Climate records are crashing at a breakneck pace. The world has just experienced its hottest day on record, amid a string of record-breaking months that followed the planet’s hottest year on record. But how does this cascade of new highs in the era of modern record-keeping relate to Earth’s deeper history?
Those who reconstruct what the climate was like in the past, in eras before thermometers and satellites – a practice known as paleoclimatology – find that today’s temperatures, when examined closely, are nothing special. For example, the Eocene, an epoch that lasted from 56 million years ago to 34 million years ago, was “screamingly hotter” than today, by about 10-15 degrees Celsius, according to Matthew Huber, an expert on historical climates at Purdue University in the US.
But, crucially, in the time span in which humans evolved and formed organised societies, the current global climate – just over 1C warmer on average than it was in the pre-industrial period, before humans began burning vast amounts of fossil fuels – is unprecedented. It has not been this hot for at least 125,000 years, before the last ice age, and probably for much longer, possibly at least 1 million years ago.
“Humans have never experienced a climate like this in our long history; we are starting to reach temperatures that are unprecedented,” Huber said. “It’s not that we’re all going to go extinct, but we’re messing with a thermostat that [us] outside a window we have been sitting in throughout human civilization.”
The Earth has experienced numerous climate fluctuations in its long history, punctuated by ice ages, but fortunately for humanity, conditions have been relatively stable for the past 10,000 years, a kind of Goldilocks zone. The pleasant temperatures and stable coastlines have allowed humanity to flourish, developing large coastal cities, highways and ploughed tracts of fertile farmland.
“The climate stabilized, people could settle in one place and civilization emerged,” Huber said.
But we are now being torn out of our epoch, the Holocene – though some scientists prefer a new term, the Anthropocene. With global temperatures 1.5C warmer than pre-industrial times, it looks more like the climate of the Pleistocene, a time of woolly mammoths and giant sloths until 2.5 million years ago. If we extend that a bit further, to 3C warmer, which could happen this century if emissions are not curbed quickly, it will be in a similar territory to the Miocene, which began about 23 million years ago.
This is a disturbing comparison, because in the past, sea levels were tens of meters higher than they are now, there was little ice at the poles, and the fauna and flora were completely different. For example, there were few grass species used to produce crops such as corn and wheat, which billions of people now depend on for food.
Even the question of whether temperatures are the highest in 1,000 years or 1 million years is almost irrelevant when you consider how new the infrastructure people rely on is – sewers that are 50 years old and have to handle extreme rainfall that was never anticipated at the time, for example.
“There is no perfect temperature for the Earth, but there is one for us humans,” as Katharine Hayhoe, a leading climate scientist at the Nature Conservancy, put it. “We are perfectly adapted to our current circumstances. Two-thirds of the world’s largest cities are within a meter of sea level.
“What happens when sea levels rise a meter or more, as is likely to happen this century? We can’t pick up Shanghai, London or New York and move them. Most of our arable land has already been carefully allocated and cultivated.”
Scientists who study past climates — by analyzing tree rings, deep ice cores, ocean sediments and other evidence and then reconstructing conditions — say what’s even more remarkable than the temperature itself is how quickly it has changed.
During a period called the Paleocene-Eocene Thermal Maximum, which occurred about 55 million years ago, temperatures jumped by at least 5 degrees Celsius as carbon dioxide poured into the atmosphere—but this change occurred over thousands of years. In contrast, the modern world has warmed by more than 1 degree Celsius in just over a century.
“A hundred years or so is less than the blink of an eye in Earth’s history,” said Lina Pérez-Angel, a paleoclimatologist at Brown University. “There’s nothing in Earth’s history that shows a change happening that fast, it’s just happening so, so fast. Usually these changes take a long time, things can adapt. Right now, the rate of change is one of the biggest concerns we have.”
It’s “hard to find analogs” where the rate of change has been this fast, says Jason Smerdon, a climate scientist at Columbia University. “If the rate of temperature change after an ice age is comparable to a pedestrian walking down the street, then the rate of change for us to get to 3C of warming by 2100 is comparable to a car traveling at least 100 mph,” Smerdon says.
Another departure from the past is the reason for the temperature change. Volcanic activity, proximity to the sun, and other factors have influenced climate change in the past, but a major means of temperature control is the release and absorption of carbon dioxide, a heat-trapping gas.
In the past, natural forces sucked carbon up into oceans and forests, or released it in long pulses, shrinking or growing ice sheets and affecting sea levels. But now, for the first time, a single species is radically and rapidly changing the amount of carbon released by burning oil, coal and gas, as well as by deforestation.
“The long-term burial of carbon changes on long timescales, but humans have reversed natural processes,” Huber said. “We are now digging up carbon and oxidizing it. We are essentially digging up ancient global warming.”
The last time carbon dioxide levels were this high, causing the atmosphere and oceans to warm, was about 3 million years ago. Before about 800,000 years ago, atmospheric concentrations of CO2 was never more than 300 ppm – that percentage is now well above 400 ppm.
According to experts, all of this should be cause for urgent action.
“The change [in global temperatures] “It’s not a surprise,” Smerdon said. “What is a surprise is that we continue to do this without acting in an emergency to address the challenge. This is within our control. It’s a bit like hitting yourself in the face with a hammer — you can choose to stop doing that.”