By Stephen Beech via SWNS
Last summer was the hottest in 2,000 years, according to a new study.
Researchers have found that 2023 was the warmest summer in the Northern Hemisphere in the past two millennia – dating back to the height of the Roman Empire.
It was almost four degrees Celsius warmer than the coldest summer during that period, say scientists.
Although 2023 has previously been reported as the hottest year on record, the instrumental evidence only goes back as far as 1850 at best, and most records are limited to certain regions.
Now, by using past climate information from annually resolved tree rings over two millennia, researchers at the University of Cambridge and Johannes Gutenberg University Mainz, Germany, have shown how exceptional the summer of 2023 really was.
Even allowing for natural climate variations over hundreds of years, 2023 was still the hottest summer since the height of the Roman Empire, exceeding extremes of natural climate variability by 0.5 degrees Celsius.
Study co-author Professor Ulf Büntgen, of Cambridge’s Department of Geography, said: “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, published in the journal Nature, also show that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 1.5C above pre-industrial levels has already been breached.
Early instrumental temperature records, from 1850-1900, are sparse and inconsistent.
However, the research team compared early instrumental data with a large-scale tree ring dataset and found the 19th-century temperature baseline used to contextualize global warming is several tenths of a degree Celsius colder than previously thought.
By re-calibrating the baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07C warmer than mean summer temperatures between 1850 and 1900.
Büntgen said: “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?
“Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”
He says that tree rings can provide that context since they contain annually-resolved and absolutely-dated information about previous summer temperatures.
Büntgen explained that using tree-ring chronologies allows scientists to look much further back in time without the uncertainty associated with some early instrumental measurements.
The available tree-ring data reveals that most of the cooler periods over the past 2,000 years – such as the ‘Little Antique Ice Age’ in the 6th Century and the ‘Little Ice Age’ in the early 19th Century – followed large sulfur-rich volcanic eruptions.
Those eruptions spewed huge amounts of aerosols into the stratosphere, triggering rapid surface cooling.
The coldest summer of the past 2,000 years, in 536AD, followed such an eruption and was 3.93C colder than the summer of 2023.
The researchers said that 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 (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.
While El Niño events were first noted by fisherman in the 17th Century, they can be observed in the tree ring data much further back in time.
However, data shows that, over the past 60 years, global warming caused by greenhouse gas emissions is causing El Niño events to become stronger, resulting in hotter summers.
Scientists say the current El Niño event is expected to continue into early summer 2024, making it likely that this summer will smash temperature records once again.
Study lead author Professor Jan Esper said: “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, so we end up with longer and more severe heat waves and extended periods of drought.”
Esper, of Johannes Gutenberg University Mainz, added: “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”
The researchers said it is difficult to obtain global averages for the same period since data is sparse for the Southern Hemisphere.
