Researchers have found that the summer of 2023 was the warmest in the Northern Hemisphere in the past two thousand years, with temperatures nearly four degrees Celsius higher than the coldest summer during the same period.
Unprecedented Heat
Although 2023 has been reported to be the hottest year on record, instrumental evidence only goes back as far as 1850 at best, and most records are limited to certain regions.
Past Climate Data and Tree Rings
Now, using past climate information obtained from tree rings at annual resolution over two millennia, scientists from the University of Cambridge and Johannes Gutenberg University Mainz have shown just how exceptional the summer of 2023 was.
Even accounting for natural climate variations over hundreds of years, 2023 remains the hottest summer since the peak of the Roman Empire, exceeding the extremes of natural climate variability by half a degree Celsius.
The Dramaticity of Recent Global Warming
When you look at the long sweep of history, you can see how dramatic recent global warming is. 2023 was an exceptionally hot year, and this trend will continue unless we dramatically reduce greenhouse gas emissions.
Professor Ulf Büntgen, Department of Geography, Cambridge.
Failure to comply with the Paris Agreement
The results, published in the journal Nature, also show that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 1.5°C above pre-industrial levels has already been surpassed.
Early instrumental temperature records, from 1850 to 1900, are sparse and inconsistent. The researchers compared early instrumental data to a large tree-ring dataset and found that the 19th-century temperature baseline used to contextualize global warming is several tenths of a degree Celsius cooler than previously thought. By recalibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07 °C warmer than average summer temperatures between 1850 and 1900.
The Importance of Climate Reconstructions
Many of the conversations we have about global warming are tied to a 19th century base temperature, but why is this the baseline? What is normal, in the context of a constantly changing climate, when we only have 150 years of meteorological measurements? Only when we look at climate reconstructions can we better account for natural variability and contextualize recent anthropogenic climate change.
Ulf Buntgen
Tree rings can provide that context, as they contain annual, absolutely dated information about last summer’s temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.
Impact of Volcanic Eruptions and El Niño
Available tree-ring data reveal that most of the coldest periods of the past 2,000 years, such as the Old Little Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large eruptions. sulfur-rich volcanics. These eruptions expel large amounts of aerosols into the stratosphere, triggering rapid surface cooling. The coldest summer in the last two thousand years, in 536 AD, followed one of these eruptions and was 3.93 °C colder than the summer of 2023.
Most of the warmer periods covered by tree-ring data can be attributed to the El Niño climate pattern, or the El Niño-Southern Oscillation (ENSO). El Niño affects global climate due to the weakening of trade winds in the Pacific Ocean and often results in warmer summers in the Northern Hemisphere. Although El Niño events were first observed by fishermen in the 17th century, they can be observed in tree-ring data much further back in time.
El Niño Amplification and Global Warming
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 once again break temperature records.
It is true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is further amplified by El Niño conditions, so we end up with longer and more severe heat waves and prolonged periods of drought. When you look at the bigger picture, it shows how urgent it is that we reduce greenhouse gas emissions immediately.
Professor Jan Esper, lead author of the study from Johannes Gutenberg University Mainz in Germany.
Limitations of the Study
The researchers point out that although their results are robust for the Northern Hemisphere, it is difficult to obtain global averages for the same period since data is scarce for the Southern Hemisphere. The Southern Hemisphere also responds differently to climate change, as it is much more covered by oceans than the Northern Hemisphere.
The research was supported in part by the European Research Council.
Via www.cam.ac.uk
More information: www.nature.com
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