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You are here: BBC > Science & Nature > TV &†Radio†Follow-up > Horizon
London in an ice age?
BBC Two, Thursday 13 November 2003, 9pm
The Big Chill
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The Big Chill - transcript

NARRATOR (JACK FORTUNE): In thirty years time they say Britainís summers could be like the South of France. By the end of the century we could be as hot as Greece. This they say is what global warming could bring us. But a growing number of scientists believe we could have our future climate completely wrong.

PROF BILL MCGUIRE: I can only describe it as catastrophic. Itís clearly going to influence every single one of us every day of our lives.

Dr RICHARD WOOD: You could expect to see sea ice off the coast of South East England, probably several miles off shore.

BOB GASGOSIAN: The implications are huge. The economic implications, the political implications, and the national security implications, for all countries.

NARRATOR: If these scientists have it right forget the Riviera, Britain could be heading for a climate like Alaskaís. And it could all happen in the just the next twenty years.

NARRATOR: In the Atlantic waters north of Scotland a fisheries researcher was the first to record a warning from the deep. Bill Turrell has spent his career studying the ocean currents. Until recently his concern has been how they affect fish stocks. But ten years ago he began to see something in the water that really alarmed him.

DR BILL TURRELL (Marine Laboratory, Aberdeen): We really worried when we saw these results, weíd never seen a change like this ever before. Changes that donít occur quickly and don't stop quickly.

NARRATOR: Turrell believes that he has found evidence that a climate catastrophe could be heading right towards us.

DR BILL TURRELL: These changes are fundamental, theyíre substantial. They are going to impact our climate and the climate our children have to live in.

NARRATOR: If he is right then Britain could be heading for a massive drop in temperatures. It seems we could be heading for something like an ice age. The ice ages were one of the greatest forces nature has unleashed on our planet.

PROF BILL MCGUIRE (University College London): They simply were the most dynamic, destructive phenomenon thatís ever hit the planet. They involve glaciers hurtling down from mountainous regions, pouring out from the polar regions and devastating everything in their paths, grinding rock to nothing, lowering mountains, filling valleys.

NARRATOR: More than twenty times in the earthís history ice sheets have come down from the North Pole, the last one struck a hundred thousand years ago. Britain was buried in a tomb of ice.

PROF BILL MCGUIRE: Effectively they wiped the slate clean in the northern hemisphere, over twenty times in the last two million years.

NARRATOR: These events were so terrible that for years scientists wondered what had caused it, and could they ever threaten us again.

NARRATOR: In the search for answers they turned to the sun, our ultimate source of heat. They discovered that the pattern of ice ages matched strange wobbles in the earthís orbit around the sun. These altered how the sunís heat shone upon the earth. They allowed the ice to grow and retreat.

Prof RICHARD ALLEY (Penn State University): And these wiggles in earthís orbit are very regular, theyíre very predictable, they make sense and so thereís this long slow bumpy slide in to an ice age and then a climb out of an ice age.

NARRATOR: They found that the ice ages didnít happen at random. They followed a slow and predictable pattern. It took ninety thousand years to grow an ice sheet and about ten thousand years to melt it. That predictability allowed scientists to calculate when the next one was due. It was reassuring news, not for thousands of years.

Prof RICHARD ALLEY: For a big ice age, a big climate change, we donít expect anything to happen on human time scales of a few dozen generations.

NARRATOR: The message from the sun seemed to be clear. We had no reason to fear a massive drop in temperatures, at least thatís what conventional theory said.

NARRATOR: Then fifteen years ago Richard Alley came to Greenland to study how our climate had changed since the last ice age.

Prof RICHARD ALLEY: When we started working in Greenland we knew we were on to something big. We really expected that we were going to find things that might surprise us.

NARRATOR: Greenland is like an ancient thermometer, a unique record of what has happened to the weather. Every single year for the last a hundred thousand years. Itís all because the ice is preserved in layers, the further down you drill the older the snow.

Prof RICHARD ALLEY: If you took all these pieces of core that have been collected and put them end to end theyíre about two miles long and this is a sort of two mile time machine.

