Keeping the global average temperature rise (since pre-industrial times) below 1.5C is widely regarded as critical if we are to sidestep dangerous, all-pervasive climate change. This idea of a 1.5C temperature threshold is in the news again because just-published research has revealed that several catastrophic climate tipping points are in danger of being crossed at around this level of warming, including collapse of the Greenland and west Antarctic ice sheets, which would lock in about 12 metres of sea-level rise. To have a fair chance of keeping this side of 1.5C, emissions have to fall by 45% in little more than 90 months, and I am on record as saying that this is practically impossible. But it’s worse than that. It is perfectly feasible that we will crash through the 1.5C guardrail even earlier. The UK Met Office, for one, forecast in 2021 that there was at least a 40% chance that 1.5C would be breached temporarily at least once in the following five years. This means the average temperature would be above 1.5C one year, but likely return below it the next – and we will fluctuate around that number before crossing it permanently some time in the future. In both 2016 and 2020, the Earth was 1.36C hotter than during pre-industrial times, so we are already getting disturbingly close. The development of El Niño conditions in the Pacific Ocean in the next year or two, which typically ramps up the level of global heat, could well provide the final push that breaches the threshold. The idea of breaching “temporarily” opens a whole can of worms. Does one year of 1.5C mean we have breached the barrier or not? How many years of 1.5C or more of heating does there need to be before we have officially crashed through the guardrail? And how critical would this really be, anyway, in terms of real-world consequences?? Maybe we are too fixated with this precise temperature rise. The fact is, while not exactly picked out of a hat, the 1.5C figure is an arbitrary one. The exact level of temperature rise at which climate change becomes dangerous is simply not known. Indeed, the 33 million people displaced from their homes in Pakistan might justifiably say we have reached it already. As for tipping points, any or all of those flagged in the new research could happen at some point below 1.5C, so we may have crossed one or more already – only time will tell. Just as easily, we might need a 1.6C, 1.7C or even higher rise before the first runaway impacts of global heating are encountered. The key point, then, is not the precise value of the global average temperature rise, but the simple fact that it is continuing to rise. The climate system is so sensitive to additional heating that every fraction of a degree rise counts, so that every 0.1C rise is just as important as every other. Global heating is now translating into extreme weather rapidly: there has been a huge hike in these events over the last few years, during which time the global average temperature climbed by one- or two-tenths of a degree at most. The bottom line is that 1.5C is not sacred. Whether we crash through it or – by some miracle – stay below it, we cannot be certain what the consequences will be. The number has been a useful metric in the global heating story, marking a somewhat concrete focal point. But we mustn’t become obsessed with a single target figure. On the contrary, we need to knuckle down as much as we can to prevent every 0.1C rise, both below this figure and above, in order to rein in climate breakdown as best we can. You never know, we might just get lucky. Bill McGuire is professor emeritus of geophysical and climate hazards at UCL, and the author of Hothouse Earth: an Inhabitant’s Guide