2023 Marks Hottest Summer in Two Millennia, Study Reveals

A recent study has found that the summer of 2023 was the hottest in the Northern Hemisphere in the past two thousand years, with temperatures nearly four degrees Celsius warmer than the coldest summer during the same period. Researchers from the University of Cambridge and the Johannes Gutenberg University Mainz used climate data from tree rings spanning two millennia to highlight the exceptional nature of this year's summer temperatures.

While 2023 has already been reported as the hottest year on record based on instrumental data, those records only date back to around 1850 and are geographically limited. The new study, published in the journal Nature, uses tree-ring data to extend the climate record much further back, providing a clearer context for recent temperature extremes.

The research team found that, even accounting for natural climate variability over centuries, the summer of 2023 was unprecedented, exceeding the extremes of natural climate variability by half a degree Celsius. This finding underscores the dramatic impact of recent global warming.

The study also highlights that the 2015 Paris Agreement target to limit warming to 1.5 degrees Celsius above pre-industrial levels has already been breached in the Northern Hemisphere. Early temperature records from 1850-1900 are sparse and inconsistent, and the researchers found that the 19th-century baseline used for global warming comparisons is several tenths of a degree Celsius colder than previously thought. Recalibrating this baseline revealed that the summer of 2023 was 2.07 degrees Celsius warmer than the average summer temperatures between 1850 and 1900.

Tree rings provide annually-resolved and precisely-dated information about past summer temperatures, offering a valuable context that extends beyond the uncertainty of early instrumental records. The data reveals that cooler periods over the past 2000 years, such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large volcanic eruptions that injected sulfur-rich aerosols into the atmosphere, causing rapid surface cooling. The coldest summer of the past two millennia, in 536 CE, was nearly four degrees Celsius colder than the summer of 2023.

Conversely, warmer periods in the tree ring data are often linked to the El Niño-Southern Oscillation (ENSO). This climate pattern, which weakens trade winds in the Pacific Ocean, results in warmer summers in the Northern Hemisphere. While El Niño events have been noted since the 17th century, tree ring data show their influence much further back in time.

In recent decades, however, global warming driven by greenhouse gas emissions has intensified El Niño events, leading to even hotter summers. The current El Niño is expected to continue into early summer 2024, likely setting new temperature records.

The researchers note that while their findings are robust for the Northern Hemisphere, obtaining comparable data for the Southern Hemisphere is challenging due to sparse historical records and different climate responses, influenced by its larger oceanic coverage.