Scientists have determined that the summer of 2023 was the hottest summer in the Northern Hemisphere in the last two thousand years, almost four degrees hotter than the coldest summer in the same period.

Although it is often reported that 2023 is the hottest recorded year, instrumental evidence goes back only to 1850, and most records are limited to specific regions.

Using historical climate data from annual tree rings over two millennia, scientists from the University of Cambridge and Johannes Gutenberg University in Mainz have shown how extraordinary the summer of 2023 was.

Even taking into account natural climate variations over hundreds of years, 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.

"When you look at the long course of history, you can see how dramatic recent global warming is," said co-author Professor Ulf Büntgen from Cambridge's Department of Geography. "2023 was an extremely hot year, and this trend will continue unless we drastically reduce greenhouse gas emissions."

The results, published in the journal Nature, also show that the Paris Agreement of 2015 on limiting warming to 1.5°C above pre-industrial levels has already been breached in the Northern Hemisphere.

Early instrumental temperature records, from 1850 to 1900, are scarce and inconsistent. Researchers compared early instrumental data with a large dataset from tree rings and found that the 19th-century temperature baseline used to contextualize global warming was several tenths of a degree Celsius cooler than previously thought. By recalibrating this baseline, researchers calculated that summer conditions in 2023 in the Northern Hemisphere were 2.07°C warmer than the average summer temperatures between 1850 and 1900.

"Many discussions we have about global warming are linked to the mid-19th-century temperature baseline, but why is that the baseline? What is normal, in the context of a constantly changing climate, when we only have 150 years of meteorological measurements?" said Büntgen. "Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate changes into context."

Tree rings can provide that context, as they contain annually resolved and absolutely dated information about past summer temperatures. By using tree ring chronologies, researchers can look much further into the past without the uncertainties associated with some early instrumental measurements.

Available tree ring data reveal that most of the colder periods in the last 2000 years, such as the Little Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed major sulfur-rich volcanic eruptions. These eruptions eject enormous amounts of aerosols into the stratosphere, causing rapid surface cooling. The coldest summer in the last two thousand years, in 536 AD, followed one such eruption and was 3.93°C cooler than the summer of 2023.

Most of the warmer periods covered by tree ring data can be attributed to the El Niño climate pattern, or the El Niño-Southern Oscillation (ENSO). El Niño affects weather worldwide due to weakened trade winds in the Pacific Ocean and often results in warmer summers in the Northern Hemisphere. Although El Niño events were first recorded by fishermen 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 has made El Niño events stronger, resulting in warmer summers. The current El Niño event is expected to continue until early summer 2024, likely breaking temperature records again.

"It is true that the climate is always 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 heatwaves and prolonged drought periods," said Professor Jan Esper, lead author of the study from Johannes Gutenberg University in Mainz, Germany. "When you look at the big picture, it shows how urgent it is to immediately reduce greenhouse gas emissions."

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

The research was partially supported by the European Research Council.

Source: University of Cambridge