20080921_CW

People:

• Dr Tim Barnett: Research Marine Physicist, University of California, San Diego
• Joseph L Bast: President, The Heartland Institute
• Senator Michael Jungbauer: Republican Member of the Minnesota Senate, United States
• Professor Richard Lindzen: Atmospheric physicist and Professor of Meteorology at MIT
• Professor Michael Mann: Climatologist, director of the Earth System Science Center, Pennsylvania State University
• Professor Pat Michaels: Climatologist and distinguished senior fellow at George Mason University
• Lord Christopher Monckton: 3rd Viscount Monckton of Brenchley
• Professor Reginald Newell: Meteorologist, Massachusetts Institute of technology
• Professor Fred Singer: Emeritus professor of Environmental Science at University of Virginia
• Dr Konrad Steffen: Arctic climatologist, University of Colorado
• Professor Iain Stewart: Geologist and Professor of Geoscience Communication at the University of Plymouth
• Margaret Thatcher: Prime Minister of Great Britain, 1981 - 1990
• Professor Jim White: Professor of Geological Sciences, University of Colorado

Iain Stewart: The last 30 years have seen an epic battle over climate change.

[Footage from separate conversations featured in Earth: The Climate Wars: Fightback.]

Senator Michael Jungbauer: As close to scientific fraud as you can get.

Michael Mann: ...defamation...

Joseph L Bast: It's a scandal! It's absolutely outrageous!

Michael Mann: ...slander...

Lord Monckton: ... I'm going to say it bluntly - deliberately bent!

[End of footage.]

Iain Stewart: It's made front page news, it's dragged scientists out of their ivory towers and into the messy world of politics and big business. But after a monumental struggle, what's emerged is one of the most rigorously tested theories in the history of science. Now there really is no serious scientific doubt. Even some die-hard sceptics agree.

Pat Michaels: When somebody says that there's no such thing as warming, I squirm, yes.

Iain Stewart: The world is getter hotter and humans are to blame. That doesn't mean the story's over. Far from it. Now there are new and challenging questions for science and for every one of us to answer. In fact, they're the most vital questions of all.

My name's lain Stewart, lecturer in geology at Plymouth University. I've been following the scientific journey that's revealed how our emissions of carbon dioxide are heating up the planet. In this programme, I explore why it's been so hard, though, to know exactly what the consequences of this will be. How the climate's complexity poses huge challenges for scientists. And why this has led to so much confusion over just what we should do about it. This film is about the science of predicting our future climate and what it means for all of us.

Right here, right now, everything looks just fine. This is living the dream. It's what people around the world aspire to. If the end of the world is nigh, it certainly isn't reflected in our everyday life.

Yet the apparent normality is deceptive, because the world we take for granted IS changing. Global warming IS happening. Some scientists are saying that we're hurtling towards disaster. Mind you, you'd never guess it from the way that we're living. And there's a reason why it's hard to make sacrifices to stop global warming. I think the problem is that I don't know what it means for me and I don't know how it's gonna change my life. So many confusing, sometimes completely contradictory stories about what the future might hold.

[Audio footage of clips from radio programmes, as Iain Stewart is driving a car.]

Male radio commentator 1: Global warming is generating drastic and dangerous weather changes...

Male radio commentator 2: Expect to see wine being made in vineyards from the Midlands to Scotland, harvests...

Male radio commentator 3: Wetter winters and drier summers because of the changing weather...

Iain Stewart: One moment we're told it's gonna be hotter and drier...

Female radio commentator 1: Soon it'll be too hot for us to visit the Med...

Iain Stewart: ...the next moment, it's wetter and windier...

Male radio commentator 4: More floods, and now scientists say "Get ready for hurricanes."

Iain Stewart: ...that warming is gonna cause cooling...

Male radio commentator 5: The Gulf Stream could stop, leaving Britain with a climate resembling Newfoundland.

Iain Stewart: ...and even maybe, that nothing will change at all.

Female radio commentator 2: Before we join Brian Hanrahan for the World Tonight, here's the weather forecast...

[End of footage.]

Iain Stewart: In the face of so much uncertainty, it's tempting just to tune out and do nothing. It feels like the more scientists look at the consequences of global warming, the more complicated and unpredictable they become. But that's not a reason for us or science to give up. Here we are in the middle of the biggest scientific experiment on the planet. And most people would believe, like I do, that science should be able to predict a result to that experiment. So the challenge for science is to grapple with the complexity of the climate, and try and answer the question we want answering most - just what is global warming going to bring?

