Arctic permafrost vanished in the past - will it happen again?

Long before modern climate records began, a very different Earth existed – one where the icy edges of the Arctic were free of permafrost. This wasn’t a minor shift. It was a Northern Hemisphere without frozen ground, stretching all the way to the Arctic Ocean. That past could be a preview of our future.

A new study shows that when Earth’s average temperature was just 4.5˚C warmer than it is today, the Asian continent had no permafrost – not even in its far north. This extent of climate warming is within the range we could reach again in the near future if emissions remain unchecked.

Researchers from Northumbria University, the University of Oxford, Bern University, and the Geological Surveys of Israel and the U.S. worked together to trace the ancient climate shift. They found their evidence in an unlikely place: inside caves beneath the Siberian tundra.

Frozen clues hidden underground

The team focused on cave formations known as speleothems – stalagmites and stalactites – which only grow when water can seep through the ground. In regions locked in permafrost, the soil stays frozen solid. That makes speleothem growth impossible.

So, if these formations exist in caves beneath modern-day permafrost, it means the ground had to be thawed when they formed.

The researchers collected more than 60 mineral samples from caves in the Lena River delta, one of the coldest and most remote areas of northeastern Siberia.

Using uranium-lead dating, a precise method for measuring tiny traces of radioactive decay, the team determined that the cave minerals had formed about 8.7 million years ago, during the late Miocene period.

This means the ground in that region was thawed at the time. Existing global temperature records show that Earth was 4.5˚C warmer than it is today.

The fate of Arctic permafrost

If Earth warms by that same amount again, the findings suggest that all but the deepest or highest-altitude permafrost would melt.

“Our findings provide direct quantitative evidence that if our climate warms by 4.5˚C, the permafrost currently covering Canada, Siberia, Mongolia, America – in fact much of the Northern Hemisphere – would thaw. Only permafrost in high mountains and deep underground would survive,” said Dr Sebastian Breitenbach of Northumbria University.

“This thaw would release billions of tonnes of carbon from the ground into the atmosphere, enhancing further warming. This finding is a real warning to us all. It shows how sensitive our climate system is and where we might be headed if we don’t act to limit our climate emissions now.”

What makes this especially concerning is the sheer volume of carbon at risk. Scientists estimate that up to 130 billion tonnes are locked in today’s permafrost. If that carbon is released as the ground thaws, it won’t just add to the warming – it will accelerate it.

When the Arctic was permafrost-free

“After much searching, we were fortunate to find well-preserved datable cave deposits in the heart of today’s Siberian permafrost,” said Dr Anton Vaks of the Geological Survey of Israel.

“We can see that this present-day tundra region experienced a warmer climate, with mean annual global temperatures above 0°C and with permafrost-free conditions.”

According to Dr. Vaks, the evidence from the cave formations indicates that most of the Siberian landmass and likely similar regions in the Northern Hemisphere were permafrost-free when the deposits formed at Taba-Ba’astakh.

Caves as climate time machines

Professor Gideon Henderson of the University of Oxford noted that caves are like time machines.

“They capture a history of the climate and environment for millions of years of Earth history, which we can now read accurately using precise chemical analyses,” said Henderson.

“By doing so, we can predict the future, using past conditions as an analogue for the future to understand the impact of the warmer world we are heading into.

“This new study provides valuable new constraints on the magnitude of warming required to completely destroy permafrost in the northern hemisphere and remove one of the biggest continental stores of carbon.”

The full study was published in the journal Nature Communications.

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