Ice ages, they come and go. Until they don’t.

Humans have pumped so much carbon dioxide into the atmosphere we’ve overridden the natural and astronomical factors that cause ice ages to occur. 

“So this is a big deal, as a species. Through polluting and changing the level of greenhouse gasses, we have essentially knocked our climate system off its natural pattern,” says paleoclimatologist Professor Tim Naish from Victoria University of Wellington in New Zealand.

“If we keep carbon dioxide above 350 parts per million (ppm) … and if it stays there long enough, then we can no longer go into a natural ice age,” he says.

For around three million years, ice ages have occurred in regular cycles triggered by astronomical factors – variations in radiation from the sun and changes in the Earth’s orbit – as well as tectonic activity and fluctuations in carbon dioxide.

But now, says Naish, if greenhouse gas concentrations in the atmosphere remain at current levels – around 415 ppm – it’s virtually certain that we can no longer go back to a natural ice age. 

Scientists have been able to reconstruct the pattern of the Earth’s climate and its ice ages going back almost 50 million years, Naish says.

In the 1940s, Serbian astrophysicist and mathematician, Milutin Milankovich, developed a theory about how Earth’s orbit around the Sun changes on long timescales, controlling variations in solar radiation on Earth and affecting its climate.

In the 1970s, scientists, including geologist Sir Nicholas Shackleton, pioneered a process of drilling the ocean floor and taking sediment cores – covering millions of years – to analyse something called the ‘oxygen isotopic composition’ of the microfossils in the sediment. This works as a proxy for the volume of ice on the planet at different points in history. And it turns out those records align with Milankovich cycles. 

“So the combination of what we call the orbital forcing, the changes in Earth’s orbit around the sun, and the different concentrations of carbon dioxide in the atmosphere, we think we can now explain Earth’s climate going back 50 million years. 

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Read more: A barcode of Earth’s climatic past

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“Which is really important, because we need to understand the natural pattern, the natural variability before we can extract, what we as humans are doing on top of that natural climate variability,” Naish says.

Learning about past climate can help scientists understand the future effects of global warming, he says. 

For instance, roughly 125,000 years ago during the last interglacial period, when the Earth was naturally 1.5 – 2 degrees warmer than today, sea levels were a lot higher. “We know there are fossil coral shorelines that are six to nine metres higher,” he says. 

“You have to go back 3 million years when geological records show us, for the last time CO₂ in the atmosphere was 400ppm. And then, sea levels were 20 metres higher.”

In contrast, Naish says, during the last ice age, sea levels were around 120 metres lower, “because all the water that’s currently in the ocean was on land, forming ice sheets”.

Carbon dioxide is a very important greenhouse gas that’s currently in the driver’s seat, controlling our climate, he says. 

“This is why the 1.5^oC target, the Paris Agreement, target is so important. Because the science tells us if we get above 1.5 degrees, or closer to two degrees, we may cause irreversible melting of both the Greenland and parts of the West Antarctic Ice Sheet and be committed to multi-metre sea level rise.”

For more listen to “Which climate extreme would you rather – an ice age or a very warm interglacial?”

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