Scientists Disagree About Drivers of September’s Global Temperature Spike, but It Has Most of Them Worried

The month’s shocking surge is likely to make 2023 the hottest year on record and drive extreme impact around the globe. It could also be a harbinger of even higher temperatures next year.

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A woman reacts as a wildfire burns at Palem Raya Regency in Ogan Ilir, South Sumatera, Indonesia on September 18, 2023. Indonesian authorities are struggling to put out forest and land fires that have been engulfing many parts of the country, including fire-prone regions in Sumatra and Borneo, as the country enters the hottest day of this year's El Nino-induced dry season Credit: Muhammad A.F/Anadolu Agency via Getty Images
A woman reacts as a wildfire burns at Palem Raya Regency in Ogan Ilir, South Sumatera, Indonesia on September 18, 2023. Indonesian authorities are struggling to put out forest and land fires that have been engulfing many parts of the country, including fire-prone regions in Sumatra and Borneo, as the country enters the hottest day of this year's El Nino-induced dry season Credit: Muhammad A.F/Anadolu Agency via Getty Images

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September’s stunning rise of the average global temperature is all but certain to make 2023 the warmest year on record, and 2024 is likely to be even hotter, edging close to the “red line” of 1.5 degrees Celsius of warming above the pre-industrial level that the 2015 Paris climate agreement is striving to avoid. 

As of Oct. 10, the daily average Northern Hemisphere temperature had been at a record high for 100 consecutive days. At least 65 countries recorded their warmest Septembers on record, and even after record heat in July and August, the September spike was a shock, said Carlo Buontempo, director of the European Union’s Copernicus climate change service.

The truth is that I think many climate scientists were absolutely flabbergasted by the plot,” he said of September’s worldwide temperature reading as Copernicus released its monthly global climate report. “This is just beyond anything we’ve ever seen. The anomaly is so incredibly large. You can call it a global heatwave.”


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September’s sudden spike to 1.7 degrees Celsius above the pre-industrial global temperature for the month is so far above the trend of rising temperatures and previous records for the month that Buontempo expressed concern that it could indicate a more rapid shift of the climate system to a warmer state.

“I’m not saying that has happened, but I’m saying it is an indication of a process that may not actually be linear at all,” he said.

Many climate scientists say they don’t know exactly why Earth’s fever suddenly spiked so high in September, and there “may never be a clear attribution” to a specific cause, said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies. Most likely, it’s a combination of factors, he said, a climate recipe with “a little bit of everything combining in ways we haven’t seen before.”

 The uncertainty sent a shiver of unease through parts of the climate science community, especially about extremes like droughts, floods and heatwaves that might be unleashed in the coming year if even warmer temperatures materialize.

The explanations for the temperature surge range from the shift to the warm, El Niño phase in a Pacific ocean cycle, to a continued drop in the concentration of tiny sulfur-based particles of pollution called aerosols.

The scramble to explain the September readings even resulted in scientists at least partly contradicting one another about the possible causes; in some cases scientists who have authored important climate science research together. A few even discounted each other’s explanations, which may reflect a growing climate debate between two camps. 

On one side are self-described climate realists—sometimes pejoratively called doomers—who say we’re already in a worst-case warming scenario leading toward existential challenges for civilization. The others subscribe to what could still be described as the mainstream belief that current and future policies will be enough to reach global climate goals. 

Pushing the Upper Edge of Climate Projections

The steady long-term warming trend of the past half century is clearly caused by carbon dioxide emissions from burning fossil fuels, said Stefan Rahmstorf, a climate expert at the Potsdam Institute for Climate Impact Research.

“For four decades it’s been going as predicted,” he wrote on X (the platform formerly known as Twitter). “But we don’t understand the surprise upward leap that is happening now. And that worries me.”

But the September reading doesn’t mean the countries of the world have failed in the effort to cap global warming at 1.5 degrees Celsius, said Michael Mann, a climate scientist at the University of Pennsylvania. The Paris Agreement is aimed at preventing warming from staying above that mark for the long-term, defined by “something like a 20-year running average, or something to that effect,” he said. If greenhouse gas emissions continue on their current trajectory, that would happen in about 15 years. But a permanent breach of the 1.5 degree threshold could be avoided by cutting emissions 50 percent by 2030 and reaching zero emissions by 2050, Mann said.

A few months, or even a year, barely above the line “does not, in any way, imply that the warming trend has crossed the 1.5C warming threshold, which is what policy efforts address,” he said. 

Some of the confusion over when the limit would be breached is because “there is no official definition” of what crossing the 1.5 degree Celsius threshold means, said Robert Rohde, a climate scientist with Berkeley Earth, an independent climate think tank. “Scientists and others sometimes create precise definitions for the purpose of analysis, but the Paris Agreement itself is imprecise on how the threshold is defined,” he said.

In 2021, the United Nations Framework Convention on Climate Change sharpened the definition to make it clear that it’s based on multidecadal average, “So at least 20 years,” said Andrew Ferrone, a co-coordinator of the European Union negotiation team on science issues for the United Nations Framework Convention on Climate Change.

The September temperature spike is eye-catching, and “certainly pushing the boundaries of model expectations,” Berkeley Earth climate scientist Zeke Hausfather wrote on X, posting a graph showing the monthly temperature soaring well above the range projected by some climate models.

Mann said the focus should be on annual average temperatures and noted that 2023 “is almost certain to be within the range predicted by the models,” he said. “I’m frustrated by the hyperventilating going on over this. It’s frustrating that so many continue to miscommunicate about that and mislead the public as to where we are. The truth is bad enough.”

