The summer of 2023 was the hottest in the northern hemisphere in the past 2,000 years, researchers have found.
It was almost four degrees warmer than the coldest summer during the same period.
Last year was reported as the hottest year on record, but instrumental evidence only reaches back as far as 1850 at best - and most records are limited to certain regions.
Now scientists from the University of Cambridge and the Johannes Gutenberg University Mainz have used past climate information from annually resolved tree rings over two millennia to show how exceptional the summer of 2023 was.
Even allowing for natural climate variations over hundreds of years, it was the hottest summer since the height of the Roman Empire.
The researchers report in the journal Nature that it exceeded the extremes of natural climate variability by half a degree Celsius.
“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said co-author Prof Ulf Büntgen, from Cambridge’s Department of Geography. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
They show that in the northern hemisphere, the 2015 Paris Agreement to limit warming to 1.5C above pre-industrial levels has already been breached.
Early instrumental temperature records from 1850-1900 are sparse and inconsistent so the researchers compared it with a large-scale tree ring dataset. They found the 19th-century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought.
By recalibrating this baseline, the researchers showed that summer 2023 conditions in the Northern Hemisphere were 2.07C warmer than mean summer temperatures between 1850 and 1900.
Prof Büntgen said: “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?
“Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”
Tree rings contain annually-resolved and absolutely-dated information about past summer temperatures.
Available tree-ring data reveals that most of the cooler periods over the past 2,000 years - such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century - followed large-sulphur-rich volcanic eruptions, which threwed huge amounts of aerosols into the stratosphere, triggering rapid surface cooling.
The coldest summer of the past two thousand years was in 536 CE, following one such eruption, and was 3.93C colder than the summer of 2023.
Meanwhile, most of the warmer periods covered by the tree ring data can be attributed to the El Niño climate pattern, or El Niño-Southern Oscillation (ENSO).
El Niño affects weather worldwide because it weakens trade winds in the Pacific Ocean and often leads to warmer summers in the Northern Hemisphere.
El Niño events were noted by fishermen in the 17th century but can be observed in the tree ring data much further back.
But in the past 60 years, global warming caused by greenhouse gas emissions are causing El Niño events to become stronger, resulting in hotter summers.
And the current El Niño event is expected to continue into early summer 2024, meaning this summer is expected to break temperature records once again.
“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” said Prof Jan Esper, the lead author of the study from the Johannes Gutenberg University Mainz in Germany. “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”
It is difficult to obtain global averages for the period because data is sparse for the southern hemisphere, which also responds differently to climate change as it is far more ocean-covered than the northern hemisphere.
The research was supported in part by the European Research Council.