Accurately predicting the climate is a problem scientists have struggled to solve for over 60 years. Scientists have a tried and tested method for making predictions - they do experiments. The trouble is, there's only one Earth, and it doesn't fit on a laboratory bench. So for years, scientists have faced the single big challenge - how to make a simplified version of the climate system which they could use for their experiments. They began by taking the idea rather literally. The plan was to create a simplified, small-scale version of the climate system and use it to discover some of the fundamental laws that govern how the climate works. To do that, they turned to something near at hand - the common-or-garden dish pan. This is the recipe for the climate.

First you need a globe or, in this case, half a globe. This is one hemisphere, say the southern hemisphere. So if we're looking down on the southern hemisphere, what we need is a polar icecap... There we go. To simulate the atmosphere itself, we're gonna fill this with water. It's not as strange as it seems. The atmosphere behaves basically like a fluid. Now, in order to see what's going on, it's best to put something into the water. This is just a silvery liquid. Oh, it looks lovely! Now we've gotta take it for a spin, because of course our planet rotates, it spins around. That'll do it. Now, we've got a sun here in the form of a Bunsen burner. Essentially what we've got here... The heat's coming up. And this is gonna be the equator, if you like, where it gains heat, and it's gonna lose heat in the centre, at the icecap. It's dead simple. 'If a bit basic! And if you give it long enough, then you do begin to see 'some of the familiar parts of the climate system begin to materialise. ' OK, now we're starting to get some eddies and currents right in... right at the edge of Antarctica. I can see some bits getting pushed out, swirling round and getting dragged back in again. What's beginning to develop are some of the classic patterns of the world's weather system.

After a little while, the patterns get even more distinct and they actually do bear some resemblance to the weather you see above Antarctica. Not bad for a first effort! But in the 1950s, scientists became real experts at this technique. They used it to recreate complex circulation patterns in the atmosphere. proved something vital - the behaviour of the atmosphere followed some simple fundamental laws. It was the beginning of a new science - climate modelling. Thing is though, there's a limit to where a dish pan, a Bunsen burner and ice can take you. Simple experiments like these may have helped scientists understand the climate, but they weren't gonna be able to predict the future.

Fortunately, there was an alternative on the horizon - computers. And they would become central to forecasting the outcome of global warming. Scientists could now take the fundamental laws that they'd begun to discover with the dish pan models and use them to help create a computer model of the climate system. First, they set to work using computers to try and predict the weather.

Male voice: By feeding the reports into computing machines, the new system hopes to obtain, electronically, a more detailed and accurate picture of what the weather's cooking up for us tomorrow and perhaps a bit further ahead.

But for all their enthusiasm, there was a problem. The complexity of the climate system overwhelmed their computers. As one researcher admitted, the calculation time for a 24-hour forecast was about 24 hours! But the computers weren't gonna be left behind for long. By the 1970s, they had finally outpaced the weather. Now scientists were able to move on to their greatest challenge - predicting the climate years into the future. Climate modelling had finally come of age. It was in the nick of time. In the 1960s, researchers had revealed that pollution was raising carbon dioxide levels in the atmosphere. They knew that carbon dioxide was a greenhouse gas, which should warm the planet up. But how much? This was the first big test for the climate modellers. They started with a simple question - what would happen to the Earth's climate if carbon dioxide levels in the atmosphere were to double? So the modellers turned to their computerised climates. They doubled the level of carbon dioxide in their model atmospheres and they pressed go! So just how would the world's climate change? The models churned through thousands of equations that simulated the climate system, from cycles of rain and snow on land, to sea ice around the poles. And they calculated what effect the extra carbon dioxide would have. The result was surprising. The model suggested that a doubling of carbon dioxide would warm the planet by about three degrees. It may not sound like much, but the last time the world was that warm, melting ice meant that large parts of London would have been under water.

By the late '80s, the model predictions had helped turn global warming into a major political issue.

Margaret Thatcher: The danger of global warming is as yet unseen, but real enough for us to make changes and sacrifices so that we do not live at the expense of future generations.

Iain Stewart: But the predictions also fuelled a backlash. In the 1990s, the whole idea of global warming came under attack and the models were a prime target. Critics said they were just too simplistic - little better than computerised crystal ball gazing.

Fred Singer [Science and Environment Policy Project, speaking in 1991]: Global climate models are simply not yet sophisticated enough to include all of the physical factors that make up the climate. Policymakers have started asking themselves "Why should we rely on such primitive models?"

Richard Lindzen [in The Greenhouse Conspiracy, produced by Hilary Lawson, Channel 4, 1999]: I don't think we could speak of the models as being accurate at this point - they're experimental tools.

Reginald Newell: I don't know why they're taking the models seriously.

