A recent study has found that the summer of 2023 was the hottest in the Northern Hemisphere in 2,000 years.
Scientists from the University of Cambridge and Johannes Gutenberg University Mainz used data from tree rings to show that this summer was nearly 4°C warmer than the coldest summer in that period.
This summer’s heat exceeded natural climate variations by half a degree Celsius, making it the hottest since the peak of the Roman Empire.
Even though 2023 is the hottest year on record, instrumental temperature records only go back to 1850, and many of these are incomplete.
Tree rings, however, provide a clearer picture, as they contain detailed information about past summer temperatures.
Professor Ulf Büntgen from Cambridge’s Department of Geography emphasized the importance of this long-term perspective, saying, “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 findings were published in the journal Nature and show that the Northern Hemisphere has already exceeded the 2015 Paris Agreement goal of limiting warming to 1.5°C above pre-industrial levels.
The researchers discovered that the 19th-century temperature baseline used to measure global warming was colder than previously thought. By adjusting this baseline, they found that the summer of 2023 was 2.07°C warmer than the average summer temperatures from 1850 to 1900.
Tree rings are particularly useful for studying climate because they provide precise, annually-resolved data. They show that the coldest periods in the past 2,000 years often followed volcanic eruptions that released large amounts of aerosols into the atmosphere, causing rapid cooling. For instance, the summer of 536 CE was 3.93°C colder than the summer of 2023, following a significant volcanic eruption.
Warmer periods in the tree ring data are usually linked to the El Niño climate pattern, which causes warmer summers in the Northern Hemisphere. El Niño events have been documented by fishermen since the 17th century, but tree ring data shows they have occurred much earlier.
In the past 60 years, however, global warming from greenhouse gases has made El Niño events stronger, leading to hotter summers.
Professor Jan Esper from Johannes Gutenberg University Mainz noted, “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.
This results in longer and more severe heat waves and extended periods of drought. When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”
While the results are clear for the Northern Hemisphere, it is harder to get the same data for the Southern Hemisphere due to less available data and its larger ocean coverage.
However, the urgency to address climate change remains critical worldwide.
