“Even after taking all plausible explanations into account, the divergence between expected and observed annual mean temperatures in 2023 remains about 0.2 °C — roughly the gap between the previous and current annual record,” he wrote in his report.

Reached by phone, Schmidt said he thinks one of three things could be going on.

It’s possible that 2023 was a “blip” — a perfect storm of natural variables and Earth cycles lining up to create one freakishly hot year. Should that prove to be the case, “it won’t have huge implications for what we’re going to see in the future, because it would have been just such a rare and unlikely thing that is not going to happen again anytime soon,” he said.

However, he indicated that’s unlikely, as those elements “have never lined up to give us a blip this large.”

Another possibility is that scientists have misunderstood the driving forces of climate change. While greenhouse gases, volcanic eruptions and aerosols are known to affect global temperatures, perhaps the full extent of their effects have been underestimated or miscalibrated. Should that be the case, he said, research and data sets will hopefully catch up soon.

The last explanation he offered is that the system itself is changing — and changing in ways that are faster and less predictable than previously understood.

“That would be worrying because science is really all about taking information from the past, looking at what’s going on, and making predictions about the future,” Schmidt said. “If we can’t really trust the past, then we have no idea what’s going to happen.”

Not everyone agrees with his assessment, however. Michael Mann, Presidential Distinguished Professor in the Department of Earth and Environmental Science at the University of Pennsylvania, said the premise that 2023’s warmth cannot be explained — or that it is inconsistent with model simulations — is “simply wrong.”

“The situation is extremely similar to what we saw during the 2014-2016 period as we transitioned from several years of La Niña conditions to a major El Niño event, and then back to La Niña,” Mann said in an email.

In fact, he said some recent modeling shows the global temperature spike in 2016 was even more of an outlier than that of 2023.

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