Iain Stewart: The critics had a point. The models were struggling to cope with the complexity of the earth's climate system and they were still unproven. The problem was that the models predicted so far into the future it was impossible to tell if they worked. What the modellers needed was an event - something big, something immediate, something to test their mettle against. They finally got their chance in June 1991.

Newsreader: Mount Pinatubo volcano has erupted, throwing rocks and molten lava miles into the air. Hundreds of thousands of people have fled their homes, but there are no immediate... '

Iain Stewart: It was one of the largest and most violent eruptions of the 20th century. Millions of tons of debris and gases were thrown into the air. For Jim Hansen, one of the world's top climate modellers, it was the chance he'd been waiting for. Finally he could test his model. He knew that the Pinatubo eruption was so big it might well have an effect on global temperatures. Huge volcanic eruptions on this scale can cool the climate, because they send a cloud of dust and gases into the upper atmosphere, which reflect the sun's rays back into space. The question was, could his model accurately predict Pinatubo's cooling effect on the climate over the next few years? If it could, then his predictions about global warming would have much greater credibility. Meetings were summoned, data was inputted and the model was run.

It came up with three predictions. First it said that the global climate would cool by half a degree. It also said that this cooling would last a couple of years. And finally it suggested that the regions that would feel it most would include Europe and the USA. Now, after all that furious activity, all they had to do was sit back and wait. As the temperature records came in from around the world, Hansen could afford to smile.

Newsreader: All around the country, people are talking about the wild weather this summer, and in a lot of places what they're saying is, "Baby, it's cold outside!"

Iain Stewart: Cold it was. In fact, around the world, the two years following the Pinatubo eruption were indeed half a degree cooler, and the places that suffered most included the US and southern Europe. Hansen's model was spot-on. This was a huge triumph for the modellers. It proved that, in terms of the big picture, models could accurately see the future. This wasn't science fiction - it was science fact. The climate modellers had been vindicated. And by the late 1990s, climate models had gained the respect of scientists around the world. This is what a global climate model produces - a map of the earth in which the colours show how climate changes as carbon dioxide levels rise. In this case, we're looking at temperature, and you can see by the changing shades of the warmer tones, the yellows, the oranges and the reds, that there's quite a bit of variation. But down here in the corner, that's the average temperature of the globe. And it's thanks to models like this that we've got a good idea of what global warming will mean for the planet as a whole.

The models give a range of predictions, but overall the message is consistent - if we double the level of carbon dioxide in the atmosphere, the earth will gradually warm by around three degrees - pretty much what they were saying 30 years ago. These models also give us a broad idea about regional changes. For example, you can see that the Arctic warms much more than the Equator. You can also see changes on an even finer scale, because in this model it looks like Britain and Ireland warm much less than our continental neighbours. But if you want to zoom in for even more detail, it just isn't there. These models struggle to give us what we really want - information about what's likely to happen right on our doorstep. Powerful as they are, computer models are still wrestling with the complexity of the climate system. One of the key challenges is scale. Even today, the models can't take into account all the local small-scale processes that happen across the planet. More computer power will help, but that alone isn't enough. Sometimes there's just no substitute for some proper field research.

I've come to Greenland to see how scientists are working to fill the local gaps in the models' calculations. Because to accurately predict the future, models need to understand what's happening on the ground, now. More than 80% of Greenland is covered in a huge ice sheet. Outside Antarctica, this is the largest body of ice on the planet. And one of the biggest unknowns about global warming is how this ice sheet is responding to rising temperatures. Climatologist Konny Steffen is taking me to a glacier that he's visited often. It's one of many glaciers that flow from the ice sheet down to the sea. When Konny first came here, the surface of the ice was smooth and gently sloping. It was known as the "Dead Glacier".

Konrad Steffen, University of Colorado: "Dead Glacier" was an old name, because the glacier didn't move for a long time.

Iain Stewart: Right. And have you worked there before?

Konrad Steffen: We had actually a station. We put up a station on that glacier in '97.

Iain Stewart [voice over]: But just three years later, Konny had to abandon his research station. The glacier had changed out of all recognition. Its previously smooth surface had been transformed into a crinkled chaotic landscape of deep crevasses.

Iain Stewart [to Konrad Steffen]: So is it dangerous there where we're going?

Konrad Steffen: Well, I would call it not dangerous with a helicopter, because we can land between the crevasses. But definitely danger if you actually walk around. I don't think this is normal. All this was a smooth area. We were driving skidoo on this upper part. Yeah.

Iain Stewart: Yeah, you wouldn't do that now.

Konrad Steffen: No, definitely not.

