Global warming has erased 6,500 years of cooling
Temperatures have increased by 1 degree Celsius since the mid-1800s, meaning they are higher now than at any point in the last 12,000 years, researchers say. Photo by Pexels/Pixabay

June 30 (UPI) -- New paleoclimate research suggests the last 150 years of global warming have erased 6,500 years of cooling.

For the study, an international team of scientists applied a variety of statistical models to paleoclimate data sets in order to reconstruct global temperature averages during the Holocene Epoch, the period that followed the last ice age and began roughly 12,000 years ago.

The analysis -- published Tuesday in the journal Scientific Data -- showed temperatures peaked in the middle of the Holocene, roughly 6,500 years ago, topping out at 0.7 degrees above the mid-19th century temperature average.

"Before global warming, there was global cooling," lead study author Darrell Kaufman, a professor of paleoclimatology at Northern Arizona University, said in a news release. "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."

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"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," he said.

Kaufman was the lead author a study published early this year that catalogued the most extensive set of Holocene paleoclimate data yet assembled.

At 659 different sites around the globe, Kaufman and his research partners recorded hundreds of measurements of both marine and terrestrial samples, including lake deposits, marine sediments, peat and glacier ice.

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Illustration by Victor O. Leshyk/Northern Arizona University

For the new study, researchers used several statistical methods to synthesize the global data sets, revealing the rates of warming and then cooling that characterized the post-glacial period.

"The rate of cooling that followed the peak warmth was subtle, only around 0.1 degrees Celsius per 1,000 years," said study co-author Michael Erb, an assistant research professor at Northern Arizona.

"This cooling seems to be driven by slow cycles in the Earth's orbit, which reduced the amount of summer sunlight in the Northern Hemisphere, culminating in the 'Little Ice Age' of recent centuries," Erb said.

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Since the middle of the 19th century, global temperatures have increased by 1 degree Celsius, which means temperatures are higher today than at any time during the last 12,000 years.

"It's possible that the last time the sustained average global temperature was 1 degree Celsius 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.

By tracing the rise and fall of pre-industrial temperatures during the Holocene, scientists can more accurately predict the future of climate change, researchers say.

"Our future climate will largely depend on the influence of human factors, especially the build-up of greenhouse gases," said co-author and assistant research professor Cody Routson.

"However, future climate will also be influenced by natural factors, and it will be complicated by the natural variability within the climate system," Routson said. "Future projections of climate change will be improved by better accounting for both anthropogenic and natural factors."