Climate overshoot and irreversibility: Implications for the social cost of greenhouse gases and emission metrics

The Climate Rollercoaster and the Price of a Hot Day

Imagine the Earth's climate as a giant, complex rollercoaster. For a long time, scientists and policymakers have been trying to design a track that stays within a safe, comfortable zone—specifically, keeping the planet's average temperature from rising more than 1.5 degrees Celsius above where it was before the industrial age. This is the "safety rail" of the Paris Agreement. But here's the twist: the train might not be able to stop at exactly that height. It might shoot up a little higher, cresting over the safety rail, before the engineers can hit the brakes and pull it back down. This is called "overshoot."

The big question this paper tackles is about what happens if the train goes too high, even for a little while. In the real world, some things that break when the temperature gets too hot might never fully fix themselves, even if the temperature drops back down. Think of it like a soufflé: if you bake it too hot, it collapses. Even if you turn the oven down, the soufflé doesn't magically rise back up to its perfect shape. In climate science, this is called "irreversibility." If we overshoot our temperature target, some damage—like melting glaciers or lost species—might stay broken forever. This paper asks: If we know some things might stay broken, how does that change the price tag we put on polluting the air? It looks at the "Social Cost" of greenhouse gases, which is basically a way of calculating how much money the world loses for every extra ton of pollution we dump into the sky.

The Hot Ticket: Why Short-Lived Pollutants Get a Price Hike

This study, led by a team of researchers from institutions like Chalmers University of Technology and New York University, uses a sophisticated computer model (a digital version of the global economy and climate) to simulate these overshoot scenarios. They wanted to see how the "price tag" on pollution changes if we assume that some climate damage is permanent. They looked at three main types of pollutants: Carbon Dioxide (CO2), which hangs around for centuries; Methane (CH4), which is powerful but disappears in about a decade; and Contrail Cirrus, the white trails left by airplanes that act like a temporary blanket over the sky.

The researchers simulated two different "rollercoaster" tracks: a "Low Overshoot" where the temperature peaks at 1.8°C and comes back down by 2100, and a "High Overshoot" where it peaks at 2.3°C and doesn't return to the safe zone until around 2200. They then ran the numbers with different assumptions about how much damage is permanent (0%, 25%, or 50% irreversibility).

Here is the surprising twist they found: The price tag on short-lived pollutants skyrockets just before the temperature peak.

In a world where all damage is reversible (like a rubber band that snaps back), the price of polluting stays relatively steady. But in their simulations, where 50% of the damage is permanent, the cost of emitting Methane (SCM) jumps dramatically as the world approaches its hottest point. In the "Low Overshoot" scenario, the cost of a ton of Methane in 2025 jumps from about $8,344 to $11,211 if we assume half the damage is permanent. For Contrail Cirrus, the effect is even wilder: the cost can become 20 times higher than usual just a decade before the temperature peak.

Why? Think of it like a party. If you know the party is going to get messy and stay messy for a long time, you don't want to add any more trouble right before the worst part. Methane and contrails are short-lived; they burn out quickly. If you release them right before the temperature hits its maximum, they add a little extra heat right at the worst moment. Because that peak heat causes permanent damage (the broken soufflé), adding that extra heat right then is incredibly expensive. The paper suggests that in these overshoot scenarios, short-lived pollutants become much more valuable to cut right now than traditional math (like the standard 100-year Global Warming Potential) would tell us.

However, the price tag on Carbon Dioxide (SCC) doesn't change nearly as much. In 2025, the cost of a ton of CO2 only went up by about 4.6% to 7.1% in their simulations. This is because CO2 stays in the air for so long that its effect is spread out over centuries; it's already "locked in" to the system, so the timing of the peak matters less for its price tag.

The "After the Peak" Drop

The story gets even more interesting after the temperature starts to drop. Once the rollercoaster has crested and is heading down, the price tags for all these pollutants actually drop if we assume irreversibility. This sounds counterintuitive, but the logic is sound: if the damage is already permanent because of the peak temperature, then emitting a little more gas after the peak doesn't cause any new permanent damage. The "broken soufflé" is already broken. So, the marginal cost of adding more pollution goes down because the worst part of the damage is already done.

The authors are careful to note that these are results from computer simulations, not a crystal ball. They used a specific model (M-DICE) and made some simplified assumptions, like assuming a certain percentage of damage is irreversible (0%, 25%, or 50%) just to see how the numbers move. They acknowledge that in the real world, irreversibility might depend on how long the heat stays high, not just how high it gets. A brief spike might be less damaging than a long, hot plateau.

The Big Takeaway

The main lesson from this paper is that timing is everything. If we are on a path where the Earth gets hotter than we want before we cool it back down, the usual rules for valuing pollution might be wrong. Specifically, the paper suggests that we might be underestimating the cost of short-lived pollutants like Methane and airplane contrails in the years leading up to the temperature peak.

In the past, we've used standard metrics (like the 100-year Global Warming Potential) to compare different gases. But this study suggests that if we are overshooting, those old metrics might make short-lived gases look "cheaper" than they really are. As we get closer to the peak temperature, the relative value of cutting Methane and contrails rises sharply. It's a "peak-shaving" strategy: if we can cut these short-lived pollutants right before the temperature hits its max, we might prevent some of that permanent, irreversible damage.

The paper doesn't say we have solved the problem or that we know exactly how much damage is permanent. Instead, it highlights a critical gap in our current thinking. It suggests that if we plan to overshoot and then cool down, we need to rethink our pricing tools to make sure we aren't ignoring the high cost of polluting right at the moment the planet is most vulnerable.