Iain Stewart [looking into crevasse]: It goes down quite a way, Konny!

Konrad Steffen [laughs]: I have long legs!

Iain Stewart [voice over]: Konny realised this change in the glacier's appearance could only have been caused by one thing - it had speeded up. The "Dead Glacier" was no longer dead and it wasn't alone. Many other glaciers around Greenland have also speeded up as temperatures have risen. These faster-moving glaciers could have a significant effect on global sea level, but it was a process that had never been predicted.

Konrad Steffen: We are surprised every year by a new process, you see. A process we did not anticipate, a process that certainly has not been modelled.

Iain Stewart [voice over]: The priority now is to understand these processes, so they can be included in the models. So Konny is investigating the link between rising temperatures and the accelerating glaciers. He believes that warmer temperatures mean that where the glaciers reach the sea, they are producing many more icebergs than before, in a process known as calving. To understand how this speeds up the glacier, I'm joining Konny in the birthplace of an iceberg - down a crevasse.

[They abseil down into a crevasse.]

Konrad Steffen: More to the right, yes. Doing fine, thanks. Yes, OK.

Iain Stewart: How far have we got to go? Whoa! Yah!

Konrad Steffen: Just push out.

Iain Stewart: Whoa! Yes! That was a bumpy landing!

Konrad Steffen: Good!

Iain Stewart: Good! Hey! Hey, hey, hey!

Konrad Steffen: Down it goes!

Iain Stewart: Wow! [Voice over.] Konny explains how increased calving can actually help make the glacier move faster. [To Konrad Steffen.] So we're in a crevasse, and eventually it'll make its way down to the sea and calve off. How does that calving process drive the glacier too?

Konrad Steffen: Oh, we can stay here a couple of weeks and then experience it ourself, because it will happen along this line. Where you have the crevasses, and it moves towards the ocean, then it is floating on the ice, and that gets a weak point. And that breaks off, and that is a major release of ice into the ocean.

Iain Stewart: And what does it mean for the glacier upstream, then - these big icebergs breaking off?

Konrad Steffen: When a big iceberg breaks off, more ice can move down. And if the ice moves faster, the ice gets thinner as well. And the thinner ice can break even faster.

Iain Stewart: In other words, it could become a chain reaction, with faster glaciers leading to thinner ice, which creates more icebergs and an even faster glacier. Nearby, scientists have for the first time filmed the whole front of a glacier breaking off. This speeded up footage shows how icebergs up to a kilometre across turn upside down as they break off into the sea. The work of Konny and other scientists is helping improve the models. Modellers can now factor in details that until recently they've been unable to include - details that may radically alter their predictions. It now seems that as global temperatures rise, Greenland is losing ice at an ever-increasing rate. The implications are serious. It means that sea level could rise much more quickly than the models have predicted up until now, threatening the homes of millions around the globe. But this isn't the only challenge the modellers face.

There's a fundamental process in the climate system which means that no model can predict the future with absolutely certainty. It was discovered entirely by accident when a climate modeller took a shortcut and went off for a cup of coffee. The year was 1961. A meteorologist called Edward Lorenz wanted to rerun an experiment on his simple climate model.

[In cafeteria, buying cup of coffee.] Thanks for that. Lovely. For the second run, instead of using numbers that ran up to six decimal places, he used numbers that had been rounded up to just three decimal places. And then he left the computer noisily chugging its way through the sums. Now, you wouldn't have thought that would have mattered - after all, rounding up the numbers hardly changed them. But it did. When he came back, Lorenz was astonished! Compared to the first run, this new version produced a completely different forecast. I mean, it was just plain weird. The numbers had hardly changed, and yet the result was transformed. Now, for a lesser scientist he might have blamed his equipment or just dismissed the result as a fluke. But for Lorenz, this was a moment of revelation. He concluded this wasn't a flaw in the model, but something intrinsic to the climate system. Tiny events could grow in significance over time, eventually having a huge impact. It led Lorenz to ask one of the most famous questions in science - does the flap of a butterfly's wings in Brazil set off a tornado in Texas? Lorenz's celebrated phrase means simply this - the tiny changes in the atmosphere triggered by the flapping of a butterfly's wings, changes so imperceptible that we can't hope to measure them, could still have a huge effect on the weather. He had discovered chaos.

