Image caption Dinosaurs once foraged beneath the Southern Lights in Antarctica

It may be hard to believe, but Antarctica was once covered in towering forests.

One hundred million years ago, the Earth was in the grip of an extreme Greenhouse Effect.

The polar ice caps had all but melted; in the south, rainforests inhabited by dinosaurs existed in their place.

These Antarctic ecosystems were adapted to the long months of winter darkness that occur at the poles, and were truly bizarre.

But if global warming continues unabated, could these ancient forests be a taste of things to come?

One of the first people to uncover evidence for a once greener Antarctic was none other than the explorer, Robert Falcon Scott.

Toiling back from the South Pole in 1912, he stumbled over fossil plants on the Beardmore Glacier at 82 degrees south. The extra weight of these specimens may have been a factor in his untimely demise.

Yet his fossil discoveries also opened up a whole new window on Antarctica's sub-tropical past.

Forests in the frost

Professor Jane Francis of the University of Leeds is an intrepid explorer who has followed in Scott's footsteps. She has spent 10 field seasons in Antarctica collecting fossil plants and received the Polar Medal from the Queen in 2002.

"I still find the idea that Antarctica was once forested absolutely mind-boggling", she told the BBC.

"We take it for granted that Antarctica has always been a frozen wilderness, but the ice caps only appeared relatively recently in geological history."

One of her most amazing fossil discoveries to date was made in the Transantarctic Mountains, not far from where Scott made his own finds.

She recalled: "We were high up on glaciated peaks when we found a sedimentary layer packed full of fragile leaves and twigs."

These fossils proved to be remains of stunted bushes of beech. At only three to five million years old, they were some of the last plants to have lived on the continent before the deep freeze set in.

Image caption Annual rings in fossil wood reveal Antarctica’s subtropical past

However, other fossils show that truly subtropical forests existed on Antarctica during even earlier times. This was during the "age of the dinosaurs" when much higher CO2 levels triggered a phase of extreme global warming.

"Go back 100 million years ago and Antarctica was covered in lush rainforests similar to those that exist in New Zealand today," said Dr Vanessa Bowman who works with Francis at the University of Leeds.

"We commonly find whole fossilised logs that must have come from really big trees."

Professor Francis has been polishing thin slices of these logs to reveal the "annual rings" in the wood. Studying these tree-rings sheds light on ancient climate.

Dark secrets

Possibly the weirdest and most baffling feature of the polar forests was their adaptation to the Antarctic "light regime". Near the pole, night reigns all winter long while in the summer, the sun shines even at midnight.

Professor David Beerling of the University of Sheffield, and author of Emerald Planet, explained the challenge that Antarctic trees must have faced in this unusual environment: "During prolonged periods of warm winter darkness, trees consume their food store," he said. And if this goes on for too long, they will eventually "starve".

To understand how trees survived against the odds, Professor Beerling has been investigating the kinds of plants that once grew on Antarctica. These include trees like the Ginkgo, a living fossil.

Image caption Experiments show how Ginkgo survived the polar night

"What we did was grow seedlings of these trees in blacked-out greenhouses where we could simulate Antarctic light conditions", he told the BBC.

"We also raised temperature and CO2 concentration to match ancient growing conditions."

His experiments showed that trees could cope remarkably well with the strange environment. Although they used up food stores in the winter, they more than made up for this by their ability to photosynthesise 24 hours per day in the summer.

In fact the main problem seems to have been that trees did not know when to stop.

"We found that trees made so much food during the summers… that this eventually caused photosynthesis to slow down," Professor Beerling explained. "As a result they couldn't fully take advantage of the long hot summers for photosynthesis".

Dinosaurs in the dark

However, it wasn't just trees that had to find ways to cope with the unusual polar conditions. Amazing fossil discoveries show that dinosaurs foraged in the tangled undergrowth.

Professor Thomas Rich of the Victoria Museum, Australia, is a world-famous dinosaur-hunter, responsible for finding several polar dinosaurs.

Over the past 20 years, he has meticulously excavated fossil sites in southern Australia. This region was positioned just off the east coast of Antarctica, 100 million years ago.

Image caption Global warming may signal a greener future for Antarctica

His finds raise an interesting question: did Antarctic dinosaurs migrate during the winter, or were they adapted to living in the dark forests of the polar night?

Professor Rich thinks he knows the answer: "The only complete dinosaur skeleton that we've found is Leaellynasaura. What's really unusual about that specimen is the skull. It indicates that the animal had enlarged optic lobes," he explained

This suggests that polar dinosaurs might have possessed acute 'night-vision' and were well suited to the prolonged winter darkness.

What might an encounter with a polar dinosaur have been like?

"Had you seen Leaellynasaura as a silhouette at dawn, you would have been justified in confusing her with a [small kangaroo]," remarked Professor Rich.

"She was bipedal with large hind limbs and a long tail." However, it would have been no threat because its teeth show that it fed exclusively on plants.

Emerald Antarctica

Visiting the frozen wasteland of Antarctica today, it is hard to believe that rainforests haunted by small dinosaurs once flourished where 3km thick ice-sheets now exist. However, the geological record provides irrefutable evidence that dramatic climate fluctuations have occurred throughout our planet's history.

Indeed, over the past 50 years, the Antarctic Peninsula has warmed by an alarming 2.8C, faster than any other part of the world. So, if this warming were to continue unabated, could an emerald Antarctica be reborn?

"It is just possible," said Professor Francis. "However, that assumes that plant species are able to migrate across the Southern Ocean from places like South America or Australia," she said.