Why September’s record-warm temperatures have scientists so worried

Sept. 23, 2023 

After months of record planetary warmth, temperatures have become even more abnormal in recent weeks — briefly averaging close to 2 degrees Celsius above preindustrial levels, a global warming threshold leaders are seeking to avoid.

"I thought we had seen exceptional temperatures back in July," said Zeke Hausfather, climate research lead for the payment company Stripe. "What we've seen this week is well above that."

The trend adds to near-certainty that 2023 will be Earth's warmest on record, and heightens threats of the extreme conditions the heat could fuel around the world.

The warmth is likely to be the fingerprints of a deepening El Niño climate pattern and a sign that temperatures will continue to accelerate beyond old norms in the year ahead, scientists said. El Niño, which began to appear this spring, is known for raising global temperatures by releasing vast stores of Pacific Ocean heat into the atmosphere.

"The El Niño won't peak until later this year and there is plenty more heat waiting in the wings," Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration, said in an email. "So expect more records to be set in the coming months."

One such analysis produced by the Japanese Meteorological Agency shows that, this month, global temperatures have persistently diverged from 1991-2020 averages by 1 degree Celsius (1.8 degrees Fahrenheit).

Hausfather said that the 1991-2020 average is, itself, about 0.9 degrees warmer than levels observed before the Industrial Revolution and the widespread burning of fossil fuels. That means temperatures are inching closer, at least briefly, to warming thresholds that global leaders have pledged to avoid.

Already, the planet is on the brink of the most ambitious climate target, to limit warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial averages. Scientists have said it is not out of reach, however, and said the latest spike of heat underscores the importance of climate action.

"Decades of burning fossil fuels and deforestation have pumped heat-trapping gases in the atmosphere, and the vast majority of that heat is absorbed by the oceans. We're now seeing the wrath of that heat as it's unleashed back to the atmosphere," Jennifer Francis, a senior scientist at the Woodwell Climate Research Center, wrote in an email. "We must act swiftly and boldly to reduce emissions of these gases to prevent as much warming as possible and the impacts of that heat."

The temperature anomalies are remarkable for being even more aberrant than the extreme heat observed around the world in July and August. "By a large margin" of 0.71 degrees, this summer was the planet's warmest three-month stretch in record books going back nearly two centuries, European climate scientists said earlier this month.

Hausfather called it "a foregone conclusion" that September marks a third consecutive month of record-setting average global temperatures.

If temperatures remain as abnormally warm as they are now, the planetary average could for the first time on an annual basis surpass 1.5 degrees Celsius of warming above preindustrial temperatures, Hausfather calculated, though he called that "very unlikely."

The temperature anomalies have grown even as, in absolute terms, the planet is cooling ahead of the Sept. 23 equinox, which marks the start of fall in the Northern Hemisphere. Conditions in the Northern Hemisphere have a larger effect on planetary averages because it contains more land than the Southern Hemisphere, and land warms and cools more quickly than oceans do.

El Niño is nonetheless likely a major driver behind the warm trend because it creates Pacific trade wind patterns that encourage more heat to be released from the ocean and trapped by greenhouse gases in the atmosphere. With that El Niño pattern only expected to strengthen, reaching a peak during the Northern Hemisphere winter, that warmth may become even more anomalous over the coming year, said Michael Mann, a climate scientist and professor at the University of Pennsylvania.

El Niño is known for boosting planetary temperatures by one to two tenths of a degree Celsius. The last strong El Niño pushed 2016 to the current record for average global warmth, and also triggered a rise in extreme heat and storms.

Other factors may be contributing to the warming, Hausfather said: Reduced emissions from shipping liners, allowing more sunlight to reach the oceans; the 2022 eruption of the South Pacific underwater volcano Hunga Tonga, which sent vast amounts of water vapor into the atmosphere; and an ongoing upswing in solar activity, slightly increasing the sun's warming effect on Earth.

But Claudia Tebaldi, an earth scientist at the Pacific Northwest National Laboratory, said the latest surge in temperature anomalies could be a sign that human influences and natural fluctuations are acting in concert to raise global temperatures.

During a stretch of the early 2000s when rates of global warming appeared to slow, Tebaldi said, natural fluctuations had a cooling effect that acted to dampen human-caused warming. That appears to have changed.

"It is not surprising that the pendulum is now oscillating in the other direction," she wrote in an email.

[Top photo: City council employees distribute bottles of water to combat a heat wave in São Paulo on Sept. 20. (EPA-EFE/Shutterstock)]