The Hottest Day in 125,000 Years?

Monday was likely the hottest day on Earth since modern recordkeeping began. On that day, the planet was 17.16 degrees Celsius, or 62.89 degrees Fahrenheit, on average, according to the European climate service Copernicus, narrowly beating out the previous record, set just the day before, by about 0.1 degrees. That news, like previous records of its kind, was quickly characterized as the hottest day in millennia—since the peak of the last interglacial period, about 125,000 years ago. That claim is true, in a way: Ice cores and sediment cores can tell us with remarkable precision what the world was like back then—a very hot and radically different place. Hippos lived in the British Isles, and the seas were 20 to 30 feet higher than they are today.

Comparing July 22, 2024, to the peak of a prehistoric hot era isn’t quite fair: Those ancient temperatures, deciphered through sampling layers of ancient ice or soil, are at best one-year averages, not one-day averages, like that of the record-breaking days this week. But average annual temperatures are rising fast too, and are approaching, if not yet surpassing, those ancient highs. For example, the world was about 2.45 degrees Fahrenheit warmer in 2023 than the average of the late 19th century, which is used as a benchmark for the preindustrial climate. The long-term average temperature during the last interglacial period was something like 2.7 degrees Fahrenheit warmer than the 19th-century benchmark. We are not yet living in the world of 125,000 years ago. We have simply, ominously, visited it.

Jacquelyn Gill, a paleoecology professor at the University of Maine, thinks of these visits as “dipping your toe” into that ancient climate. If temperatures held at this level for a week or two each year, we’d be ankle-deep. But we could be fully immersed in that climate soon. If the long-term temperature averages—say, over decades—begin to resemble the current short-term ones, we’ll have succeeded in traveling back to the interglacial period from a climate perspective. “We have a couple of degrees to go,” Gill told me. “But we’re certainly on track. That’s where we’re headed by the end of the century.”

Gill specializes in that precise time period, the warm era between the last two glacial periods when the seas rose by some estimates an average of eight feet a century, submerging large areas of land. Studying that time is a useful window into what may lie ahead for us. Arctic summers were probably ice-free, and were four to five degrees Celsius warmer than they are today. There’s evidence that although some ice likely persisted year-round in Antarctica, the rapid melting of the Antarctic ice sheet likely played a major or even starring role in the oceans’ rise at the time. This detail is especially worrying, given that the Copernicus analysis pointed to an unusually warm Antarctic winter as one of the main factors that pushed the global temperature into record-breaking territory earlier this week. Antarctic temperatures have been as much as 18 degrees Fahrenheit warmer than normal this winter. Scientists still don’t fully understand Antarctic sea-ice dynamics, making insights about how ice sheets reacted the last time the world warmed this much particularly useful.

The climate 125,000 years ago was very unstable, Gill said. The hotter air temperatures may have fueled stronger storms, including hurricanes, a parallel to the rapidly intensifying hurricanes the planet is experiencing now. Some models of that ancient period suggest that, because warmer air holds more moisture, the Asian monsoon was more intense, echoing the rainfall increase projected for the modern monsoon season in the nearer future. (Other studies of that period suggest a weakened interglacial monsoon, which goes to show how hard it can be to look back that far.) But by all accounts, in our modern climate, the relationship between heat and rain is clear: As the world warms, wet places will get wetter, possibly as they did the last time the world warmed like this.

Forests at the time extended well into the Arctic circle. As the tree line moved northward, so did animal species, according to the fossil record. The Neanderthals, our closely related human relatives, followed. They “started going further north than they had been hanging out previously, tracking resources up,” Gill said. Northward migration of both humans and other species is already a foregone conclusion of climate change today. But every organism living now is dealing with temperature change that’s likely happening far faster than creatures alive back then did. Whereas the world warmed over thousands of years in the last interglacial period, human activity has warmed the planet rapidly in just the past 150 years or so. Many species simply cannot move toward newly more habitable places fast enough to survive.

One might look at the last interglacial period as an indication of the naturalness of climate change. Indeed, the planet is no stranger to major temperature swings. "The Earth’s history is full of these shake-ups, these abrupt events,” Gill said. Yet our current period of rapid change is distinctly unnatural. “We’re taking over as the dominant force in Earth’s climate system,” Gill said.

While Neanderthals certainly had no say in the chaotic and overheated climate they contended with, modern humans are in no such bind. If humanity ceased to burn fossil fuels and emit greenhouse gases, the planet would, within a few years or decades, likely begin to cool. “We get to decide how much time travel we’re doing,” Gill said. “I don’t want to go to the interglacial. It’s fun to visit in my mind, but it’s not the planet I want to live on.”

For most people, from a climate perspective, Monday was just a typical day. None of us can feel the slippage of a global average into record-breaking territory, and even if we could, a fraction of a degree would not likely faze anyone. And yet, all the same, we are hurtling remarkably fast toward a world distinct from the one our societies developed in. Paleoclimatic data exist precisely for this moment: to show us what might be ahead, should history be allowed to repeat itself.