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CAMBRIDGE, United Kingdom — The summer of 2023 will go down in history as more than just a scorcher - it was the hottest summer the Northern Hemisphere has experienced in the last 2,000 years, according to a new study by researchers from the University of Cambridge and Johannes Gutenberg University Mainz. If that's not enough to make you break into a sweat, consider this: 2023 exceeded the natural variations in climate over the past two millennia by nearly a whole degree Fahrenheit.
So, how do scientists know this? After all, reliable instrumental temperature records only go back to about 1850. The answer lies in a surprising source: tree rings. Yes, those concentric circles you see when you cut into a tree trunk are more than just a way to tell a tree's age - they also contain valuable information about past climates.
Each year, trees add a new ring of growth, and the width of these rings can vary depending on the environmental conditions the tree experienced that year. Wider rings generally indicate favorable conditions like ample moisture and warmth, while narrower rings suggest less ideal growing conditions.
By analyzing the patterns in these tree rings, scientists can reconstruct temperature records that stretch back centuries, if not millennia. That's exactly what the researchers did in this study, using a large-scale dataset of tree rings to piece together a picture of Northern Hemisphere summers over the past 2,000 years.
What they found was striking. Even when accounting for natural climate variations over the centuries, the summer of 2023 still stood out as the hottest since the height of the Roman Empire.
“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” says co-author Professor Ulf Büntgen from Cambridge's Department of Geography in a media release. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
The study also revealed that the 1.5°C warming limit set by the 2015 Paris Agreement has already been breached in the Northern Hemisphere. By comparing early instrumental temperature data with their tree ring dataset, the researchers found that the baseline used to contextualize global warming - the average temperature from 1850-1900 - is actually several tenths of a degree Celsius colder than previously thought. When they re-calibrated using this new baseline, they calculated that the summer of 2023 was a staggering 2.07°C warmer (3.7°F) than the average summer temperature from 1850-1900.
“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,” Prof. Büntgen adds, emphasizing the importance of looking at these longer-term climate reconstructions to truly understand the context of our current warming.
The tree ring data also shed light on some of the cooler and warmer periods over the past two millennia. Most of the cooler summers, like the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large volcanic eruptions that spewed sulfur-rich aerosols into the stratosphere, triggering rapid cooling at the surface. In fact, the coldest summer in the past 2,000 years, way back in 536 CE, was a frigid 3.93°C colder (7.07°F) than 2023's summer scorcher.
On the flip side, many of the warmer periods can be linked to the El Niño climate pattern, which affects weather worldwide and often brings hotter summers to the Northern Hemisphere. While El Niño events have been noted by fishermen as far back as the 17th century, the tree ring data allows scientists to track their influence much further back in time.
However, the researchers note that over the past 60 years, El Niño events have been getting stronger due to greenhouse gas-driven global warming, leading to even hotter summers. With the current El Niño event expected to continue into early summer 2024, we may well see temperature records shattered once again.
“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,” says lead author Professor Jan Esper from Johannes Gutenberg University Mainz. “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”
While the study's results are robust for the Northern Hemisphere, the researchers caution that obtaining global averages for the same period is more challenging due to sparse data from the Southern Hemisphere, which also responds differently to climate change given its higher proportion of ocean coverage.
Nevertheless, the message from this groundbreaking study is clear: our planet is heating up at an unprecedented rate, and unless we take swift action to curb greenhouse gas emissions, we can expect many more record-breaking summers in the years to come. As we swelter through the dog days of summer, let's hope this research serves as a wake-up call to policymakers and individuals alike. The time to act is now - before we're all left feeling the heat.
EdNews Editor-in-Chief Steve Fink contributed to this report.