It was an insight that would go on to revolutionise scientists' understanding of everything from irregular heartbeats to stock markets, and it explains why a weather forecast isn't always accurate and why climate models give a range of predictions. But this doesn't mean models are useless, because the modellers have a way of dealing with chaos - run the models again and again. While the results might vary, clear trends do emerge. And repeated runs confirm the general direction that climate change is taking us - a gradually warming world. That's worrying enough, but for scientists, chaos was also a wake-up call. It alerted them to a new possibility. They realised there could be instabilities hidden in the climate system which they still knew nothing about. It's a fear that's been proved absolutely right. Something's been discovered in an entirely different branch of science that's got some really unnerving implications. It's come from looking at what happened to the climate in the past.

The story begins with a series of intriguing discoveries almost 40 years ago. It would end with a scientific revolution and the realisation that global warming could deliver some very nasty surprises. The first inkling of something amiss was in the 1960s, with the discovery of some rather puzzling dead beetles. These beetles were found in soil that was over 11,000 years old, laid down at the end of a cold period called the Younger Dryas. What's strange is that in layer after layer of soil built up over years, there's lots of beetles like this. Now, these flourish in cold conditions. And then suddenly the cold-loving beetles disappeared and were replaced by critters like this. This loves the warmth. In Switzerland, another researcher was astonished to discover, in mud dug from an ancient lakebed, layer upon layer of pollen from cold-loving plants and then a sudden change to warm-loving plants. What's more, the pollen came from the same period as the beetles - the end of that strangely named era, the Younger Dryas. It was intriguing, because it seemed to be that the climate switched from cold to warm incredibly quickly. And yet surely that was impossible? The scientific theory was clear - Earth's climate changed with glacial slowness. It was a view that went back to Charles Darwin and beyond. "Nature," he said, "doesn't make leaps." So these unsettling reports from the field were quietly filed away. Yet a nagging question remained - just what happened to the climate over 11,000 years ago?

In the early 1990s, a group of scientists decided to take a closer look. They set up camp on top of the Greenland ice sheet. It was a major undertaking. Conditions could be brutal. But for scientists like Jim White, it was the experience of a lifetime.

[TV footage from 1990s.]

Interviewer [on moving sled]: Dr White, do you expect to vacation here in the future?

Jim White: Yes. Yes, I'm bringing all my friends next time.

Interviewer: Oh, wonderful, wonderful!

[End of footage.]

Iain Stewart: Jim was there to travel back in time and finally solve the mystery of what happened to the climate at the end of the Younger Dryas. Buried in the Greenland ice sheet was just what he needed - a record of past temperatures stretching back tens of thousands of years. Greenland's ice sheet is made up from layer upon layer of snow that builds up over time. As each layer gets buried, it gradually turns to ice. The deeper down you go, the further back in time you travel. Crucially, by analysing the chemical signature of the ice, it's possible to discover the temperature at the time the snow fell. So Jim's team drilled down into the ancient ice, bringing up long, thin cylinders of ice called ice cores. What they found inside the ice would not only solve the mystery of what happened at the end of the Younger Dryas, it would transform our understanding of what global warming could mean for our future. Today, the Greenland ice core is stored in a warehouse in Denver at 35 degrees below zero. I've come to meet Jim and discover what the ice revealed about the end of the Younger Dryas.

Iain Stewart [to Jim White]: So, how cold is it gonna be? Is it gonna be criminally cold?

Jim White: It's gonna be beyond criminally cold!

[They put on warm clothing, prepare to enter cold room where ice cores are stored.]

Iain Stewart: That's the business.

Jim White: [Inaudible.]

Iain Stewart: That's it.

Jim White: All right.

Iain Stewart [voice over]: Each ice core is made up of distinct layers. A clear layer, which is laid down in late summer, followed by a cloudy layer, which is laid down in late winter. Each pair of layers represents a single year's snowfall. So scientists were able to count the years all the way back to the end of the Younger Dryas and by analysing the chemistry of the ice, they could construct a graph of the temperature back in time. And this is the priceless section of ice that finally provided the answer to what happened at the end of the Younger Dryas. Pieces of ice have been carved away for extensive tests and analysis.

Iain Stewart [to Jim White]: So we've come out of the last ice age, it's started to warm up, then we dip into this 1,000 year-old cold stage, the Younger Dryas. What does this core tell us about how it ends?

Jim White: As you come along, in this particular core, you see layers that are roughly half a centimetre thick, and then right here, these layers become about a centimetre thick all the way up the core...

Iain Stewart: Ah, yeah!

Jim White: And that is a fundamental change in the amount of snowfall that occurs. This is the Younger Dryas cold period. Right here is the end of the Younger Dryas. We can actually put a line right there in the ice.

Iain Stewart: So, right on that divide there?

Jim White: Right on that divide right there.

