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The global temperature numbers for September are in, and they are not good. “This month was, in my professional opinion as a climate scientist—absolutely gobsmackingly bananas,” Zeke Hausfather posted Tuesday on X (formerly known as Twitter).
Kristina Dahl, principal climate scientist at the Union of Concerned Scientists, read that post yesterday. “I've been sitting at my desk trying to think of a better way to describe that, but I can't,” Dahl says. “It's just shocking.”
“Concerning, worrying, wild—whatever superlative you want to use,” says Kate Marvel, senior scientist at Project Drawdown, a nonprofit that fights climate change. “That's what it is.”
The graph below, created by Hausfather, a researcher at the climate group Berkeley Earth, shows temperature anomalies, meaning how high each month was above a historical average baseline temperature. Each multicolored line represents a previous year, color coded by decade. (The 1990s, for example, are the lines in yellow.) The solid black line is 2023, and it has been soaring above the others since May. It stops in the month of September, which beat the prior monthly record by more than 0.5 degrees Celsius.
This September was on average 1.8 degrees C hotter than preindustrial levels, well above the Paris Agreement’s goal of keeping temperatures from rising more than 1.5 C. (Important caveats on that in a moment.)
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“We've already seen a summer of extreme temperatures, so my threshold for being surprised was a bit higher,” Hausfather tells WIRED. “But just how extreme September was, it's kind of bananas; 0.5 C is just off the charts. We’ve never seen a month with that level of jump before.”
“It's astounding to see the previous record broken by so much,” agrees Dahl. “And astounding to see that the global temperature this September is on par with what we normally see in July—the hottest month of the year, typically. So it really just illustrates how profoundly our climate is shifting.”
What’s unfolded all summer has been a mixture of climate science factors, some of which are well understood and others that are more uncertain. It’s a certainty that the more greenhouse gases we pump into the atmosphere, the more warming we get. “We should expect not just record-breaking extremes, but record-shattering extremes,” says Marvel. “Things that break previous records by incredible margins.”
A little bit of uncertainty—along with some extra heat—is being injected into the process right now by El Niño, the band of warm water that forms in the Pacific Ocean off the coast of South America. Typically, that phenomenon can add a couple tenths of a degree Celsius by transferring heat from the oceans to the atmosphere. The exact effects or arrival date of El Niño can be unpredictable—and this one is still developing. But it’s a cyclical natural process, so climate scientists are used to working it into their calculations.
“Then you get in the realms of things that yeah, sure, they probably have an effect, but I don't think we've done the sort of really rigorous science to quantify them,” says Marvel. For one, scientists are investigating whether new regulations have had a weird side effect for the climate. In 2020, the International Maritime Organization dramatically limited the amount of sulfur in shipping fuel. That was great for reducing air pollution, but that sulfur had actually been brightening the clouds over shipping lanes, bouncing some of the sun’s energy back into space. Less cloud cover could be helping raise temperatures.
Last year’s massive Hunga Tonga volcanic eruption may also still be having a small warming influence. Volcanoes can cool the climate dramatically by firing aerosols into the atmosphere and blocking the sun. But not Hunga Tonga. “Because it was so big, and because it was underwater, it put a bunch of water vapor in the stratosphere,” says Marvel. Water vapor is actually a planet-warming greenhouse gas. “This might have had a slight warming effect, but we're talking less than a tenth of a degree. But added on to everything else that's going on, that could be a factor.”
All of these variables added up to September reaching 1.8 degrees C above preindustrial levels. That does not mean, though, that we’ve blown through the Paris Agreement’s 1.5 degree limit. That goal represents sustained temperatures, not those for a single month.
Still, such extremes are alarming to scientists, both in terms of how quickly we’re approaching the Paris threshold and how gnarly the effects of climate change already are: fiercer rainfall, like the precipitation that flooded New York City in late September. More massive hurricanes, like this season’s Lee and Idalia. More vicious wildfires, like the one that obliterated Maui’s city of Lahaina in August. The proliferation of bacteria and fungi that thrive in a warmer world. Ever more extreme heat.
“This is not about our grandchildren, this is not about the polar bears, this is not about someplace far away. This is affecting us right now,” says Marvel. “What the science says is that every tenth of a degree matters. Every ton of emissions that can be avoided matters. If the world passes 1.5, then you shoot for 1.6. If it passes 1.6, you shoot for 1.7. And I think we now know after this year how 1.5 is not safe.”
