“The plot shows that the surface warming of the planet is proceeding almost precisely as predicted,” Mann said. “And the models show that the warming will continue apace as long as we continue to burn fossil fuels and generate carbon pollution.”

When asked about this interpretation, Schmidt said it’s true that the 2014 to 2016 period was similarly anomalous. But there is a key difference between then and now, he said.

The 2016 temperature spike came on the heels of an El Niño event, with the biggest anomalies in February, March and April of the year following its peak, he said. He noted that similar patterns occurred after previous El Niños in 1998 and 1942.

Conversely, last year’s spike arrived in August, September, October and November — before the peak of El Niño — “and that has never happened before,” Schmidt said. “It never happened in the temperature record that we have. It doesn’t happen in the climate models.”

Alex Hall, a professor of atmospheric and oceanic sciences at UCLA, said he largely agrees with Schmidt’s assessment that the hypothesized factors alone can’t account for the large temperature anomaly experienced in 2023 and early 2024. He likened it to the emergence of megafires, or extreme wildfires, in the last decade, which wasn’t entirely foreseen.

“What we’ve learned is that there’s an aspect of this that isn’t fully predictable — that we don’t fully understand — and that we are tempting fate here a little bit by continuing to interfere with the climate system,” Hall said. “It’s going to do things that we don’t understand, that we don’t anticipate, and those are going to have potentially big impacts.”

Hall said the rapid transition from a persistent La Niña to a strong El Niño last summer likely played a role, as did the change in aerosol regulations.

He also posited that the rapid loss of Antarctic sea ice in 2023 — itself an outcome of the warmer planet and oceans — could have created a kind of feedback loop that contributed to more warming. Ice and snow are reflective, so when they melt, it can result in a darker ocean that absorbs more heat and sunlight. (Antarctic sea ice coverage dropped to a record low in 2023, according to the National Oceanic and Atmospheric Administration.)

“It’s sort of a planetary emergency for us to figure out what’s going on when we see these types of changes,” Hall said. “There should be large teams of people working on it to try to understand it, and we don’t really have those kinds of efforts, so I think there’s lessons, too, for the need for focus on this particular topic.”

While he and other scientists may not agree on just how extraordinary 2023 was — or what was behind its exceptional warmth — they all acknowledged the clear signs of a planet being pushed to its limits.

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