NARRATOR: Each layer records what was happening in the earthís atmosphere at the time the ice was formed. Whether itís the traces of a huge natural disaster or pollution from human activity, itís all frozen and perfectly preserved.

Prof RICHARD ALLEY: We can see this, this ice core is beautifully layered and we can ask of it what happened, what was coming through the atmosphere at that time. Is there ash and acids from a big volcano, is there lead from Roman lead refining or what have you? So thereís this history of what was blowing through the air and piling up on top of the Greenland ice sheet, sitting here on these beautiful layers.

NARRATOR: But the most important measurement preserved in the layers is the temperature. The ice that Ally brought back to the lab was in effect the annual weather reports since the beginning of the last ice age. What he was looking for was changes in the amount of so called heavy water held in the ice. The basic rule, the more heavy water the warmer the climate. If the conventional wisdom about climate change was true then when he plotted the results he would expect to see slow changes in heavy water as the world warmed and cooled during the last ice age. But thatís not what he found.

Prof RICHARD ALLEY: This flabbergasted us, I think this flabbergasted a lot of people.

NARRATOR: The changes were anything but slow. He saw that temperatures could drop suddenly and catastrophically. And it happened far more often than was predicted by the passage of ice ages.

Prof RICHARD ALLEY: The world sometimes did change in the slow grand sleeps and sometimes it changed like a light switch.

NARRATOR: The earthís past was full of devastating climate jilts, none as bad as a full ice age but enough to turn Britain in to Alaska. The search was on to find out what could trigger these climatic disasters. They searched through the ice record for clues. Had huge volcanoes blotted out the sun? There was no evidence for that. A succession of asteroid impacts? Again, no evidence. More wobbles in the sunís orbit? That didnít fit. In fact no one could account for what Ally had discovered. Except for one man who thought he could. Wally Broecker is the guru of climate science. He was convinced that it was all to do with the oceans.

Prof WALLY BROECKER (Columbia University): Iím convinced that the ocean is at the core of the whole thing. The trigger lies in the ocean.

NARRATOR: Not everyone saw it his way.

Prof WALLY BROECKER: And of course you started out was three allies in a hundred people, who think itís nuts.

NARRATOR: Broeckerís attention was drawn to one ocean current in particular, the gulf stream. Britain bathes in its heat. It begins south of the equator and as it flows along the gulf of Mexico it absorbs heat from the tropics. It continues on past the coast of Britain.

Prof WALLY BROECKER: Itís roughly equal to all the rain in the world. Fifteen million cubic meters per second.

NARRATOR: The current carries the heat of a million power stations. It means we can swim in the sea at the same latitudes that Canada has polar bears. But the most important thing about it happens further north. It sinks.

Prof WALLY BROECKER: When it gets up in to the northern regions of the Atlantic, cooled and make denser and it sinks in to the deep sea and goes back the other way.

NARRATOR: This sinking is caused by salt in the water. When the salty water cools near Greenland it becomes so dense that it plummets to the bottom of the ocean. The water then heads back south to where the gulf stream began, and the whole process begins again. Itís a continuously circulating belt of water and heat, thatís why itís called the conveyor. The sinking off Greenland is vital, this is what keeps Britain forever warm. So Wally began to speculate what would happen if the conveyor ceased to flow. Could that explain those dramatic falls in temperature? Trouble was most scientists were convinced that the oceans never changed at all.

Prof WALLY BROECKER: People tended to think about it as something that went on and on and on and on. We assume that the properties of the ocean are at a steady state, but if itís changing then you can't assume that anymore and it makes real chaos.

NARRATOR: Then came something that shocked everyone out of their complacency. Something so small it might have been completely overlooked. Lloyd Keigwin spends his time examining mud samples from the bottom of the ocean. Mud cores just like the ice in Greenland can tell the history of activity in the ocean and what was going on in the conveyor. In particular Keigwin was looking for tiny sea shells called forams. These lurk on the ocean floor feeding on nutrients that sink to the sea bed.