Many other climate scientists expressed their astonishment at September’s global temperature reading in social media posts, including Mika Rantanen, a climate researcher with the Finnish Meteorological Institute, who wrote, “I’m still struggling to comprehend how a single year can jump so much compared to previous years. Just by adding the latest data point, the linear warming trend since 1979 increased by 10%.”

Hansen Points to Aerosols, and a “Helluva a Ride” to Come

The September temperature spike is at the very highest edge of what recent climate models projected as possible, and even absent intentional miscommunication, there are divergent views about how fast Earth will warm during the next few decades. That’s partly because even the newest climate models don’t include some of the climate feed backs that amplify warming, like the huge surge of greenhouse gas emissions from wildfires. 

And it’s also because scientists haven’t been able to study some of the most important atmospheric processes that will determine the pace of climate change, like the interaction between clouds and tiny particles called aerosols that come from industrial sources, primarily from burning fossil fuels, as well as natural sources, said James Hansen, the former NASA climate scientist whose 1988 testimony to Congress put climate change in the political spotlight. 

In 2021, Hansen warned that the rate of warming could double over the next 25 years, heating the planet by somewhere close to 2 degrees Celsius over pre-industrial levels (3.6 degrees Fahrenheit) by 2050. 

Aerosols affect cloud particle size, cloud cover and cloud brightness, which, in turn, affect how much heat energy the atmosphere absorbs. Even without directly measuring the climate effect of aerosols, Hansen said other satellite data, including from NASA’s CERES program, show changes in clouds that account for the rapidly increasing energy imbalance in the climate system, with increasing amounts of heat staying in the atmosphere.

Hansen said those measurements show that greenhouse gases can be ruled out as the main cause of September’s anomalous temperature increase.

“The data are all consistent with aerosol reduction being the cause of accelerated global warming,” he said, reiterating his recent warnings about a sudden spike of global warming . “So hang onto your hat. We are in for a helluva ride. Global warming will rise above 1.5 degrees Celsius within several months and above 2 degrees Celsius within a couple of decades, unless we take purposeful actions to restore Earth’s energy balance and climate, in addition to phasing down fossil fuel emissions as rapidly as practical.”

Those actions include geoengineering and nature restoration, as well as technologies to remove CO2 from the atmosphere, he said.

“We need to research, urgently, all of the above because plausible emission reductions will not be enough,” he said. “We have a very limited time to avoid locking in consequences for today’s young people that they will consider unacceptable, and we have allowed the problem to reach a magnitude such that strong actions will be required.”

Preemptively ruling out any method of mitigating climate change “would be the height of irresponsibility” with regard to young people and must be rejected, he said. 

Other Possible Causes and Impacts

Schmidt, the NASA climate scientist, said the September temperature increase can at least partly be attributed to the emerging El Niño, when a big slice of Pacific Ocean around the equator heats up between 1 and 2 degrees Celsius above normal, which also raises the global average. But that doesn’t completely explain it, he said.

It usually takes about three to six months for an El Niño to “really make a difference to the global mean,” he said. The full impacts will be felt next year, affecting the global average by about .1 degrees Celsius, depending on how strong the El Niño becomes.

“The drivers of the current spike are going to be a little of everything in ways that we haven’t really seen before,” he said. “We may not ever have a clean attribution though.”

He said most of this year’s global temperature sets will still “all be substantially below” 1.5 degrees Celsius warming compared to the late 19th century. 

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“Even with the El Niño boost next year, that’s likely to be true then as well,” he said.

Kevin Trenberth, formerly with the climate analysis team at the National Center for Atmospheric Research in Boulder, Colorado, said El Niño may also be a factor in the September temperature surge because it increases the amount of heat-trapping water vapor in the atmosphere.

During El Nino, he said, the warming equatorial Pacific dissipates its heat through evaporation, ”adding more water vapor to the atmosphere that eventually finds its ways into storms and it pours with rain while releasing the latent heat,” he said. “That is what really gives the mini-global warming with El Nino.”

Regardless of the exact cause, the surging average global temperature is cause for concern, because even small increments of warming can exponentially increase the risk of some climate extremes, such as with recent heatwaves and floods, sometimes in the same place

As the global average temperature increase approaches 1.5 degrees Celsius, studies show that increasingly large swaths of the planet will experience heat waves that will kill unprotected people after several hours of exposure. Other research identifies clear climate tipping points between 1.5 and 2 degrees Celsius that will wipe most coral reefs and nearly all mountain glaciers and raise sea level rapidly by melting ice sheets in Greenland and Antarctica. 

The incremental additional warming from El Niño raises more specific concerns, since observations and research show regional extreme impacts, including drought in the Amazon rainforest and across parts of Africa

During recent El Niños, there have also been megafires in Indonesia that emitted massive quantities of carbon dioxide, and such fires have already started this year. In January 2018, Cape Town, South Africa, nearly ran out of water, which climate researchers attributed to a combination of an El Niño that preceded the dry spell and climate change. And this year has already been disastrously bad for mountain glaciers. Switzerland’s glaciers may have lost as much as 10 percent of their mass during the past year alone, according to recent survey results.

But even though patterns of regional El Niño impacts are documented, the current level of warming puts the planet in “uncharted waters,” said Maarten Van Aalst, director of the Royal Netherlands Meteorological Institute.

“The standard impacts of El Nino are pretty well known of course and I do expect many of these to appear,” he said. “But on top of the current levels of warming, we’re clearly also in for some surprises.”

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