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September's sizzling global average temperature ensures that 2023 will be the warmest year on record.

Why it matters: Confirmed by data from NASA, NOAA and Copernicus Climate Change Service in the E.U., the annual record is a surprise, and has only become clear in the past few months.

It may signal a continuation of a trend towards an acceleration of global warming in recent years, occurring about 40% faster during the past 15 years when compared to any other period since the 1970s.

By the numbers: September, with a temperature anomaly of 1.44°C (2.58°F) above average, was the most unusually warm month ever recorded in NOAA's 174 years of instrument records.

This beat the previous warmest September by a staggering margin of 0.46°C (0.83°F), and the prior largest temperature anomaly, which occurred in March 2016, by 0.09°C (0.16°F), NOAA concluded. (NASA pegged September as a bit warmer, at 1.47°C (2.64°F) above average.)
The most likely suspects for the astonishing spike in the globe's fever are an El Niño event taking place in the tropical Pacific Ocean, which tends to increase global average temperatures, as well as the long-term human-caused increase in temperatures due to the burning of fossil fuels for energy.
Last month follows the hottest June, July and August, and is the 535th straight month with warmer-than-average temperatures, NOAA found.

What they're saying: "With the huge September data in, we can confirm that we expect 2023 to be the warmest year in the record (99% probability)," NASA climate researcher Gavin Schmidt posted on X.

"The expectation for 2024 is warmer still."

Zoom in: Climate scientists also point to other factors to explain the record warmth of late, and the apparent acceleration in warming. One is an increase in the planet's energy imbalance, i.e. the ratio of solar radiation coming into the atmosphere to the energy radiating back out.

That imbalance has been on the rise.

The other factor is something that longtime climate scientist James Hansen has pointed to, among other researchers. For decades, emissions of aerosols from power plants, cars, ships and other fossil fuel-powered machines have actually helped to slow the pace of climate change.

But countries have been moving to curb such pollution, since it contributes to poor air quality that kills millions each year.
Because of aerosol reductions, continued greenhouse gas emissions and other factors, Hansen and his coauthors predict "At least a 50% increase of the post-2010 global warming rate compared to the 1970-2010 rate of 0.18°C (0.32°F)."

The intrigue: "This is a partial payment in return for the Faustian bargain that humanity made when it chose to build its economies on fossil fuel energy," Hansen and his co-authors wrote in mid-September and detailed further late last week.

Climate scientist Zeke Hausfauther largely backed Hansen's findings, citing evidence of a clear climate change acceleration.
"While many experts have been cautious about acknowledging it, there is increasing evidence that global warming has accelerated over the past 15 years rather than continued at a gradual, steady pace," he wrote in the New York Times on Friday.
"That acceleration means that the effects of climate change we are already seeing — extreme heat waves, wildfires, rainfall and sea level rise — will only grow more severe in the coming years."

Yes, but: Some prominent climate scientists disagree with the idea that Earth's warming rate is speeding up, pointing to a linear rise in ocean heat content, for example.

Even with an acceleration, if global emissions were brought down to net zero, a goal the U.S. and other countries have agreed to meet by 2050, warming would stop within a few years.
However, it would take centuries to bring temperatures down, barring breakthroughs in direct air capture and other technologies.