How Much CO2 Will the World Have to Remove from the Atmosphere?

Scientists increasingly agree that the world may need negative emissions to prevent catastrophic warming.

Scientists increasingly agree that it might be impossible to cap global temperatures at 1.5 degrees Celsius over preindustrial levels—without first overshooting it and then using technology to siphon carbon dioxide out of the atmosphere, causing temperatures to fall again.

The problem is there are no rules under the Paris climate accord, or anywhere else, for how badly the target can be missed and what techniques might be used to lower the planet's temperatures.  And that's a big weakness in the global fight against climate change, some experts argue.

"Without clearly defined constraints to overshoot, politicians cannot fail and thus cannot be held accountable for insufficient action," say climate experts Oliver Geden of the German Institute for International and Security Affairs and Andreas Löschel of the University of Münster in Germany in a comment published this week in Nature Geoscience.

In other words, global temperatures could continue to rise without any clear stopping point, and policymakers could still claim that they will be brought back within acceptable limits eventually.  Without clearly outlined rules and action plans for overshoot scenarios, there's no exact definition of what constitutes a failure to meet global climate goals—which makes it harder to plan for the type of action needed for success, they argue.

Although rarely mentioned by policymakers, "negative emissions" technology—the idea of sucking carbon dioxide out of the air to bring global temperatures down—is essentially built into the models that scientists use to explore different climate scenarios.  Meeting a 1.5-degree temperature target almost certainly relies on it, even with aggressive emissions reduction efforts starting now.  And many modeling scenarios assume that a 2-degree threshold may require negative emissions, although it might be possible to reach that goal with heavy-handed mitigation efforts.

A major problem is that the technology isn't developed enough to be useful on a global scale.  In fact, scientists disagree on exactly what methods could be used.  Some have suggested a future in which machines are used to chemically scrub carbon dioxide out of the sky.  But while some projects have demonstrated this type of technology on a small scale, it's nowhere near ready to be deployed at the levels required.

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