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September 2023 Was the Hottest On Record — and ‘Gobsmackingly Bananas’

A conversation with Zeke Hausfather, the climate scientist and lead researcher at Frontier, on why last month was so appallingly warm.

Melting ice and a graph.
Heatmap Illustration/Getty Images

Congrats! You just lived through the hottest September ever recorded.

“This month was — in my professional opinion as a climate scientist — absolutely gobsmackingly bananas,” said Zeke Hausfather, who leads research at the carbon-removal initiative Frontier.

In many parts of the world, last month saw temperatures that would not have been out of place in July. It smashed the previous record for hottest September ever by nearly 1 degree Fahrenheit (or half a degree Celsius). And it was a gobsmacking 3 degrees Fahrenheit — that is, 1.8 degrees Celsius — warmer than what would have been historically normal.

Earlier today, I called up Zeke to ask him why September saw such appalling warmth. Our conversation has been edited for length and clarity.

The chart looks crazy. Does it seem as crazy to you as it would seem to us?

It feels kind of crazy. I mean, people are going to write dissertations about 2023, and just how unusual this year has been for the climate.

Where has this year been worse?

The North Atlantic is the place that really stands out. It’s just so far above anything we’ve seen in that region — it’s hard to know what’s going on there. But we’ve also seen the warmest year to date over China. We’ve seen South America be exceptionally warm. We’ve seen some winter temperatures in Brazil that rival the hottest summer temperatures ever seen there — although it’s in the tropics, so there’s less seasonal variability there, generally. Australia’s been unusually warm.

Why has this year, in particular, been so hot?

Part of the reason that these summer charts look so crazy is that the most recent big El Niño events that we’ve had have primarily boosted winter temperatures. 1998 and 2016 both had really high December, January, and February temperatures. And we’re probably on track for that as well this year — El Nino is still growing.

But this year, we saw a very dramatic shift from a moderate La Niña — a very unusually long, “triple dip” moderate La Niña that lasted from late 2020 to the start of this year — to strong El Niño conditions over the course of a few months. And so it’s not just the transition from neutral to El Niño that affects temperatures, it’s the swap from La Niña to El Niño. And that’s been part of the story this year, and one of the reasons why you’ve seen such high temperatures this summer.

There’s a bunch of other contributing factors that we’re still in the early stages of precisely quantifying. Those include an uptick in the solar cycle that happens every 11 years — that has a small effect, 0.05 degrees Celsius maybe. There’s the phase-out of sulfate shipping fuels by 2020, which shouldn’t suddenly affect the summer of this year but which certainly has contributed to more recent warming. And that’s on top of the broad decline in forcing from aerosols — and sulfur dioxide, in particular, that’s fallen about 30% since the year 2000.

And then there’s a bit of a wild card with this Tonga volcano that erupted last year that put a huge amount of water vapor in the atmosphere. Again, most of the early modeling of that shows somewhere in the range of an increase of 0.05 to 0.08 C — a boost to warming, but not the main cause. But I think if you combine the rapid switch from La Niña to El Niño and all these smaller contributors on top of the 1.3 degrees Celsius or so of human-driven warming that we’ve had to date, you can get temperatures this extreme.

So if I understand correctly, the last big El Niño that we had — in 2016, I think — developed during the big Northern Hemisphere summer. Is that right?

It developed a little later than this one developed. But more importantly, it switched more gradually from neutral conditions to El Niño conditions. What is a bit unusual this year is just how rapidly we’ve transitioned from La Niña to El Niño.

Why would making the leap from La Niña to El Niño make the temperature leap worse than switching from neutral to El Niño?

The last three years have been slightly cooler than we would normally expect because of La Niña conditions. And so the jump we see this year is somewhat relative to what we’ve seen in previous years — if the previous summers had been much warmer, than this summer’s jump on the chart would seem less extraordinary.

I see. So part of what we’re seeing is the past three years of warming sort of getting unmasked, so to speak?

Yeah, on top of a big El Niño and those other factors.

I assume August sealed it, but 2023 will definitely be the hottest year on record, right?

It would be extremely unlikely to not be the hottest year on record. Barring an asteroid hitting the planet or maybe a Pinatubo-sized volcano erupting tomorrow, 2023 is definitely going to be the hottest year on record.

Do we have any sense of how this compares to baselines after this El Niño ends? Is this a new normal?

I think that 2024 will probably be fairly similar. The El Niño that’s evolving now will — should, actually — have its bigger effects next year. But this year has been so weird, it’s hard to say what’s going on. We do expect temperatures to fall down below 2023-2024 levels in 2025 or 2026.

One way I like to think about this is we have this long-term human-driven warming. And then on top of that, there’s plus or minus two tenths of a degree Celsius in any given year due to internal climate variability, primarily due to La Niña or El Niño. So when we have all the stars align, as we do this year, we get a peek of what the new normal is going to be a decade from now.

Robinson Meyer profile image

Robinson Meyer

Robinson is the founding executive editor of Heatmap. He was previously a staff writer at The Atlantic, where he covered climate change, energy, and technology.


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