We all remember it as a scorching summer, characterized by a succession of long heat waves. And in fact, according to scientists, summer 2023 was the hottest in the last two thousand years in the Northern Hemisphere. Almost four degrees higher than the coolest summer of the same period. To do the calculationusing past climate information stored in tree rings year after year over two millennia, scientists from the University of Cambridge and the Johannes Gutenberg University of Mainz succeeded, demonstrating the exceptional nature of last summer. 2023 had already been reported as the hottest year on record, but instrumental evidence dates back at best only to 1850. Now researchers have gone much further back, concluding that even taking into account natural climate variations over hundreds of years, last summer is the hottest since the Roman Empireexceeding the extremes of natural climate variability by half a degree Celsius.
“When you look at such a long span of history, you can see how dramatic recent global warming is – underlines the co-author of the work, Ulf Büntgen, of the Cambridge Department of Geography – 2023 was an exceptionally warm year and this trend will continue unless we dramatically reduce greenhouse gas emissions.”
The findings, published in the journal Nature, also demonstrate that the 2015 Paris Agreement to limit warming to 1.5°C above pre-industrial levels has already been violated in the Northern Hemisphere.
Available tree-ring data, cross-referenced with the first instrumental temperature records, reveal that most of the coldest periods of the last 2,000 years, such as the ‘little ice age’ in the 6th century and that in the early 19th century, they followed large sulphur-rich volcanic eruptions. These eruptions emit huge amounts of aerosols into the stratosphere, triggering rapid surface cooling. The coldest summer of the last two thousand years dates back to 536 ADafter one of these eruptions, with a temperature almost 4 degrees C lower than last year.
Most of the hottest periods in these 2,000 years can be attributed to the influence of El Niño, but over the last 60 years global warming caused by greenhouse gas emissions has caused an increase in these events, with summer months increasingly warm. The current El Niño event is expected to continue into early summer 2024, which is likely to break new temperature records.
“It is true that the climate is continually changing, but the warming in 2023, caused by greenhouse gases, is further amplified by El Niño conditions, so we end up with longer and more severe heat waves and prolonged periods of drought,” he said Jan Esper, lead author of the study and professor at Gutenberg University Mainz in Germany. “If you look at the big picture, you see how much it is It is urgent to immediately reduce greenhouse gas emissions“, he remarks. The research was partly supported by the European Research Council.
