In little more than 150 years, humans have reversed a 6,000-year pattern of global cooling, a major study into historic temperatures has found.
A 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.
The findings show that global cooling began around 6,500 years ago, when the long-term average global temperature topped out at around 0.7C warmer than the mid-19th century - just as the Industrial Revolution, which had begun in Britain, began to take hold across Europe and America.
The period before industrialisation 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 industrialised economies have contributed to global average temperatures 1C above the mid-19th century.
Northern Arizona University's School of Earth and Sustainability’s Regents’ professor Darrell Kaufman, lead author of the study, said: “Before global warming, there was global cooling.”
“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.”
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.
At each site, researchers analysed ecological, geochemical and biophysical evidence from both marine and terrestrial archives, such as lake deposits, marine sediments, peat and glacier ice, to work out past temperature changes. Countless scientists, working around the world over many decades, conducted the basic research that contributed to the global database.
Assistant research professor Michael Erb, who analysed the temperature reconstructions, said: “The rate of cooling that followed the peak warmth was subtle, only around 0.1C per 1,000 years. 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.”
The average global temperature has increased by just over 1C (2F) since 1880, suggesting the global average temperature of the last decade (2010-2019) was warmer than anytime during the post-glacial period.
The study’s co-author, associate professor Nicholas McKay, noted that some individual decades are not resolved in the 12,000-year-long temperature reconstruction, making it difficult to compare it with any recent decade.
He said: “On the other hand, this past decade was likely cooler than what the average temperatures will be for the rest of this century and beyond, which are very likely to continue to exceed 1C above pre-industrial temperatures.”
Professor Kaufman added: “It's possible 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.”
Professor Cody Routson said the study helps scientists quantify the processes causing the climate crisis.
He said: “Investigating the patterns of natural temperature changes over space and time helps us understand and quantify the processes that cause climate to change, which is important as we prepare for the full range of future climate changes due to both human and natural causes.
“Our future climate will largely depend on the influence of human factors, especially the build-up of greenhouse gases. 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 and natural factors.”
The research is published in Nature Research’s journal Scientific Data.