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.

Other experts have proposed combining bioenergy with carbon capture and storage technology, an approach nicknamed "BECCS." Under this strategy, trees would soak up carbon dioxide from the atmosphere as they grow and then be harvested for fuel. The bioenergy power plants they feed would be equipped with carbon-capturing technology to trap the emissions.

Even this approach may have its shortcomings. Multiple studies in the last year or two have indicated that it's not feasible to grow enough trees, even on plantations. In one such paper, published earlier this year in Science, the authors recommend that scientists and policymakers seriously manage their expectations about negative emissions and avoid "cavalier assumptions of future technological breakthroughs."

Developing negative emissions technology to the point that it's actually capable of meeting global climate goals is essential for temperature overshoot scenarios to be considered viable. But in their new paper, Geden and Löschel say policymakers generally "refrain from any political commitment to developing and deploying negative emissions technologies" at the scale needed for success.

And the lack of urgency may be tied to the fact that there are no clearly defined goals for when or how the technology should be deployed. What is the last year by which global temperatures should be back below a 1.5- or 2-degree threshold? How acceptable is it to overshoot either goal?

Setting these limits is essential, not just for developing negative emission technologies, but for motivating political action in the first place, Geden and Löschel say. "Otherwise, climate policymakers, and even more so other branches of governments, could easily miss the urgent need for drastic mitigation, because they are under the impression that even inadequate action will never result in political failure," they write.

Geden and Löschel point to several clear parameters for overshoot scenarios, including setting a date for temperature stabilization at the end of the century: "If targets agreed upon in 2015 cannot be met by 2100 then it should be called failure," they write. And they also say that future reports from the Intergovernmental Panel on Climate Change should exclude any overshoot scenarios associated with the 2-degree threshold, another effort at keeping global climate goals strict and their required action clearly defined.

Still, there are plenty of other concerns about overshoot scenarios, even with their limits clearly outlined. Both the 1.5- and 2-degree goals were established to avoid triggering catastrophic climate effects in the future. But Geden and Löschel point out that it's "unclear what the overshoot effects would be on issues such as sea-level rise, ice-sheet loss or thawing permafrost, and whether such impacts might be reversible when global mean surface temperature falls below the threshold again."

And they're not the only ones to caution against potential unintended consequences of overshoot scenarios. Other scientists have warned about the dangers of assuming that negative emissions will solve the climate problem.

In a paper published last year in Science, climate experts Glen Peters of the Center for International Climate and Environmental Research and Kevin Anderson of the University of Manchester in the United Kingdom warn, "If we rely on these [technologies] and they are not deployed or are unsuccessful at removing CO2 from the atmosphere at the levels assumed, society will be locked into a high-temperature pathway."

Instead, they say, policymakers should proceed with their climate mitigation efforts as though negative emissions technology will fail. Then, if it proves useful in the future, it will only add to an already aggressive global climate action plan.

The same spirit of caution is presented in this week's comment. Overshoot scenarios may be a useful way of looking at the global climate problem if their limitations are understood, the authors suggest. That could prevent global climate targets from becoming "mere benchmarks that can be crossed for extended periods of time."

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.