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2023 was the northern hemisphere’s hottest summer in 2,000 years
While 2023 was already the hottest year on record, new research suggests that Europe hasn’t had as warm a summer since the time of the Romans.
Another climate record may have been broken, as new research suggests that the northern hemisphere had its hottest summer in thousands of years.
By studying tree rings from the past 2,000 years, researchers have revealed that the summer of 2023 was exceptionally hot. A combination of rising greenhouse gas levels and a climate event known as El Niño drove land temperatures to as much as 2.2⁰C higher than they were millennia ago.
Despite regular climatic events, these recent high temperatures are due to global warming. As more greenhouse gases are put into the atmosphere, sweltering summers are likely to become increasingly common.
Professor Jan Esper, the lead author of the study, says, “By putting 2023 in a context stretching back to the year 1 CE, we found that the northern hemisphere’s summer was more than half a degree outside the range of natural variability.”
“This is the continuation of a stepwise trend of warming. I don’t know when the next step will come, but I wouldn’t be surprised if it was in the next 10 to 15 years.”
While the study, published in the journal Nature, only covers the non-tropical regions of the northern hemisphere and not the whole world, the scale of its impacts are concerning. The researchers described the temperature rise as “unparalleled” and called for immediate action to move towards net zero.
Trees form new rings as they grow, providing a record of our planet's past. Image © The Trustees of the Natural History Museum, London.
Telling the temperature with trees
Working out exactly how much the Earth is warming by is a difficult and often controversial business. To start with, scientists need to work out the normal temperature of the planet, a task which is surprisingly hard.
While older measurements were taken when our climate wasn’t as altered, the measuring equipment used wasn’t as precise. There are also disagreements over when human actions started to warm the planet, with different organisations using different years.
For example, the Intergovernmental Panel on Climate Change which advises the United Nations puts the baseline as the average temperature between 1850 and 1900, whilst NASA compares its climate measurements to the years 1951-1980. Despite these differences all of their findings agree that climate change is real and happening right now.
To try and provide greater clarity on these changes, researchers have turned to natural climate records such as tree rings. The width of a ring is related to how much a tree grew in a particular year, which in some species is related to the temperature.
By comparing the pattern of the rings to temperature records from the area, researchers can see what influence climate is having on their growth. Using samples from dead and subfossil trees, the scientists can then use this information to estimate the area’s temperature hundreds – or even thousands – of years ago.
In the current study, the researchers combined just over 10,000 tree ring series from 15 sites in the northern hemisphere to get an average temperature for every summer back to 1 CE. Their findings confirm that the Earth has warmed substantially over the last century, hitting levels that are unprecedented in the common era.
The coldest summers on record, like 536 CE, were all linked to the eruption of volcanoes which can temporarily block out sunlight. Image © Putu Artana / Shutterstock.
History’s hottest summers
Looking back at the past 2,000 years, the team searched for the warmest summers on record to see how they compared to 2023. They found that the hottest June to August in the pre-industrial era was in 246 CE when temperatures were around 0.88⁰C above average.
This record stood for over 1,000 years, before being broken repeatedly since the late 1990s. Over the last 28 summers in the northern hemisphere, 25 were hotter than 246 CE. Of these, 2023 was the hottest to date.
This exceeded the previous hottest summer of 2016 by around a quarter of a degree. It is thought that both of these summers were made warmer by a natural fluctuation in sea temperatures known as El Niño, which is currently predicted to last until the summer of 2024.
If this is the case, then it’s likely that this year will once again break the temperature record.
Counterintuitively, the research also suggests that air pollution may have slightly reduced the impact of increasing greenhouse gases by blocking some of the Sun’s light from reaching Earth and limiting the effects of warming, although its impact on human health and natural world are still incredibly dangerous.
The researchers now want to go further back in time by using preserved wood to look up to 8,000 years into the past, as well as expanding the number of sites they have tree rings from.
However, this work is currently being held up by red tape.
“It can take half a year to get permissions to take tree cores, and even then it’s a question of whether we get it at all,” Jan says. “If we can’t get permission to help understand how our planet is changing, then I think the world is a bit upside down.”
While scientists continue to understand how our planet was in the past, it’s important to put it on a better path for the future. It is still possible to avoid the most damaging impacts of climate change, but only rapid cuts in greenhouse gas emissions will make this hope a reality.
