Over the past 150 years, human-caused global warming has erased the natural global cooling that occurred over the previous 6,500 years, according to a study published Tuesday.
Scientists reconstructed the global average temperature over the past 12,000 years to reach their conclusion. They studied "paleoclimatic" sources from around the world – such as lake deposits, marine sediments, peat and glacier ice – to infer past temperature changes.
"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," he said.
The rate of cooling that followed the peak warmth was subtle, only around 0.1 degree Celsius per 1,000 years, according to assistant research professor and study co-author Michael Erb, also of Northern Arizona University, who analyzed the temperature reconstructions.
The cooling seems to be driven by slow cycles in the Earth's orbit around the sun, which reduced the amount of summer sunlight in the Northern Hemisphere, culminating in the "Little Ice Age" of recent centuries, Erb said.
Since the mid-19th century, global warming has climbed about 1 degree Celsius (1.8 degrees Fahrenheit), suggesting that the global average temperature of the last decade (2010-2019) was warmer than any time during the past 12,000 years.
"It's possible," Kaufman said, "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."
The study is important because by tracking global temperatures over the past 12,000 years, researchers can better forecast future climate changes, scientists say.
"Our future climate will largely depend on the influence of human factors, especially the build-up of greenhouse gases," said study co-author Cody Routson, also an assistant research professor at Northern Arizona University.
"However, future climate will also be influenced by natural factors, and it will be complicated by the natural variability within the climate system. Future projections of climate change will be improved by better accounting for both anthropogenic (human-caused) and natural factors."
The study was published in Nature Research's Scientific Data, a peer-reviewed journal.
This article originally appeared on USA TODAY: Global warming has wiped out 6,500 years of cooling, study says