The summer of 2023 was hotter than any other in the Northern Hemisphere for the past two millennia, according to a study published Tuesday in Nature. And as scorching as 2023 was, the coming summer could be even hotter - largely because of manmade climate change heating the planet, compounded by an El Niño weather cycle.
Scientists previously determined that 2023 was the hottest year since 1850, when global modern temperature records began. The researchers managed to establish a 2,000-year record by combining instrumental measurements with climate reconstructions. They found the extreme warmth of last summer not only smashed modern records but also exceeded the warmest summer prior to the instrumental record - in the year 246 - by more than half a degree Celsius, with almost all natural climate variations taken into account. And it was almost 4C warmer than the coldest summer (in 536).
A pedestrian in Paris wears a head covering to shield from the sun at a 'Paris Plage' temporary beach, Aug. 14, 2023.
"When you look at the long sweep of history, you can see just how dramatic recent global warming is," Ulf Büntgen, a study co-author from the University of Cambridge in the UK, said in a statement. "2023 was an exceptionally hot year, and this trend will continue unless we dramatically reduce greenhouse gas emissions."
Büntgen and his colleagues limited their analysis to the landmasses between the 30th parallel north and the North Pole, because that's where most of the world's long-standing meteorological stations are. They also reconstructed historical climate conditions in this zone by studying thousands of tree rings from nine regions in the Northern Hemisphere. Weather influences how trees form the layers of wood in their trunks, so tree rings contain key clues to past temperatures. Given the strong correlation between tree rings and summer temperatures, the researchers focused on June through August.
They found a lack of consistency between tree-ring-enabled climate reconstructions and instrument-based measurements during the second half of the 19th century, raising the question of whether older thermometers produced inaccurately high temperature readings. The consequence of that, according to the researchers, is a "systematic warm bias" in early instrumental observations, which are used widely as the baseline for global climate science.
Pedestrians hold umbrellas for protection from the sun during a heat wave in New York, July 27, 2023.
The tree-ring data also reveals that most of the cooler periods over the past 2,000 years came after major volcanic eruptions, which spewed huge amounts of aerosols into the stratosphere and triggered rapid surface cooling. Meanwhile, most of the warmer periods can be attributed to El Niño, one of three phases of a multi-year climate cycle known as the El Niño-Southern Oscillation that disrupts weather patterns worldwide and typically drives up summer temperatures in the Northern Hemisphere.
El Niño is a natural climate phenomenon, but scientists say global warming caused by burning fossil fuels and other human activities is intensifying its strength. That, in turn, creates more extremely hot summers.
An El Niño phase began in June 2023 and is ongoing, though expected to end in coming weeks.
"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," said Jan Esper, the study's lead author and a professor of climate geography at Johannes Gutenberg University Mainz in Germany.
The study notes that the 2015 Paris Agreement goal to limit temperature rises to 1.5C above pre-industrial levels "has already been superseded" in the Northern Hemisphere. Although the conclusion cannot be applied on a global scale, as the warming rate varies among high and low latitudes and from land to sea surfaces, the research findings "clearly demonstrate the unparalleled nature of present-day warmth at large scales," the authors write.
It also reaffirms what some climate scientists have warned: As climate change is amplified by an El Niño, 2024 will likely see temperature records broken again. In recent weeks, exceptional heat waves have baked many countries across Asia, with Myanmar experiencing its hottest-ever April temperature of 48.2C.
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