HQ Team
May 14, 2024: Last year was the hottest summer in the Northern Hemisphere for the past two thousand years, researchers found after foraging through tree ring width data.
Scientists from the University of Cambridge and Johannes Gutenberg University Mainz have shown that 2023 was still the hottest summer since the height of the Roman Empire.
They used past climate information from annually resolved tree rings over two millennia to arrive at their findings and to gauge how exceptional the summer of 2023 was.
Trees that depend heavily on temperature in the growing season will have narrow rings during cold periods and wider rings for warm periods.
Trees that depend heavily on moisture during the growing season will have wider rings during rainy periods and narrower rings during dry periods.
Ring-width
Weather data from nearby weather stations or sometimes larger-scale gridded data are compared against the ring-width time series.
If there is a strong correlation, the ring widths can be used to reconstruct climate conditions for the length of the tree ring record, which is often several centuries and occasionally a thousand years or more.
The 2023 heat exceeded the extremes of natural climate variability by half a degree Celsius, and was warmer by almost four degrees compared to the coldest summer during the 2,000 years, according to a University of Cambridge statement.
“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said co-author Professor Ulf Büntgen, from Cambridge’s Department of Geography.
“2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
Limited to regions
Although 2023 has been reported as the hottest year on record, the instrumental evidence only reaches back as far as 1850 at best, and most records are limited to certain regions.
The scientists said early instrumental temperature records, from 1850-1900, were “sparse and inconsistent.”
They compared early instrumental data with a large-scale tree ring dataset and found the 19th-century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought.
By re-calibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07C warmer than mean summer temperatures between 1850 and 1900.
It was difficult to obtain global averages for the same period since data was sparse for the Southern Hemisphere.
Regions respond differently
The Southern Hemisphere also responds differently to climate change, since it is far more ocean-covered than the Northern Hemisphere.
“Many of the conversations we have around global warming are tied to a baseline temperature from the mid-19th century, but why is this the baseline?
“What is normal, in the context of a constantly changing climate, when we’ve only got 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 change into context.”
The available tree-ring data reveals that most of the 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-sulphur-rich volcanic eruptions.
Coldest summer
These eruptions spew huge amounts of aerosols into the stratosphere, triggering rapid surface cooling.
The coldest summer of the past two thousand years, in 536 CE, followed one such eruption and was 3.93C colder than the summer of 2023.
Most of the warmer periods covered by the tree ring data can be attributed to the El Niño climate pattern or El Niño-Southern Oscillation.
El Niño affects weather worldwide due to weakened trade winds in the Pacific Ocean and often results in warmer summers in the Northern Hemisphere.
While El Niño events were first noted by fishermen in the 17th century, they can be observed in the tree ring data much further back in time.
Over the past 60 years, global warming caused by greenhouse gas emissions has caused El Niño events to become stronger, resulting in hotter summers.
El Niño to continue
The current El Niño event is expected to continue into early summer 2024, making it likely that this summer will break temperature records once again, according to the statement.
“Indeed, the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” said Professor Jan Esper, the lead author of the study from the Johannes Gutenberg University Mainz in Germany.
“When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”
The Northern Hemisphere houses 158 UN-recognised countries —15 of which are equatorial countries that also extend into the Southern Hemisphere.
It has about 50 non-sovereign territories such as Taiwan, Kosovo, and Puerto Rico.
