The summer of 2023 was the hottest in 2,000 years

According to a recent study, the summer of 2023 was the hottest in the Northern Hemisphere over the last two thousand years, marking it nearly four degrees Celsius warmer than the coldest recorded summer in the same period. This finding highlights a stark contrast in temperature extremes over the millennia.

While 2023 is recognized as the hottest year on record, our reliance on instrumental records only dates back to approximately 1850, and even these are often geographically limited. 

Hottest summer since the peak of the Roman Empire 

A team of scientists from the University of Cambridge and Johannes Gutenberg University Mainz has now extended this perspective by integrating data derived from tree rings, which chronicle climate changes annually and span two millennia.

Despite natural climate fluctuations over centuries, the summer of 2023 has set a new precedent, being the hottest since the peak of the Roman Empire, exceeding natural climate variability by half a degree Celsius. 

Rapid global warming

“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said Ulf Büntgen, an expert in dendroecology and paleoclimatology at Cambridge. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”

These findings also show that the thresholds set by the 2015 Paris Agreement, which aim to keep global warming below 1.5C above pre-industrial levels, have already been surpassed in the Northern Hemisphere.

Historical climate patterns 

The scientists meticulously compared sparse and inconsistent early instrumental temperature records from 1850 to 1900 with extensive tree-ring data. This comparison not only provided a more comprehensive view of historical climate patterns but also led to the recalibration of the 19th-century temperature baseline, which is now understood to be several tenths of a degree Celsius cooler than previously estimated. 

Consequently, the summer of 2023 in the Northern Hemisphere was found to be 2.07C warmer than the average temperatures between 1850 and 1900.

A better perspective on climate change 

“Many of the conversations we have around global warming are tied to a baseline temperature from the mid-19th century, but why is this the baseline? What is normal, in the context of a constantly-changing climate, when we’ve only got 150 years of meteorological measurements?” Büntgen said. 

“Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”

Tree rings revealed the hottest summer

Tree rings are particularly valuable for this type of research, providing detailed and accurately dated records of past summer temperatures. These records indicate that most cooler periods in the last 2000 years followed major volcanic eruptions, like the one in 536 CE, which resulted in the coldest summer on record, being 3.93C colder than the summer of 2023. 

In contrast, warmer periods are often linked to the El Niño climate pattern, which has been exacerbated by greenhouse gas emissions in recent decades, leading to stronger El Niño events and consequently hotter summers.

Longer and more severe heat waves 

“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gasses, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” said lead author Jan Esper, a professor of climate science at Johannes Gutenberg University Mainz. 

“When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”

The scientists argued that while their results are robust for the Northern Hemisphere, it is more difficult to obtain global averages for the same period due to lack of data for the Southern Hemisphere. Moreover, the Southern Hemisphere also responds differently to global warming, since it is far more ocean-covered than the Northern Hemisphere.

The study is published in the journal Nature.

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