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When the Paris agreement on climate change was gaveled into being in December 2015, it briefly looked like that rarest of things: a political victory for climate activists and delegates from the poorest regions of the world that, due to colonization by today's wealthy nations, have contributed little to the climate crisis—but stand to suffer its worst ravages.

The world had finally agreed an upper limit for global warming. And in a move that stunned most experts, it had embraced the stretch target of 1.5°C, the boundary that small island states, acutely threatened by sea-level rise, had tirelessly pushed for years.

Or so, at least, it seemed. For soon, the ambitious Paris agreement limit turned out to be not much of a limit at all. When the Intergovernmental Panel on Climate Change (or IPCC, the world's foremost body of climate experts) lent its authority to the 1.5°C temperature target with its 2018 special report, something odd transpired.

Nearly all modeled pathways for limiting global heating to 1.5°C above pre-industrial levels involved temporarily transgressing this target. Each still arrived back at 1.5°C eventually (the deadline being the random end point of 2100), but not before first shooting past it.

Scientists responsible for modeling the response of Earth's climate to greenhouse gas emissions—primarily caused by burning fossil fuels—called these "overshoot" scenarios. They became the dominant path along which mitigating climate change was imagined to proceed, almost as soon as talk of temperature limits emerged.

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