Iain Stewart [voice over]: In the cold, dry period of the Younger Dryas, the layers are cloudy and so thin they seem to merge into each other. But then there's a sudden transition to clearer, thicker layers. These thicker bands show there was much heavier snowfall. And when they analysed the chemistry of the ice, it revealed the temperature had jumped by five degrees.

Iain Stewart [to Jim White]: So how quick is that transition?

Jim White: Well, if you look at it, basically one year.

Iain Stewart: No?!

Jim White: Yes, the - um, there's enough noise in here that one can argue, maybe it's one to three years. But it's not one to five years and it's certainly not one to ten years. It's right around one year.

Iain Stewart: So we go from essentially an ice age in the Younger Dryas, to the warm period immediately afterwards within a year?

Jim White: In terms of snowfall, yes. Now, it takes a little longer for the climate system to warm up.

Iain Stewart [voice over]: The Earth's climate was meant to take thousands of years to change. But the ice core showed that the climate could switch from an ice age to warm conditions in less than a human lifetime. And as they looked further back in time, there was more to come.

Iain Stewart [to Jim White]: So is the only abrupt change we find?

Jim White: No. No, this ice core contains a couple of dozen abrupt climate changes that have warmings that are as fast and environmental changes that are as drastic as the one you see here.

Iain Stewart: So are these rapid shifts characteristic of the climate system?

Jim White: It's clearly not an artefact of the system. It's clearly not just, you know, a once or twice kind of thing that maybe was a meteorite or something like that, or a volcano. This is an inherent intrinsic part of climate system.

Iain Stewart [voice over]: The discovery of rapid climate change was a scientific revolution. It meant that the climate was capable of sudden jumps in a timescale that modern civilisation has never had to deal with.

Iain Stewart [to Jim White]: And do you get genuinely scared about what that possibly means for us if we encounter it in the future?

Jim White: Yes. I don't think anybody could not get scared. If you understood just how fast that was and how big this change was, just how fundamental it was... If something like that happened to us, and it's important to recognise we've not seen anything like it, if something like that happened to us today, we would probably not be able to grow enough food, we would not have enough fresh water. It would challenge even the most industrialised society to adapt. And that is scary.

Iain Stewart: Mmm.

Iain Stewart [voice over]: The most frightening thing is that no-one knows what causes the climate to change so quickly. So scientists began to worry that the changes already underway as a result of global warming could accelerate and turn out to be just as fast. Now, it's impossible not to look at that core and see that change from an ice age into a warm world over the course of a season or two, and realise that we could see climate change not in some distant future but in our lifetime. And that's made the debate much, much more urgent. Instead of the slow, steady rise in temperatures forecast by the climate models, we could face a catastrophic warming in just a few years. And there are signs some changes are already happening faster than predicted. So far, it's in the Arctic that global warming has caused the most dramatic changes. Above all, to the sea ice. Every winter, as temperatures drop, the sea ice spreads, stretching from the north pole down to Greenland and Canada. In the summer, it melts back. As the Earth warms, the models say that over the next century, the area covered by sea ice will shrink gradually. But the ice is changing well ahead of schedule.

Visit a village like Umanak on the west coast of Greenland, and you can see the impact of those changes in many different ways. For starters, each year the sea ice is melting earlier. This is the harbour! Thing is, although you can see that most of the boats are still frozen in, the sea ice has just begun to melt. In fact the first fishing boats of the year left here yesterday. Thing is, this is mid May! Five or six years ago, the boats wouldn't get out of this harbour until the middle of June. Over the last few years, the spring break-up of the sea ice has moved forward almost a month. You can really experience the effects if you travel out on the sea ice in spring.

[Travels across the ice in a sled.] Now, we tend to think of snow and ice as being a barrier to travel, but for the Greenlanders this is a vast open road. And all these bumps and lumps in the ice is because it's starting to break up. It's the spring, and this is a really dangerous time of year out in these skidoos. Every so often, you come across these deep cracks in the frozen surface where there's these open stretches of water. I've got a horrible feeling he's gonna go for a run through it! This is the Inuit version of the great escape when Steve McQueen leaps the fence. Yeah, ready! So he's just gonna rev this up, and we're gonna go straight over! Well, that's the plan. Here we go! Yah-yah! Ha-ha! And I tell you, it's salt water. Ah!

The earlier melt isn't unique to Umanak. Across most of the Arctic, the spring melt has been getting earlier since the 1980s. And it's not just that the melt is starting earlier. Scientists now know that the sea ice has been getting steadily thinner for the last 30 years as well. Yet it's only because of a bizarre historical fluke that we've any long-term records of sea ice thickness. For decades, American and Russian nuclear submarines prowled the waters that are a few metres beneath my feet, playing a game of hide and seek beneath the shifting sea ice. From the ocean depths, they measured the thickness of the ice.

