In the past 150 years, global warming has more than undone the global cooling that occurred over the past six millennia.
The findings show the millennial-scale global cooling began approximately 6,500 years ago when the long-term average global temperature topped out at about 0.7 degrees C warmer than the mid-19th century. Since then, accelerating greenhouse gas emissions have contributed to global average temperatures that are now surpassing 1 degree C above the mid-19th century.
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 throughout the world to reconstruct the global average temperature over the Holocene Epoch—the period following the Ice Age, beginning about 12,000 years ago.
“Before global warming, there was global cooling,” Kaufman said. “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.”
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