How hot? Scientists have found that 2023 was the hottest summer in the Northern Hemisphere in the past two thousand years. The year was close to four degrees warmer than the coldest summer during the same period.
Previous assessments for the hottest year on record have been limited by reliable records only going back as far as 1850 (and event then data is limited to certain regions).
A new alternative to assess climate information is based on annually resolved tree rings over two millennia. This has been developed by scientists from the University of Cambridge and the Johannes Gutenberg University Mainz.
The researchers compared early instrumental data with a large-scale tree ring dataset and found the 19th century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought.
By re-calibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07oC warmer than mean summer temperatures between 1850 and 1900.
Tree rings provide that context, since they contain annually-resolved and absolutely-dated information about past summer temperatures, allowing researchers to look much further back in time.
The available tree-ring data further reveals that most of the cooler periods over the past 2000 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. These eruptions spewed huge amounts of aerosols into the stratosphere, triggering rapid surface cooling.
The coldest summer of the past two thousand years, in 536 CE, followed one such eruption, and was 3.93oC colder than the summer of 2023.
Even considering natural climate variations, 2023 was still the hottest summer since the height of the Roman Empire, exceeding the extremes of natural climate variability by half a degree Celsius.
This has led lead scientist Professor Ulf Büntgen to comment: “When you look at the long sweep of history, you can see just how dramatic recent global warming is. 2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
The results also demonstrate that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 1.5oC above pre-industrial levels has already been breached.
The research appears in the science journal Nature, titled “2023 summer warmth unparalleled over the past 2,000 years.”
