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Are we due for another ice age?

Based on previous cycles the Earth is probably due to go into an ice age about now. In fact, conditions were starting to line up for a new ice age at least 6,000 years ago.

"If you look at what was happening prior to the industrial revolution, summers were actually getting colder in the northern hemisphere. They've been getting colder for at least the last 6,000 years, so we were definitely on that trend," Dr Phipps said.

But that trend has now been comprehensively reversed because of greenhouse gas emissions, according to Dr Phipps.

"There's no chance of us going into an ice age now because the greenhouse gases we've put into the atmosphere during the industrial era have warmed the earth."

Although scientists cannot say we have definitely prevented the next ice age, it's certainly accepted that humans have had a significant part to play.

Humans could have just walked from New Guinea to the Australian mainland.Dr Steven Phipps, University of Tasmania.

"There is actually a hypothesis that it's not just industrial society but ever since humans began practicing large scale farming at least 5,000 years ago, such as methane emissions from rice paddies," Dr Phipps said.

"So it's possibly not just greenhouse gas emissions over the last 200 years that's stopped us going into an ice age, but it's actually greenhouse gas emissions for the last 5,000 years that have collectively helped to steer us away from the next ice age."

What would happen if there was an ice age today?

We may have delayed the onset of the next ice age for now, but if another one came it would have pretty big consequences for human civilisation.

Besides the fact it would be an awful lot colder, huge regions where hundreds of millions of people live would become completely uninhabitable. They'd be covered in thick ice sheets and subject to an inhospitable climate.

"Assuming it was similar to the last one, then north America would be covered in ice, the whole of northern Europe, the whole of northern Asia would be covered in ice," Dr Phipps said.

There would be a lot less agricultural land available, so it would be very difficult to support the human population, Dr Phipps warned.

And the physical shape of the continents would look completely different across the whole planet.

A huge drop in sea level of up to 120 metres would close down marine channels - the Mediterranean Sea, Torres Strait, Bass Strait and Bering Strait - and create new areas of land that could be used for habitation or agriculture.

Ocean ports would no longer be on the ocean, and anyone wanting water views would need to relocate large distances.

What we have the last ice age to thank for...

Ice ages have had an absolutely enormous impact on human evolution.

During the last ice age, which ran from about 110,000 years ago to 10,000 years ago, the lower sea levels allowed humans to move out across the entire world.

"There was no Bering Straits, so north America and Asia were joined and that's actually how humans first roamed into the Americas, they just walked over the land bridge," Dr Phipps said.

While there was still some water between Asia and Australia it took just a few short canoe trips to bring the first humans to Australasia.

"They would have come over towards New Guinea. There was no Torres Strait so humans could have just walked from New Guinea to the Australian mainland. And there was no Bass Strait so humans could have walked from the Australian mainland over to Tasmania," he said.

The whole dispersal of humans around the world during the last 100,000 years was made entirely possible by the fact we were in an ice age at the time.

How do we know they happened in the past?

It's a fair question - how can we know so much about these major events in the past? Scientists have a variety of methods they use.

Evidence for the more recent ice ages comes from changing sea levels in the past, which can be seen by looking at coral reefs or modern landscapes.

The Quaternary glaciation

• Compared to conditions on Earth 20,000 to 30,000 years ago we are clearly not in an ice age now

• But in terms of the long history of the Earth we are actually still in an overarching ice age period - known as the Quaternary glaciation - which has been going for the last 2.6 million years. At the moment, the Earth is just in a slightly warmer period, an interglacial.

• There have been at least four other overarching ice ages in the Earth's history, the oldest started about 2,400 million years ago.

"That's how they first pieced together the evidence for glacial cycles. Looking at corals, and coral reefs and evidence of past sea level changes in the tropics, they saw there was a cycle of changing sea levels," Dr Phipps said.

Ice core records also provide invaluable information on changes in temperature and greenhouse gases over the last 800,000 years.

But going back further into the past, evidence for ice ages in the last tens of millions of years is predominantly seen in ocean sediments.

"If you go out into the open ocean you can drill a core down through the sediments into the ocean bed and that can take you back tens of millions of years," Dr Phipps said.

And for the deep time ice ages that occurred tens to hundreds of millions of years ago, scientists use the geological record where the story of sea level and climate can be unravelled by analysing rocks of various ages.