Dr LLOYD KEIGWIN (Woods Hole Oceanographic Inst.): The forams build their shells out of calcium carbonate like any sea shell youíd find on the beach. And when they take carbon and oxygen and other elements out of the water to make their shell the chemistry of their shell reflects the chemistry and the physical properties of the water.

NARRATOR: When the conveyor flows most of the nutrients get swept past the forams, and so their shells are usually poor in nutrients.

Dr LLOYD KEIGWIN: When the conveyor is on the deep water of the north Atlantic is continually being flushed out and there arenít a lot of nutrients that have accumulated. But if the conveyor went in to an off mode or a reduced mode um more nutrients would accumulate.

NARRATOR: Keigwin developed a way of measuring how much nutrients the forams must have absorbed.

Dr LLOYD KEIGWIN: We add a little more liquid nitrogen here, and this trap will capture the carbon dioxide produced as this sample falls in the hot acid. And soon weíll know was the conveyor circulation on or off at this particular time.

NARRATOR: To his surprise he found massive variations in the composition of the shells. In other words at times in the past the conveyor must have switched off. When this work was published Wally Broecker knew exactly what it meant. It was the clue he had been looking for.

Prof WALLY BROECKER: The greatest joy to a scientist is discovery. Itís almost as if nature is trying to prevent us from uncovering her secrets.

NARRATOR: So he put it all together. The huge drops in temperature seen in the Greenland ice, the shells that said the conveyor had been cut off, they had to be connected.

Prof WALLY BROECKER: Well it was, it was this merging two parts of my science, the understanding of the ocean and the understanding of the climate. It popped in to my head that one way you could do that would be by turning on and off the whatís now called the conveyor belt, the deep water formation in the north Atlantic. And so I got the idea well hell if you turned that on and off or up and down, that could make at least in Europe very large climate changes.

NARRATOR: Broecker believed this explained the massive jolts in temperature seen in Greenland, the conveyor had switched off.

STATION ANNOUNCEMENT: We can only apologise to passengers for the inconvenience caused and for the disruption to services this morning.

NARRATOR: And if it had happened before it could happen again. And if it did Britain would be plunged in to a bitterly cold climate. But then someone pointed out one obvious flaw in the theory. This isnít the age of global cooling but of global warming. Eight of the ten warmest years on record fell in the last decade. Climate experts are predicting temperatures to rise faster than at any time since the last ice age.

Dr RICHARD WOOD (Met. Office): The models that are used to make the predictions of global warming suggest that by the end of the century the warming we would see will be between about one and a half degrees and around six degrees. And really to go to see that in historical context youíd have to go back hundreds of thousands, maybe even millions of years to see a climate in the past that was that warm.

NARRATOR: The effects of global warming are being felt across the planet. The talk is not of ice but about how to prepare for hotter weather and the extremes that would bring.

PROF BILL MCGUIRE: Global warming should make the UK a more hazardous place to live. Particularly during the winter when we have not only more rain but we have more heavy precipitation, bursts of extreme rainfall. This is going to mean that for example river flooding is much more common, much more frequent and its implications will be much more serious.

Dr RICHARD WOOD: Theyíll be changes in er frequency of things like tropical cyclones. And possible storms in the er outside the tropics and there are some latitudes to the UK and Europe.

NARRATOR: These are the changes our government is urgently planning for in the coming century. But recent discoveries suggest we might have to gear up for ice, not heat and rain. But our understanding of global warming maybe too simplistic. One of the most important of these discoveries was made by a team of NASA scientists based inside the arctic circle. Bill Krabill and his team have spent the last decade monitoring the effect of global warming upon the vast Greenland ice sheet.

BILL KRABILL (NASA, Wallops Island): This particular area, you can think of it as a huge ice cube that nicely offers global climate, an ice cube thatís a thousand miles long, four hundred miles wide and two miles thick in the centre.

NARRATOR: For years accurate measurements of the effects of global warming across the ice sheet were impossible. It was just too huge and inhospitable to measure from the ground. So they took to the air. Greenland is one of the biggest blocks of frozen water in the world. And if it started to melt the effects would be felt worldwide.

