Last Year, the Northern Hemisphere Had Its Hottest Summer in 2,000 Years

Researchers used tree ring data to compare temperatures from as far back as 1 C.E. to 2023 temperatures

People cool off in front of an outdoor fan
Tourists cool off in front of a fan in Rome, Italy on July 18, 2023. Temperatures in the area at the time surpassed 104 degrees Fahrenheit. Pablo Esparza / Anadolu Agency via Getty Images

Last year, the Northern Hemisphere had its hottest summer in the past 2,000 years, a new study in Nature finds.

Previous research had found that 2023’s temperatures outstripped all other years on record, dating back to the mid-19th century. The new paper analyzes tree ring data to find that the 2023 Northern Hemisphere summer was hotter than any other summer dating back to 1 C.E.

Global temperatures have been steadily increasing due to human activity that releases greenhouse gases into the atmosphere. Last year, rising temperatures were exacerbated by El Niño, a climate pattern during which warmer water in the Pacific Ocean leads to warmer temperatures in northern North America. The findings reinforce the need for net emissions to be limited to zero, the study authors write.

“It’s pretty impressive that you can go 2,000 years back and know that we’re hotter than any of those individual years,” Vikki Thompson, a climate scientist at the Royal Netherlands Meteorological Institute who did not contribute to the findings, tells Scientific American’s Andrea Thompson.

“It’s no surprise—this really, really outstanding 2023—but it was also, step-wise, a continuation of a trend that will continue,” Jan Esper, lead author of the study and a climatologist at Johannes Gutenberg University in Germany, said to reporters on Monday, per the Los Angeles Times Hayley Smith. “Personally I’m not surprised, but I am worried.”

The summer of 2023 was marked by heat waves throughout the Northern hemisphere. In Valencia, Spain, temperatures reached 116.2 degrees Fahrenheit in August. It got as hot as 119.8 degrees Fahrenheit on the Italian island of Sicily. And Phoenix, Arizona, saw temperatures of at least 110 degrees Fahrenheit on 54 days last year.

In September, NASA announced that 2023 was the hottest summer on record since global records began in 1880. The European Union’s Copernicus Climate Change Service said in December that 2023 would be the hottest year in their recorded history. And in January, the National Oceanic and Atmospheric Administration determined that 2023 was the world’s warmest year, by far, in their 174-year climate record.

For the new analysis, the researchers focused on the months of June to August between 30 and 90 degrees latitude, an area spanning from just south of Cario, Egypt, to the North Pole.

Using aggregated measurements from thousands of meteorological stations, they found that temperatures were 2.07 degrees Celsius warmer in 2023 compared to the mean from 1850 to 1900. This demonstrates that the temperature increase since pre-industrial times—in this portion of the Northern Hemisphere—has already passed the 1.5 degrees Celsius limit set in the 2015 Paris Agreement.

The researchers also compared 2023 to summers going back to 1 C.E. using data from tree rings, according to the New York Times Delger Erdenesanaa. Trees grow wider rings during warmer periods. The researchers didn’t have tree ring records from the Southern Hemisphere, and tropical trees don’t have annual rings since they don’t experience winter, so the study authors focused on the non-tropical part of the Northern Hemisphere, per Scientific American. The analysis showed that temperatures were 2.20 degrees Celsius warmer in 2023 compared to the mean from 1 C.E. to 1890.

The summer of 2023 was also 1.19 degrees Celsius warmer than the warmest year in the older period, 246 C.E., and a whopping 3.93 degrees Celsius warmer than the coolest year, 536 C.E.

“I naively hope that we will limit our greenhouse gas emissions,” Esper tells Scientific American. “The latest numbers are not promising at all, but I do see change.”

“The longer we wait, the more expensive it will be and the more difficult it will be to mitigate [climate change],” Esper said to reporters, per ABC News’ Julia Jacobo.

Since scientists expect the influence of the El Niño period to extend into the early summer of this year, 2024 could break last year’s records.

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