Carbon Externalities: The Invoice Nobody Signed

What really happens between the gas pump and the sky...

You pull up to the pump on a Friday afternoon. About 60 litres, maybe a hundred dollars, tank full for a weekend trip. The fuel station gets paid. The government gets its excise tax. The driller got paid weeks ago, the country of origin got its share, the refinery got paid to turn crude into something your engine can use. Everyone got paid. Simple transaction, done in five minutes, and you're back on the road.

Except one thing never got paid for, because nobody has ever figured out how to pay an externality.

Six Hours to Undo 150 Million Years

That fuel in your tank didn't start as fuel. It started as a forest, an ancient one, long before there were roads or engines or people to fill either. Over millions of years, that buried organic material was slowly cooked and pressurized by the planet itself until it became crude oil, and then it sat there. Untouched. Stored. Depending on the basin it came from, that oil has been sitting in the ground for somewhere between roughly fifty million and three hundred million years, in some fields even longer. Mother Earth held onto that carbon, sealed away from the sky, for a span of time that makes human history look like a rounding error.

Your road trip starts Saturday morning. By two in the afternoon, six hours in, the tank is empty. In those six hours, an amount of carbon that took the planet on the order of a hundred million years or more to lock away came back out, all at once, into the air. Geologically speaking, that release happened instantly. Not slowly. Not carefully. Instantly.

Here's where the numbers get stranger, not simpler. About 60 litres of gasoline weighs roughly 45 kilograms. But when your engine burns it, it doesn't just disappear as smoke, it chemically combines with oxygen pulled straight out of the air around you, and what comes out the tailpipe is roughly 140 kilograms of carbon dioxide, about three times heavier than the fuel you started with. That isn't a typo and it isn't your engine being wasteful. It's just chemistry: burning carbon grabs oxygen out of the atmosphere and turns it into gas, and that gas has to go somewhere.

Now scale that one tank up to everyone. Since 1960, humanity has pulled something on the order of 215 trillion litres of oil out of the ground, and burned the better part of it as fuel. Run that math forward and you land on a number like four hundred and fifty billion metric tonnes of carbon dioxide, released into the air by ordinary people doing exactly what you did on Friday afternoon: filling up, and driving off, without a second thought.

There's a reason burning that fuel releases energy at all, and it's a simpler answer than most people expect. We'll come back to it once we've followed the carbon the rest of the way.

Instant to burn. Thousands of years to clear.

The Bill Nobody Sent

Here is the part that doesn't show up on the receipt. The hundred dollars you paid at the pump covered extraction, transport, refining, taxes, and profit for everyone who touched that barrel of oil on its way to your tank. It did not cover the actual cost of what happens when carbon that took a hundred million years to lock away gets put back into the sky in an afternoon.

Economists have a name for a real cost that nobody actually pays: an externality. It's a cost that exists, whether or not it shows up on anyone's invoice, and it gets paid eventually, just not by the person who benefited. Carbon externalities are real climate costs created by fossil fuel use, but excluded from the price paid at the pump.

In this case, the bill doesn't go to the driver, or the gas station, or even the oil company. It goes to the atmosphere, to the forests, to the oceans, to whatever is left to absorb what nobody priced in the first place. For as long as we've been pulling carbon out of the ground faster than the planet can put it back, that invoice has been quietly accumulating, unsigned, unpaid, and largely unnoticed by the people generating it one tank at a time.

Where Does It All Go?

Here's a way to picture just how little room that carbon actually has to go. Take a regular basketball and wrap it in a single layer of plastic kitchen wrap. Proportionally, that thin plastic layer is thicker, relative to the ball, than our entire atmosphere is relative to the Earth. Everything that keeps every living thing alive, all the weather, all the air we breathe, sits in a layer barely thicker than the apple's skin on an apple. And into that impossibly thin layer, your car alone just added 140 kilograms of new gas in a single afternoon.

And it doesn't leave quickly. Roughly half of the CO2 released today will be pulled back out within a few decades, absorbed by oceans and land. But a real share of it lingers far longer, a portion still measurably in the air after several centuries, and a smaller portion for over a thousand years, because at that point it's not forests doing the work anymore. It's the slow churn of ocean chemistry and rock weathering, grinding the planet's whole carbon budget back into balance. Combustion happens in a breath. Full recovery happens on a clock most of us will never see turn over even once.

So where does it actually go once it's up there? Mostly, it doesn't just float forever. Some of it gets pulled back down, slowly, by two enormous living systems that have been doing this job since long before anyone burned a drop of oil: the forests on land, and the oceans that cover most of the planet.

The Forest's Side of the Story

Picture a forest as something like a sponge for carbon. While it's alive and growing, it's constantly pulling carbon dioxide out of the air and locking it into wood, roots, and soil. Cut that forest down, and the sponge gets wrung out almost instantly. A new forest planted in its place will eventually start soaking carbon back up again, but slowly, over decades, sometimes the better part of a century, depending on the species and the region.

Here is the part that, once you notice it, you won't be able to un-notice. Drive past a hillside of planted forest almost anywhere, and from the road it reads as recovered. Green, closed canopy, mission accomplished. Walk into that same stand and the story changes. The trees are often thin, evenly spaced, a single layer deep, with dead lower branches and almost nothing growing beneath them, no understory, no young trees coming up behind, sometimes barely a shrub. Stand next to one of the original stumps left over from the harvest and the difference in girth alone tells you what the forty or fifty years since have and haven't accomplished. That hillside has been quietly giving off more carbon than it has taken in for a very long time, and it will keep doing that for years yet, long after it started looking, from a moving car, like the debt was already paid. Picture the 'Middle' sponge below. That's roughly where a stand like this sits, well past the moment of harvest, genuinely further along than it was, and still owing far more than it's repaid.

