ in the atmosphere by ~50% since the beginning of the Industrial Revolution. If so, that kinda tracks with our figures. If we assume that Americans have been emitting at the current rate (fewer but shittier cars in the past) for about 50 years, that works out to a total contribution of 2.5% just from our cars.

We know that cars are not the dominant form of CO$_2$ emissions. British Petroleum publishes an amazing, annual statistical review of global energy trends. Let’s pore over the 2022 document (link). In 2022, Americans emitted 4.701 Gt of CO$_2$ (page 12). Thus cars contributed 32.33% of our total CO$_2$ budget. In the same year, China emitted about 10.523 GT of CO$_2$ (page 12). Much of that can be seen as Americans offloading their emissions to China in the form of manufacturing. Finally, we see that the entire world’s emissions amount to about 33.884 Gt of CO$_2$ per year. American drivers are therefore responsible for about 4.485% of that budget.

If we synthesize our “2.5% of the CO2 in the air is from American drivers” number with the above figure that we’re emitting about 5% of the global budget, we get a global cumulative emission of about 50%. That also matches what Wikipedia claims: that CO2 in the atmosphere has increased by about 50% since the start of the Industrial Revolution.

So through basic analysis of public data and a couple reasonable inferences, we have arrived at the same conclusion as the “entrenched academics”: that the change in CO$_2$ in the atmosphere over the last 200 years is due to human activity.

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