Our impact on Earth’s climate might be even more profound than we realise. Before we started pumping massive amounts of carbon dioxide into the atmosphere, the planet was on the brink of entering a semi-permanent ice age, two researchers have proposed.

Had we not radically altered the atmosphere, say Thomas Crowley of the University of Edinburgh, UK, and William Hyde of the University of Toronto in Canada, the current cycle of ice ages and interglacials would have given way in the not-too-distant future to an ice age lasting millions of years.

“It’s not proven but it’s more than just an interesting idea,” says Crowley.

For much of the 500 million years or so since complex life evolved, Earth’s climate has been much hotter than it is now, with no ice at the poles. During the last of these “hothouse Earth” phases, from around 100 to 50 million years ago, the Antarctic was covered by lush forests and shallow seas submerged vast areas of America, Europe and Africa.


Oscillating wildly

Since that time, though, CO 2 levels have slowly fallen, possibly due to the rise of the Himalayas. As a result Earth has gradually cooled, with permanent ice sheets starting to form in Antarctica around 30 million years ago and later in the Arctic.

Then, 2.5 million years ago, the climate entered a curious new phase: it started oscillating wildly, see-sawing between interglacial periods with conditions similar to today’s and ice ages during which the amount of permanent ice in the northern hemisphere expanded hugely.

At the peaks of these transient ice ages, much of northern Europe, northern Asia and North America were covered in ice sheets up to 4 kilometres thick, and sea levels were 120 metres lower than today.

From a “deep time” perspective, this ice age-interglacial cycle may be just another brief transitional phase. It has been becoming ever more variable, Crowley says.

Bigger swings

When the cycle began, the climate went from ice age to interglacial and back roughly every 41,000 years. More recently, it has been happening every 100,000 years.

The temperature swings have also become greater: the interglacials have been no warmer but the ice ages have become much colder. So the overall cooling trend was continuing – until the arrival of the Anthropocene, the period in which humans have started to have a major affect on Earth’s climate and ecosystems.

According to a simple climate model developed by Crowley and Hyde, this increasing variability was a sign that the climate was about to flip into a new stable state – a semi-permanent ice age. This ice age might well have lasted for tens of millions of years or more, Crowley says.

In the model runs best resembling actual climate history, the switch to a long-lasting ice age happened as early as 10,000 to 100,000 years from now. However, Crowley stresses that not too much confidence can be placed on the results of single runs out of many.

Hello snowball

The idea of the world becoming locked in an ice age is certainly plausible, says James Zachos of the University of California, Santa Cruz, who studies past climate. It’s not that rare for the climate to switch from one state into another, he says.

And there were extensive and long-lived ice ages during the Carboniferous period, around 300 million years, points out climate modeller Andy Ridgwell of the University of Bristol, UK.

Further back, around 700 million years ago, there was an even colder period known as “Snowball Earth”, when the planet froze over nearly completely.

However, Crowley and Hyde are going to have to do a lot more work to convince their peers. Because of the vast lengths of time involved, they used a very basic model to simplify calculations. “It is not as complex as everyone wants it to be, but you can run it for a very long time,” says Crowley.

Handle with care

None of the researchers contacted by New Scientist thought the model’s predictions are worth taking seriously. It appears to have a bias to forming large and stable ice sheets, says Ridgwell. “So it does not come as a shock that they find a transition point to an even greater ice mass state.”

Still, everyone agrees that it is an intriguing idea. “It is worth delving into deeper,” says Ridgwell.

The idea that humans have averted an ice age may ring a bell with regular New Scientist readers. Climatologist Bill Ruddiman has suggested that Stone Age farmers prevented an ice age by releasing greenhouse gases.

However, the two ideas are quite distinct: Ruddiman thinks that without human intervention we would now be entering another transient ice age like all the previous ones, while Crowley thinks that at some time in the future the whole ice age-interglacial cycle would have ended.

It’s possible they could both be right – or wrong – but we will never know for sure. We have pumped so much carbon dioxide into the atmosphere in little more than a century that levels are higher now than they have been for at least 800,000 years. This will have delayed any switch to a long-lasting ice age indefinitely. “We are probably very comfortably away from it happening now,” Crowley stresses.

Instead, we are putting the planet’s climate on the opposite trajectory, back towards “hothouse Earth” conditions.

Journal reference: Nature (DOI: 10.1038/nature07365)