Some ground, once cut, struggles to become a real forest again at all, not because nobody replanted it, but because the site itself, the exposure, the soil, a shifting climate, works against it. That's not a rare exception. It's common enough that anyone who spends real time on the ground, rather than reading the number from a spreadsheet, ends up seeing it. Once you know to look for the gap between how a hillside looks from the highway and what's actually happening under the canopy, you start seeing that gap everywhere.

A concept illustration, not a measurement of any specific stand. What it shows is simply the shape of the problem: what leaves fast does not come back fast, and for a long stretch of that timeline, the debt is still open.

Instant to burn. Thousands of years to clear.

The Ocean's Side of the Story

The ocean is doing a version of the same job, at a much larger scale. Roughly a quarter of all the carbon dioxide people put into the air ends up absorbed by the ocean, partly taken up directly by seawater, partly pulled down by phytoplankton, tiny floating plants that photosynthesize just like a tree does, only by the trillions, across every ocean on Earth.

But the ocean absorbing more carbon doesn't come free either. As seawater takes in more carbon dioxide, it becomes slightly more acidic, a shift that makes it harder for corals, shellfish, and countless small organisms to build the shells and skeletons they need to survive, organisms that sit at the base of ocean food webs most sea life depends on. Think of a warm can of soda losing its fizz almost the moment you crack it open, while a cold one holds its bubbles far longer. The same basic physics is at work in the ocean: warmer water simply can't hold as much dissolved gas as cold water. So as the oceans warm, their capacity to keep absorbing our excess starts to shrink, right as we're asking more of it than ever.

Different medium, same story as the forest: a living system doing real, valuable work, under a load that's growing faster than its capacity to keep up.

The hundred dollars paid for the fuel. It never paid for putting the carbon back.

Back at the Pump

Here's the answer we set aside earlier. There's a wonderful piece of explaining, done decades ago by the physicist Richard Feynman, about why a fire gives off heat and light at all. He pointed out that a tree is mostly built out of thin air, carbon pulled straight out of carbon dioxide, and that it takes real energy to pry that carbon loose from the oxygen it's bonded to. That energy comes from sunlight. The tree spends a summer's worth of sunlight doing the work of separating carbon from oxygen, keeps the carbon to build its wood, and breathes the leftover oxygen back out. So every log in a woodpile is really a small, patient store of sunlight, saved up over however many years that tree stood in a field.

Burn the log, and the whole process runs backward in seconds. The carbon reunites with oxygen, and the sunlight that was spent separating them comes back out, all at once, as heat and light. The fire in a fireplace is, quite literally, old sunshine, arriving late.

Here's a way to actually see that for yourself. Burn a log all the way down to ash, and weigh what's left. It's a small fraction of the log's original weight, often less than five percent. Everything else didn't vanish. It went back into the air, mostly as the same carbon dioxide and water vapor the tree spent years pulling out of the sky in the first place. The tree was air, borrowed for a while. The fire is just the air being returned. That's not a metaphor, it's a fact you can put on a kitchen scale.

Oil is the exact same story, just told on a much longer clock. It's the same carbon, once part of ancient forests, once separated from oxygen by sunlight falling on leaves that no living person has ever seen. That sunlight has been sitting patiently in the ground, in some cases for a hundred million years or more, waiting. When you turned the key on Saturday morning, you weren't just burning fuel. You were finally letting a hundred million years of sunlight out, all in one six-hour trip.

So you fill up on Friday, and by Saturday afternoon, you've quietly asked a forest somewhere and an ocean somewhere else to start absorbing carbon that took the planet a hundred million years or more to put away safely. Nobody planned it that way. Nobody at the pump is thinking about paleo basins or phytoplankton. It's just an ordinary errand, done by millions of people, every single day, for decades.

Instant to burn. Thousands of years to clear.

None of this means the answer is guilt, or giving up the car, or never taking the weekend trip. It means the choice is bigger than it feels in the five minutes it takes to fill a tank, and once you can see the size of it, it becomes possible to choose to offset it, in whatever way actually fits your life. Planting trees, and giving them the decades they need rather than counting them the day they go in the ground. Driving a little less, or combining trips instead of taking three separate ones. Buying less, keeping things longer, asking what a purchase actually costs the planet before it costs you at the register.

Nobody can single-handedly repay a debt that took the whole world a century and a half to build. But every one of those choices is a small, real payment against an invoice that has been sitting unpaid for a very long time. Mother Earth kept her side of the deal for a hundred million years before anyone showed up to ask for it. The least we can do is start paying attention to what it actually costs to spend it.

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About the Author

John Profitt, B.Sc., P.Eng., GSC, LEED AP® is Principal of Nextlevel Modular Inc., a BC-licensed residential builder and USGBC member company operating from a repurposed shipping container and timber frame home in Vinsulla, British Columbia, Canada. He holds individual membership in the Canada Green Building Council and is registered as a Professional Engineer with Engineers and Geoscientists of British Columbia. He brings nearly four decades of construction and geological engineering experience across North and South America, including field work with Indigenous communities across Canada and Mexico.

AI Transparency Disclosure: This paper was prepared with AI writing assistance from Anthropic Claude (claude.ai). All observations, data collection, evidentiary interpretation, primary arguments, analytical conclusions, and professional judgments are the sole work of John Profitt, B.Sc., P.Eng. AI assistance was used for drafting, editing, and structural organisation of the written text. The author takes full professional responsibility for the content of this paper.