We’ve all seen moments and situations when a boundary was crossed, a limit was exceeded, a budget was over-run. When something like that happens, we take action to get back on the right side of things: apologies are offered, penalties are paid, use of reserves is authorized.

Sometimes, though, it’s not a simple matter of undoing an error or acquiring additional resources. If you bend a paper clip too far, trying to bend it back into shape may break it. If you run an engine with too little oil, and bearings get burned, adding oil doesn’t recondition the bearings. An overshoot may sometimes have effects that are not readily reversible – and climate may perhaps be the largest-scale realm where even a momentary excess may incur catastrophic costs.

Global warming overshoot is a topic of grave concern because “global and regional climate change and associated risks after an overshoot are different from a world that avoids it,” in words near the beginning of a research paper whose title includes the word “overconfidence.” The authors of that paper warn that “Temperature reversal could be undercut by strong Earth-system feedbacks resulting in high near-term and continuous long-term warming” – in shorter words, once this gets away from us, it gets worse more quickly and requires ever more drastic measures to contain (let alone to return things to a tolerable state).

Unfortunately, we find at least a whisper of wishful thinking, a notion of “we can hit the Undo key and get back to a safe space,” in the “First Global Stocktake” document produced from the Conference of the Parties (Fifth Session) in regard to the Paris Agreement: “The Conference of the Parties serving as the meeting of the Parties to the Paris Agreement…emphasizes the need for urgent action and support to keep the 1.5 °C goal within reach and to address the climate crisis in this critical decade.” I boldfaced the passage, “keep the 1.5 °C goal within reach,” because it’s widely felt that “within reach” is interpreted by many as “recoverable from above” – that is, capable of being achieved by working our way back down, with added effort and/or technology improvement, if that 1.5 ºC ceiling is (most likely) exceeded within a small single-digit number of years from now.

For example, in October of last year, a paper in Annual Review of Environment and Resources warned of a trend toward “reframing of what ‘keeping 1.5°C within reach’ means: the notion that even though exceeding global warming of 1.5°C is largely inevitable by now, such exceedance would be limited in magnitude and duration, and that sustained and concerted global action can bring global warming back down to 1.5°C before the end of the twenty-first century.”

Global temperature overshoot is not considered an alarmist prediction; it’s pretty much the current consensus, as exemplified by this language in a paper appearing this past June in the journal Earth System Science Data:

Carbon-dioxide-induced warming remains the dominant contribution to the anthropogenic warming rate, and, having consistently increased over the assessed historical period, reached a new historical high over the decade 2016–2025… Based on the current assessed level and rate of warming, human-induced warming will reach 1.5 °C around the year 2030.

The notion that we can work our way back down from a peak above 1.5 ºC is dangerous, note the authors of the “overconfidence” paper cited above, because

For a range of climate impacts, there is no expectation of immediate reversibility after an overshoot. This includes changes in the deep ocean, marine biogeochemistry and species abundance, land-based biomes, carbon stocks and crop yields, but also biodiversity on land. An overshoot will also increase the probability of triggering potential Earth system tipping elements. Sea levels will continue to rise for centuries to millennia even if long-term temperatures decline.

As I noted in December, the significance of “tipping points” is among the excellent features of the board game “Daybreak,” which I continue to recommend as a collaborative exercise in making the urgencies of climate more understandable. Perhaps it should be a required icebreaker at international conferences – because as of now, “the concept of overshoot allows governments to assert that their actions remain consistent with their ambition to limit warming to 1.5°C even while that warming level is being exceeded” (in the assessment of the Annual Review of Environment and Resources paper cited above).

The notion of “hit the Undo key” that I mentioned above is roughly 60 years old, having first appeared in the “File Retrieval and Editing SyStem” (FRESS) at Brown University. Once Undo became part of the Macintosh Human Interface Guidelines in 1987, the idea quickly became pervasive – and in 1996, Thomas Landauer’s book The Trouble With Computers observed that fluent understanding of “Undo” capability was one of the greatest single contributors to huge differences in different people’s task performance times.

We are all, therefore, well imprinted with the idea that mastering (and expecting to use) “Undo” is not merely useful, but actually beneficial. When it comes to climate, though, “Don’t do” may be a far more important idea.