Newsreader: The submarine Nautilus on the first transporter voyage from Pacific to Atlantic oceans. A continuous record of water depth and ice thickness is obtained by the Nautilus safely on its silent invisible cruise below polar ice, which is on average 12 feet thick, with some ridges extending down 50 feet and even farther.

Iain Stewart: This was the Cold War, and the Americans were convinced that the Russians had submarines that could break through the ice and potentially launch a surprise nuclear attack. But crucially, it depended on how thick the ice was. So for over 30 years, the military kept detailed secret records of the ice thickness. Then, when the Cold War ended, the American navy invited civilian scientists to join their missions. Once more, they headed under the Pole, measuring the thickness of the ice as they went. What they discovered was that the ice was 40% thinner than it had been when the measurements were first taken. It had gone from an average thickness of 3.3 metres, which is the height of this face of an iceberg up to about just over my head, down to a height of about 1.9 metres. In other words, in 40 years it had virtually halved. It was an astonishing change in such a short space of time. But it wasn't just that the sea ice was getting thinner and melting earlier, there was also a third change. There was a lot less of it. Over the last 30 years, satellites measuring the extent of summer sea ice have recorded its decline. By 2005, the sea ice had shrunk by almost 20%, and by 2007, it hit a new record low when an area about ten times the size of Britain disappeared. Scientists have put all these changes together, the earlier melt, thinner ice and a reduction in the extent of sea ice, and some now fear that within a little over a decade, sea ice could vanish in summer altogether. Rapid climate change is already happening here. The reality is that we may be on the verge of crossing a threshold into a world where the Arctic is ice-free every summer. And if that happens, could it lead to a further acceleration in warming, even help cause a sudden leap in temperatures like the one at the end of the Younger Dryas? The truth is, no-one knows. And there's the rub. The models say global warming will be slow and steady, and that's bad enough. But the lesson from history is that when change comes, it can be very sudden.

To get some idea of what rapid climate change can mean for a society, I've come to the Four Corners region of south-west America. It's a harsh environment. In summers, it's dry and hot. In winters, it's bitterly cold. But it was once home to one of America's most famous ancient civilisations. It was a pair of cowboys chasing stray cattle that first re-discovered buildings in this area, and they were completely blown away by them. One described exploring the nooks and crannies of these cliffs as more exciting than hunting for gold, because you never knew what you'd find next.

[Climbs up ladder to find ancient cliff settlement.] Wow! That is spectacular! That's absolutely... spectacular. It's the scale. I don't know how many dwellings are here... Well, maybe nearly 100 houses. You can hardly see them in places, because it's just the same beautiful colour as the rock. I mean, that's a small town, isn't it? Wow! What those 19th-century cattle ranchers uncovered was evidence of an extraordinary civilisation, the Anasazi, or Ancestral pueblans, who made their home in this area for more than seven centuries. What's remarkable about this place is that the people who lived here didn't just survive in this hot, dry climate, they prospered. The dry conditions preserved all the trappings of a really sophisticated culture. Things like traded goods and farmed corn kernels, beautifully woven cloths and intricately patterned murals and pottery. At its height, as many as 10,000 people lived in beautifully crafted houses across the region. The secret of the Anasazi's success was their ability to exploit and control scarce water resources in their desert home. You know, for all the beauty and sophistication of this site, this is the real heart of it. This is a spring. Out there when the rains come, they come in sudden deluges and they're gone in an instant. But above here, when the rain falls, it trickles down through the sandstone rock until it hits a clay layer that it can't get past and it oozes out and it gives you this moist bed here. You can see the lichen and mosses. And there's tree roots here that tell you that in the past there's been a lot more water here.

The Anasazi became masters at exploiting every possible source of water. But then, just before 1300 AD, the people here deserted the region. So why did they leave these beautiful homes? Well, that's the great mystery. Whatever the reason, their departure wasn't just abrupt, it was final. They never occupied these buildings again. Scientists have analysed tree rings and uncovered evidence of a sudden change in climate. And it seems that it was this that overwhelmed the Anasazi, because in the late 13th century, rainfall in the region dropped sharply. They had survived drought in the past, but this time was different. As the rains failed and there was competition for resources, the whole fabric of the community disintegrated and the result was conflict. Excavations have revealed that village fought village in a brutal and bloody struggle for survival. With their society in turmoil, the Anasazi fled. In just over 20 years, the region was completely deserted.

