Humanity has wound back the climate clock 50 million years in just a couple of centuries, according to a new study.
Between 2030 and 2040 our climate will match that of the mid-Pliocene - 3 million years ago - and by 2150, we'll be well on our way to recreating the Eocene, scientists in the US have claimed.
Fifty million years ago temperatures spiked about 13degC hotter than they are now.
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If Earth warms by just 2degC, hundreds of thousands of species could go extinct, according to another global report in October.
"The Arctic was occupied by swampy forests like those found today in the southern US," says the study published in the Proceedings of the National Academy of Sciences.
This assumes greenhouse gas emissions keep rising as they have been in the past few decades.
"In the roughly 20 to 25 years I have been working in the field, we have gone from expecting climate change to happen, to detecting the effects, and now, we are seeing that it's causing harm," said Jack Williams, professor of geography at the University of Wisconsin-Madison.
"People are dying, property is being damaged, we're seeing intensified fires and intensified storms that can be attributed to climate change. There is more energy in the climate system, leading to more intense events."
If significant efforts were made to decrease emissions, such switching to renewable energy, we could turn things around, the study says. Life has also proven to be extremely resilient, it adds.
Prof Williams notes that many countries are moving away from fossil fuels toward more sustainable sources of energy. But more needs to be done, he says.
"We've seen big things happen in Earth's history - new species evolved, life persists and species survive. But many species will be lost, and we live on this planet.
"These are things to be concerned about, so this work points us to how we can use our history and Earth's history to understand changes today and how we can best adapt."
The study findings built upon work Prof Williams first published in 2007, which compared possible future climates to historical climate data from the early 20th Century.