The findings show that the millennial-scale global cooling began approximately 6,500 years ago when the long-term average global temperature topped out at around 0.7°C warmer than the mid-19th century
Over the past 150 years, global warming has more than undone the global cooling that occurred over the past six millennia, according to a major study published on Tuesday in Nature Research’s Scientific Data, “Holocene global mean surface temperature, a multi-method reconstruction approach.”
The findings show that the millennial-scale global cooling began approximately 6,500 years ago when the long-term average global temperature topped out at around 0.7°C warmer than the mid-19th century - just as the Industrial Revolution, which had begun in Britain, began to take hold across Europe and America.
Since then, accelerating greenhouse gas emissions have contributed to global average temperatures that are now surpassing 1°C above the mid-19th century.
The team of researchers reconstructed the global average temperature over the Holocene Epoch - the 12,000 years of Earth's history since the last Ice Age.
Four researchers of Northern Arizona University’s School of Earth and Sustainability (SES) led the study, with Regents’ Professor Darrell Kaufman as lead author and Associate Professor Nicholas McKay as co-author, along with Assistant Research Professors Cody Routson and Michael Erb.
The team worked in collaboration with scientists from research institutions all over the world.
“Before global warming, there was global cooling,” said Kaufman.
“Previous work has shown convincingly that the world naturally and slowly cooled for at least 1,000 years prior to the middle of the 19th century, when the global average temperature reversed course along with the build-up of greenhouse gases.
This study, based on a major new compilation of previously published paleoclimate data, combined with new statistical analyses, shows more confidently than ever that the millennial-scale global cooling began approximately 6,500 years ago.”
The period before industrialization represented the lowest global temperatures since the last Ice Age, culminating in a so-called “little ice age” in recent centuries, the study found.
Since then, increasing greenhouse gas emissions from industrialized economies have contributed to global average temperatures 1°C above the mid-19th century, suggesting that the global average temperature of the last decade (2010-2019) was warmer than anytime during the present post-glacial period.
“That the last time the sustained average global temperature was 1°C above the 19th century was prior to the last Ice Age, back around 125,000 years ago when sea level was around 20 feet higher than today,” Kaufman said.
Earlier this year, an international group of 93 paleoclimate scientists from 23 countries - led by Professor Kaufman and his team - published the most comprehensive paleoclimate data ever compiled on the past 12,000 years, compressing 1,319 data records, based on samples taken from 679 sites globally.