It's tempting to think that what happened here has no relevance to us living today. But I think that there's a universal story here. The Anasazi were an ingenious, sophisticated people. They hung on in this landscape for nearly 700 years. The point here is, that when climate change is really rapid, the results can be brutal. So could a similar fate befall us today? If you head west to the city of Las Vegas, the answer appears to be a resounding no. Here is a city built in the middle of a desert, but full of water. A triumph of technology. Coming to Vegas, you can't help but be seduced by the feeling that our technological brilliance will enable us to cope with the effects of global warming. Vegas owes its existence to one of the greatest feats of engineering in the world - the Hoover Dam. Built in the 1930s, the Hoover Dam tamed the mighty Colorado river.

Newsreader: Just as the passenger plane symbolises man's conquest of the air, so does Boulder Dam symbolise man's determination to harness nature through this eighth wonder of the world - America's monument to progress.

Iain Stewart: At the time, it was the world's largest concrete structure, creating a lake 100 miles long - Lake Mead. Proof of its success is easy to see. This is the eighth year of a major drought. The level of the water has fallen, as you can see, thanks to the bath-tub ring of pale rock around the lake, but the water's still flowing. The possibility that technology could prove inadequate here, in one of the most sophisticated nations on earth, seems unimaginable. But now the Hoover Dam faces a new challenge - one over and above the natural variability that it's coped with so well. Man-made climate change. Some scientists believe that the current eight-year drought is just the beginning, that global warming and a rising population mean water will become ever scarcer. The question is, will the lake be able to cope? I'm here to meet one man who's done a detailed study of what the future might hold for the people who live here, Dr Tim Barnett.

Iain Stewart [to Tim Barnett]: So what's the prognosis for the lake and the dam?

Tim Barnett: If nothing's done, if we continue business as usual, there is a 50/50 chance that Lake Mead becomes ineffective in about 2021, early 2020s. Even sooner, the power intakes for the hydro-electric power generation go dry, and that goes out of commission perhaps as little as ten years from now, again with odds 50/50.

Iain Stewart: It's quite a frightening scenario.

Tim Barnett: I think the human beings in this part of the world don't understand what the word "sustainability" means. I mean, we could easily be right at the edge of civilisation, a sustainable civilisation, in the south west.

Iain Stewart [voice over]: That means not just Vegas is at risk. Many other cities built in the desert are under threat, from San Diego to Los Angeles. Predicting the future climate might be one of science's most complex challenges, but uncertainty over the detail isn't a reason for inaction, especially when there's a 50/50 chance that the water supply to 20 million people could fail, and that's even without a sudden acceleration in warming.

Tim Barnett: I think virtually everybody I know in the business that's had a look at real data and a look at models, is surprised that what the models say seems to be happening sooner than they project. It's really a nagging worry that there's something we've missed or somehow we're underestimating the speed that this is gonna go down.

Iain Stewart [voice over]: Phenomenal feat of engineering as the Hoover Dam is, exuberant and confident as life in this region is, what strikes me now is the fragility. Even the richest societies will struggle to insulate themselves against the effects of global warming, especially if that change is big and fast. Which brings me back to where I started the series - wondering what global warming means for me and my family, and what sacrifices we should make to prevent it.

Iain Stewart [to his children, while driving in family car]: Can you see the sea yet?

Daughter 1: No.

Daughter 2: Yeah!

Iain Stewart [voice over]: I'm not sure the kids have fully grasped the significance of it all just yet.

Daughter 2: Dad, you refused to let me put up that sticker saying, "Stuff global warming"!

Iain Stewart: They wanted to put it on the blooming car! [Voice over.] For the kids, global warming is something they can still have a laugh about. They're always teasing me I take things too seriously. [On narrow bridge inside Eden Project, Cornwall.] I don't think I fit, really! It's too small for me! [Voice over.] But I'm taking my cue from the scientists who study global warming. You know, I was thinking it would have been lovely to have made a programme about how science had got it all wrong, that actually we've got nothing to worry about. But unfortunately, it's the opposite. Most of the climate scientists I talked to are actually genuinely scared by the future. They're worried that it's in the nature of the climate to change far faster than we once thought possible. And my feeling is, if they're scared, so should we be, because whatever the uncertainties surrounding climate prediction, the fundamental science is pretty clear. We may not know exactly what global warming will bring, but we sure as hell know it's happening. There's just no hiding place from that simple fact. Of course, what it means for us and our families, well, that's a different matter. But if I've learned one thing in this series, it's that the stakes are so high, doing nothing simply isn't an option. Science has discovered the dangers of global warming. Now it's up to all of us - individuals, companies, governments - to decide what action to take. After all, it's our own future at stake.