How Hot Was It Last Year?

It is notoriously tricky to synthesize data from all the world’s weather stations into one measure of global warming, but all of the major efforts to do so for 2023 are now in, and they are also all in distressing alignment: Last year was the warmest recorded in modern history, and it broke that record by an exceedingly large margin, one that conventional climate science has not yet managed to adequately explain.

The big data sets are now all in, and one of them, published by Berkeley Earth last week, contained what counts as an eye-popping assertion even against the backdrop of the record-setting year: The global average temperature was more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above the preindustrial level. (Other models had it just below.)

When climate scientists and advocates talk about the risks of breaching 1.5 degrees Celsius of warming — as they have somewhat obsessively at least since it was established as the ambitious climate goal of the 2015 Paris Agreement and since the Intergovernmental Panel on Climate Change described the consequences of exceeding it in its 2018 Special Report on Global Warming of 1.5°C — this isn’t exactly what they mean. That threshold describes a long-term average rather than a single-year anomaly. But because it describes a multidecade average, the measure will always be backward-looking, with the precise moment the world crossed the 1.5 mark clear only in retrospect. This year a handful of prominent scientists have suggested that when we do look back to mark that time, we may well circle 2023.

Not that long ago, it would have been pretty contentious to suggest that the world’s most ambitious climate goal was already lost. But it has been quite a while since climate activists found themselves leading large rallies in chants of “1.5 to stay alive,” perhaps in part because it has been quite a while since many of them believed achieving it was possible. For several years, the 1.5-degree goal has been sitting in a strange state of limbo — privately acknowledged as practically out of reach and yet publicly serving as the basis for nearly all global debate about the state of the climate and the global pace of decarbonization. Perhaps 2024 is the year we may finally be ready to retire it publicly, too.

Does it matter? In some corners, discarding the 1.5-degree goal looks like a possibly welcome development, since the target was in many ways flawed to begin with. A global average temperature rise may be a useful shorthand measure of the state of play, but it probably isn’t the best guide to climate disruptions, since temperatures and expected effects vary significantly from place to place. Global average temperature is also not so tightly linked to human activity, since a certain amount of emissions could produce a relatively wide range of warming levels. (That’s why our uncertainty about climate sensitivity is worrying.)

To some, the target was somewhat arbitrary to begin with, reflecting some earlier calculations about climate safety that imply much lower thresholds of concern. The base line historical period was not exactly a climate paradise; it featured many of the modern world’s most devastating disasters and famines — a reminder that temperature is not the sole determinant of human fate or suffering. And given the state of even the most generous estimates of global carbon budgets, it may have been already close to impossible to achieve when the target was formally enshrined in the Paris Agreement of 2015. Global emissions have only grown since, making the path to 1.5 degrees so treacherously steep, it’s not really a path but a crash. To give ourselves a two-thirds chance without relying on negative emissions would now require getting from 40 billion tons of annual carbon dioxide emissions to zero by 2030, which is one reason that many climate advocates will tell you that we should move on to more realistic targets, which might offer those working to build a climate-resilient world through adaptation a more useful set of expectations.