How does today’s extreme heat compare with Earth’s past climate?

Climate records are tumbling at a galloping pace. The world has just experienced its hottest ever single day on record, amid a string of record-breaking months that followed the planet’s hottest recorded year . But how does this cascade of new highs in the era of modern record-keeping compare with the Earth’s deeper history?

Those who piece together what past climates were like in eras before thermometers and satellites – a practice known as palaeoclimatology – find that today’s temperatures are, when narrowly viewed, unremarkable. For example, the Eocene, an epoch lasting from 56m years to 34m years ago, was “screamingly hotter” than today, by about 10-15C, according to Matthew Huber, an expert in historical climates at Purdue University in the US.

But, crucially, in the timespan in which humans evolved and formed organised societies, today’s global climate – a bit more than 1C hotter on average than it was in the preindustrial period before people started burning huge quantities of fossil fuels – is unparalleled. It has not been as hot as this for at least 125,000 years, prior to the last ice age, and most likely longer, potentially going back at least 1m years.

“Humans have not faced a climate like this over our long history; we are starting to hit temperatures that are unprecedented,” said Huber. “It’s not like we will all become extinct, but we are messing with a thermostat that is pushing [us] outside a window we have been in during all of human civilisation.”

The Earth has had numerous climate fluctuations marked by ice ages over its long history but, fortunately for humanity, for the past 10,000 years or so conditions have been relatively stable, a sort of Goldilocks zone. The agreeable temperature and stable coastlines have allowed humanity to flourish, unfurling great coastal cities, highways, and ploughing tracts of fertile farmland.

“The climate settled on an even keel, people could settle in one place and civilisation started,” Huber said.

But we are now being wrenched from our era, the Holocene – though some scientists prefer a new term, the Anthropocene. As the global temperature approaches 1.5C hotter than preindustrial times, it is more resembling the climate of the Pleistocene, a time of woolly mammoths and giant sloths up to 2.5m years ago. Push it a bit more, to 3C hotter, which could happen this century if emissions are not rapidly curbed, and it will enter comparable territory to the Miocene , which started about 23m years ago.

This is an unsettling analogy as these past conditions had sea levels tens of metres higher than today, with little ice at the poles and completely different fauna and flora, including few of the types of grasses that form the crops such as corn and wheat that billions of people now depend on for food.

Also, whether temperatures are the highest in 1,000 years or 1m years is almost moot when one considers the newness of the infrastructure people rely on – sewers that are 50 years old having to cope with extreme rainfall never foreseen at the time, for example.

“There is no one perfect temperature for the Earth, but there is for us humans,” as Katharine Hayhoe, a leading climate scientist at the Nature Conservancy, has put it . “We are perfectly adapted to our current conditions. Two-thirds of the world’s largest cities are located within a metre of sea level.

“What happens when sea level rises a metre or more, as it’s likely to this century? We can’t pick up Shanghai or London or New York and move them. Most of our arable land is already carefully allocated and farmed.”

Scientists who study past climates – from analysing tree rings, deep ice cores, ocean sediments and other evidence and then reconstructing the conditions – say what is even more remarkable than the temperature itself is how quickly it has changed.

During a period called the Palaeocene-Eocene thermal maximum, which occurred about 55m years ago, temperatures jumped by at least 5C as carbon dioxide flooded into the atmosphere – but this change unfolded over thousands of years. By contrast, the modern world has heated up by more than 1C in little more than a century.

“A hundred years or so is less than a blink of an eye in Earth’s history,” said Lina Pérez-Angel, a palaeoclimatologist at Brown University. “There’s nothing in Earth’s history that shows a change happening this quickly, it’s just so, so fast. Usually these changes take a long time, things can adapt. Right now the pace of change is one of the biggest concerns we have.”

It is “hard to find analogues” where the rate of change has been this fast, says Jason Smerdon, a climate scientist at Columbia University. “If the pace of temperature change coming out of an ice age is like a pedestrian walking on the street, then the pace of change for us getting to 3C warming by 2100 would be like a car passing by at least 160mph,” Smerdon said.

Another departure from the past is the reason for the temperature change. Volcanic activity, the proximity of the sun and other factors have influenced past climate change, but a major means of temperature control has been the release and absorption of carbon dioxide, a heat-trapping gas.

Previously, natural forces have caused carbon to be sucked up by the oceans and forests, or released in long pulses, causing the ice caps to shrink or grow and influencing sea levels. But now for the first time a single species is radically and rapidly reshaping the amount of carbon being released through the burning of oil, coal and gas, as well as deforestation.

“The long-term burial of carbon changes on long timescales but humans have reversed natural processes,” said Huber. “We are now digging up carbon and oxidising it. We are basically digging up old global warming.”

The last time carbon dioxide levels were this high, causing the atmosphere and oceans to heat up, was about 3m years ago. Prior to about 800,000 years ago the atmospheric concentration of CO ₂ was never more than 300 parts per million – that rate has now sailed well beyond 400ppm .

All of this should compel urgent action, say experts.

“The change [in global temperatures] isn’t a surprise,” said Smerdon. “What is a surprise is that we’re continuing to do this without acting in an emergency to address the challenge. This is within our control. It’s a bit like if you are hitting yourself in the face with a hammer – you can choose to stop doing that.”