The price that's missing

A factory makes paint on the edge of a town. It pays for everything it uses: the chemicals, the machines, the electricity, the wages of the people who run the line. Add it up and you get the cost of a tin of paint. The factory also sends smoke up its chimney, and the smoke drifts over the town. People downwind cough more. A few get sick. The factory pays none of that. Someone else does — in doctor’s bills, in worse days, in a cost that never appears on the factory’s books.

That gap, between what the factory pays and what its paint actually costs the world, has a name. And once you can see it, you can see why the market, left alone, gets fossil fuels wrong.

A cost that lands on other people

Economists call it an externality: a cost (or sometimes a benefit) of an action that lands on people other than the buyer and the seller. It sits outside the deal — hence the name.

When the factory and a customer agree on a price, they’re settling a private deal between two parties. The factory weighs its costs; the customer weighs what the paint is worth to them. They meet at a price. But the smoke wasn’t in that negotiation. The townspeople breathing it never had a seat at the table. Their cost is real, but external to the trade, so it never enters the price.

Burning fossil fuels works exactly the same way. Buy a litre of petrol and you pay for the crude oil, the refining, the trucking, the station’s margin, and a tax or two. What you don’t pay for is the carbon. Burning that litre puts carbon dioxide into the air, and that carbon dioxide warms the planet (the mechanism you met in Module 3 — heat trapped, the average shifted, the extremes pushed further). The warming brings real harm: heat, floods, damage, crops lost. That harm is a genuine cost. But it’s spread across everyone — strangers, other countries, people not yet born — and charged to no one in particular. So it stays off the price at the pump.

Why the market under-prices it

Here’s the part that matters. A market is a remarkable machine for balancing costs it can see. Every price is a signal: it tells buyers how scarce a thing is, tells sellers what’s worth making, and quietly steers millions of choices toward the cheaper option. That’s the system working as designed.

But a price can only carry the costs that got put into it. When a real cost is left off the books, the thing looks cheaper than it truly is. And when something looks cheap, people use more of it than they would if the full cost were staring back at them. Not because anyone is being foolish — they’re responding honestly to the number in front of them. The number is just wrong. It’s missing a piece.

This is the Protocol in its plainest form. The harm from carbon is connected to the fuel — it comes from burning it. But the price treats them as separate: fuel here, harm somewhere else, on someone else’s tab. Treat a connected cost as if it were separate, and the whole system tilts toward burning too much. It isn’t a moral failing of buyers. It’s a missing signal.

Putting the cost back on the books

So what do you do about a cost that’s gone missing? You try to put it back. That’s what a carbon price is: an attempt to estimate the harm from a tonne of carbon dioxide and add it to the price of emitting it — usually through a tax on carbon, or a market where the right to emit is capped and traded.

The idea is simple even if the execution isn’t. Take the hidden cost, give it a number, and bolt it onto the price where decisions get made. Now the buyer sees something closer to the true cost. And the consequence: cleaner options that looked “more expensive” can become the cheaper choice once the missing cost is counted. The ranking can flip.

Watch it flip with numbers. Two ways to power a home for a year. The fossil way costs $1,200 at the meter and emits 4 tonnes of carbon dioxide. The clean way costs $1,400 and emits almost none — about 0.2 tonnes. At a carbon price of $0, fossil wins: $1,200 beats $1,400, and people pick it. Now set a carbon price of $100 per tonne. Fossil’s hidden cost is 4 × $100 = $400, so its true cost becomes $1,200 + $400 = $1,600. Clean’s 0.2 tonnes add only $20 — still about $1,400. The clean option is now cheaper — by roughly $200. The two cross at about $53 a tonne. Nothing about the physics changed. We just stopped leaving a real cost off one side of the comparison. (The numbers are illustrative.)

An honest difficulty

Be honest about the hard part: nobody knows the exact right number. The “social cost of carbon” — the dollar value of the harm one tonne does — is genuinely hard to pin down: it depends on damages spread over a century and across the whole world. Estimates vary widely, from tens to a couple of hundred dollars per tonne. A carbon price is a best guess at a quantity we can’t measure precisely, not a dial set to a known truth.

So treat it as what it is: an attempt to correct a known blind spot, not a cure. The point isn’t that some particular price is right — it’s that zero is almost certainly wrong, the one number that leaves a real cost entirely off the books.

The whole, one more time

A price is a signal, and a signal is only as good as what it’s told to carry. Leave a real cost out, and it points everyone the wrong way — cheaply, confidently, at scale. That’s not a flaw in any one buyer or seller; it’s what happens when a connected cost is filed as someone else’s problem.

You stand inside that market every day. The petrol, the power, the price of nearly everything you touch carries a signal that, for carbon, has long been missing a piece. Seeing the gap doesn’t tell you what the price should be — that’s a hard, contested number, and worth holding loosely. It tells you something smaller and steadier: a market can only weigh what it’s handed, and we’re all choosing on numbers that don’t yet count everything they cost.