2023 was reported as the hottest year since detailed weather records began in 1850. Now, a team of scientists who used tree ring data to work out historical temperatures, say that 2023 was, in fact, the hottest summer since the death of Jesus Christ.
The results, reported in the journal Nature, look at a timespan of the last 2,000 years in the Northern Hemisphere and show that the summer of 2023 was almost four degrees warmer than the coldest summer.
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Climate change in context
Using historical climate information gathered from tree ring analysis, scientists from the University of Cambridge and Johannes Gutenberg University Mainz say that even when factoring in natural climate fluctuations spanning centuries, the summer of 2023 was the hottest since the peak 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 the Department of Geography at the University of Cambridge.
“2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
The study suggests that the historical temperature records from 1850 to 1900 are both scarce and inconsistent. Comparing these early instrumental data with an extensive tree ring dataset, researchers discovered that the temperature baseline for the 19th century (which is used to provide context for global warming) is several tenths of a degree Celsius colder than previously believed. Through recalibration of this baseline, they determined that the summer conditions of 2023 in the Northern Hemisphere were 2.07 degrees Celsius warmer than the average summer temperatures recorded between 1850 and 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?” says Büntgen.
“Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”
Using tree rings to map temperature
Tree rings offer an annual map of information about past summer temperatures. Using these chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements, says the paper.
Based on the tree ring records at hand, the majority of cooler periods observed over the last 2,000 years – such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century – coincided with significant volcanic eruptions rich in sulphur. These eruptions emitted substantial volumes of aerosols into the stratosphere, swiftly cooling Earth's surface.
Conversely, most of the warmer periods seen in the tree ring data can be attributed to El Niño, a climate pattern that results in the warming of the ocean surface in the central and eastern tropical Pacific Ocean. El Niño events were first recorded by fishermen in the 17th century, but they can be observed in the tree ring data much further back in time.
However, over the past 60 years, global warming (caused by greenhouse gas emissions) is causing El Niño events to become stronger, resulting in hotter summers. 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.
“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 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 paper acknowledges that acquiring global averages for the same period in the Southern Hemisphere is more difficult as data is sparse and responses to climate change are different.
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