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

May 14. () –

A tree ring study suggests that last summer was the hottest in the northern hemisphere in the last 2,000 years, almost 4 degrees warmer than the coldest summer of the same period.

Although 2023 has already been reported to be the hottest year on record, instrumental evidence only dates back to 1850 at best, and most records are limited to certain regions.

PARIS AGREEMENT ALREADY BREACHED

Now, using past climate information from tree rings resolved annually over two millennia, scientists from the University of Cambridge and the Johannes Gutenberg University of Mainz have shown just how exceptional the summer of 2023 was. The results, published in the magazine ‘Nature’, They also show that the 2015 Paris Agreement to limit warming to 1.5°C above pre-industrial levels has already been breached in the northern hemisphere.

Even taking into account natural climate variations over hundreds of years, 2023 was still the hottest summer since the height of the Roman Empireexceeding the extremes of natural climate variability by half a degree Celsius.

“When you look at the long history of history, you can see how dramatic recent global warming is,” he explains. it’s a statement co-author Professor Ulf Büntgen of the Cambridge Department of Geography. “2023 was an exceptionally hot year and This trend will continue unless we dramatically reduce greenhouse gas emissions.“.

The first instrumental temperature records, from 1850 to 1900, are scarce and inconsistent. The researchers compared early instrumental data with a large-scale tree-ring data set and found that the 19th-century reference temperature used to contextualize global warming is several tenths of a degree Celsius colder than previously thought. By recalibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere They were 2.07 °C warmer than the average summer temperatures between 1850 and 1900.

“Many of the conversations we have about global warming are tied to a mid-19th century reference temperature, but why is this the reference temperature? What is normal, in the context of an ever-changing climate, when we only have 150 years of meteorological measurements? Büntgen asks. “Only when we analyze climate reconstructions will we be able to better account for natural variability and put recent anthropogenic climate change in context.”

Tree rings can provide that context, as they contain annually resolved and absolutely dated information about last summer’s temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.

Available tree-ring data reveal that most of the coldest periods of the past 2,000 years, such as the Old Little Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large volcanic masses rich in eruptions and sulfur. These eruptions spew enormous amounts of aerosols into the stratosphere, causing rapid cooling of the surface. The coldest summer in the last two thousand years, in 536 AD, followed such an eruption and was 3.93 °C colder than the summer of 2023.

Most of the warmer periods covered by the tree ring data can be attributed to the El Niño or El Niño-Southern Oscillation (ENSO) climate pattern. El Niño affects weather around the world due to weakening trade winds in the Pacific Ocean and often causes warmer summers in the Northern Hemisphere. While fishermen first noticed El Niño events in the 17th century, They can be observed in tree ring data much further back in time.

However, over the past 60 years, global warming caused by greenhouse gas emissions is causing El Niño events to become stronger, resulting in hotter summers. The current El Niño episode is expected to continue until early summer 2024, so it is likely that this summer will once again break temperature records.

“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, will be further amplified by El Niño conditions, so we will end up with longer, more severe heat waves and prolonged periods of drought,” says Professor Jan Esper, lead author of the study from Johannes Gutenberg University Mainz in Germany. “When you look at the big picture, shows how urgent it is that we reduce greenhouse gas emissions immediately.

The researchers note that while their results are robust for the Northern Hemisphere, it is difficult to obtain global averages for the same period as data is sparse for the Southern Hemisphere. The Southern Hemisphere also responds differently to climate change, as it is much more covered in ocean than the Northern Hemisphere.