Tree Ring Records Reveal 2023 Hottest Summer in 2000 Years

In 2023, weather records keep falling like dominoes as heat records after heat records are broken every month. Copernicus, the E.U.’s climate monitoring service, and NOAA, a U.S. weather and climate monitoring agency, declared that April 2024 is the hottest April on record.

The month also is the 11th month in a row of record warmth worldwide. Copernicus notes that April’s 2024 average temperature is 1.58°C warmer than the pre-industrial (1850-1900) average temperatures for April, and NOAA says that the month is 1.32°C above the 20th-century average.

Based on Copernicus’s data, the Earth has already surpassed the Paris Agreement’s 1.5°C temperature threshold in the past 12 months, from May 2023 to April 2024, as this period records average temperatures that are 1.61°C above the 1850-1900 pre-industrial average.

April 2024 also saw a severe heatwave sweep across South and Southeast Asia, with temperatures pushing past 40°C. In its May analysis, the World Weather Attribution finds that climate change made the heatwaves hotter and more likely.

While scientists have already established 2023 as the hottest year since the pre-industrial period, when temperature records began, a new study published in Nature on 14 May 2024 finds that summer 2023 is the hottest across the Northern Hemisphere in the last 2000 years. Scientists called last year’s warmth “unprecedented.”

Last summer’s high temperatures exacerbated the deadly wildfires in Canada and Hawaii, fueling the intense heat waves in Europe, the U.S., Japan, and South America.

Scientists from the University of Cambridge and Johannes Gutenberg University Mainz have reconstructed the ancient climate using tree ring patterns, and their work revealed how exceptional the 2023 summer temperatures were.

Their analysis shows that summer 2023 temperatures were at least 0.5°C above 246 C.E. (AD 246), which registered the hottest summer before direct measurements began, and humans started to raise global temperatures by burning fuels.

Based on tree ring data, the authors find that 536 C.E. (AD 536) is the coldest summer in the past two thousand years; the summer 2023 temperature is 3.93°C hotter. This cold period is associated with a large volcanic eruption spewing vast amounts of sulphur into the atmosphere, helping it cool rapidly.

Most of the warmer periods recorded in the tree rings are ascribed to the El Niño climate pattern, but in the last 60 years, rising temperatures from GHG emissions have caused El Niño events to become stronger, leading to hotter summers.

The study shows that not only was the 2023 summer the hottest in two millenniums, but even if climate variability, such as the ENSO oscillation, is accounted for, the 2023 summer temperatures have already breached the Paris Agreement’s 1.5°C temperature threshold.

Furthermore, based on these tree ring data, average temperatures of the Industrial Period between 1850 to 1900, used as temperature baseline to contextualise global warming, are many tenths of a degree colder than previously thought, making the summer of 2023 temperatures in the Northern Hemisphere 2.07°C warmer than mean summer temperatures between the pre-industrial period.

The study based its analysis on tree ring records from the Northern Hemisphere between 30 and 90 degrees north latitude because tree ring records from the Southern Hemisphere are lacking. Besides, tropical trees do not display the same annual rings as those in the Northern Hemisphere, so the researchers note that their findings are only robust for the Northern Hemisphere.