The global warming experienced by the planet over the past 150 years has undone all the global cooling that happened over the past six millennia, according to a new study. The findings reinforce the role of mankind in climate change and the urgency to take action to avoid its worst consequences.
The millennial-scale cooling of the planet 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, the researchers argued – just as the Industrial Revolution, which had begun in Britain, began to take hold across Europe and America.
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 1ºC above the mid-19th century.
Researchers from Northern Arizona University’s School of Earth and Sustainability (SES) led the study and worked with scientists from research institutions all over the world, reconstructing the global average temperature over the Holocene Epoch. This was the period following the Ice Age and beginning about 12,000 years ago.
“Previous work showed that the world naturally and slowly cooled for 1,000 years prior to the middle of the 19th century, when temperatures reversed course due to growing greenhouse gases,” said Darell Kaufman, led author, in a statement. “This study shows more confidently than ever that the millennial-scale global cooling began approximately 6,500 years ago.”
An international group of 93 paleoclimate scientists from 23 countries, including Kaufman and his team, published earlier this year the most comprehensive set of paleoclimate data ever compiled for the past 12,000 years, compressing 1,319 data records based on samples taken from 679 sites globally.
At each site, the researchers analyzed ecological, geochemical and biophysical evidence from both marine and terrestrial archives, such as lake deposits, marine sediments, peat and glacier ice, to infer past temperature changes.
“The rate of cooling that followed the peak warmth was subtle, only around 0.1°C 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,” said Michael Erb, co-author who analyzed the temperature reconstructions.
Since the mid-19th century, global warming has climbed to about 1°C, suggesting that the global average temperature of the last decade (2010-2019) was warmer than at any time during the present post-glacial period. Nicholas McKay, a co-author, said individual decades are not resolved in the 12,000-year-long temperature reconstruction, making it difficult to compare it with any recent decade.
“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 1°C above pre-industrial temperatures,” McKay said.
The researchers argued that investigating the patterns of natural temperature changes over space and time helps understand and quantify the processes that cause the climate to change, which is important as the world prepares for the full range of future climatic shifts due to both human and natural causes.
Countries have agreed in the Paris Agreement in 2015 to avoid temperature increase past 2ºC degrees. Nevertheless, the plans so far presented are far from ambitious and put the world in a 3ºC to 4ºC global warming trajectory.