BOB THOMAS (NASA, Wallops Island): It comprises enough water to raise sea level by about six or seven metres if it all were to melt.

NARRATOR: They mapped the ice with a combination of global positioning satellites and lasers. The satellite measures the height of the plane and the laser measures the distance from the plane to the ice.

BILL KRABILL: There are five thousand individual beams per second that are being projected in to that scan there and then down in a pattern on the surface, measures the surface at ten centimetre accuracy.

NARRATOR: At five year intervals they have flown the same route across the island, each time they have measured the height of the ice. By comparing the two measurements they can see if the ice is growing or shrinking.

BILL KRABILL : Thereís definitely changes taking place here, all over the margin of the Greenland ice sheet it is thinning. Itís equivalent to fifty cubic kilometres of ice and snow that are disappearing off the Greenland ice sheet each year.

NARRATOR: This fifty gigotonnes of water melting from Greenland was the first evidence that global warming might be effecting the ice sheet here. But one change really shocked them. They started to measure one of the islandís biggest glaciers.

BOB THOMAS: Less than ten years ago, five years ago, it was moving at about six, seven kilometres per year. And that was more or less in balance with the snowfall. Now in the five years since then the speed is almost doubled.

NARRATOR: Itís now advancing at twelve kilometres a year. The increase seems to be linked to global warming. Itís the fastest moving glacier on the planet. It dumps enough fresh water in to the sea each day to supply London for several months. Global warming seems to be reshaping the whole landscape of one of the biggest ice sheets on earth.

BOB THOMAS: Well I myself am convinced that global warming has affected the dynamics of the Greenland ice sheet and um that is possibly because increased melt water is creeping to the bed through crevasses and boullans and lubricating the bed and making it far more easy for the er, for the ice to flow.

NARRATOR: NASA have recently doubled their estimates of how much fresh water is coming off Greenland. To a hundred cubic kilometres per year. And much of that fresh water is flowing towards the sinking zone of the conveyor. Scientists began to wonder just what could be the effect of all that fresh water on the conveyor. They began to realise that because the conveyor was driven by the salty water sinking then too much fresh water would dilute the salt and so if the salt was diluted too much the conveyor wouldnít sink.

Dr TERRY JOYCE (Woods Hole Oceanographic Inst.): Whatís amazing about this is that how well fine tuned the system is. Where only a one percent chance in the salinities maybe significant, may tip us over in to this regime where the water is too fresh at higher altitudes to sink and the conveyor will stop.

NARRATOR: Cut off the conveyor and a climate catastrophe would happen. The only question was just how much fresh water would it take. The truth is that no one quite knows. What they do know is that Greenland isnít the half of it. There is another even bigger source of fresh water heading straight for the sinking zone. Predictions are that global warming will lead to a much wetter world, because warm air can hold more moisture. And when it heads north and cools it should lead to more rain. In 2000 a team of American scientists travelled to Siberia. They studied the effects of global warming on some of the biggest rivers in the world.

Dr BRUCE PETERSON (MBL, Woods Hole): If you took the Ob, the Enesai and the Elena rivers and put them side by side youíd have a volume of discharge equivalent to three Mississippi rivers.

NARRATOR: The rivers collect rain that has fallen over a vast area of land. They already carry nearly twenty times the amount of fresh water towards the sinking zone, then drains from the ice in Greenland. Peterson wanted to find out if global warming had led to more water in these rivers. The warming in the last hundred years is only about half a degree. He measured how the flow of water had changed.

Dr BRUCE PETERSON: An increase in annual discharge of a hundred and twenty eight cubic kilometres per year. You probably can not imagine what a hundred and twenty eight cubic kilometres per yearís like. Itís a large volume of water.

NARRATOR: And in the coming century weíre expecting up to ten times more global warming. So Peterson decided to calculate how much more fresh water that could bring.

Dr BRUCE PETERSON: At the end of the next hundred years we expect approximately fifty percent increase in the discharge that these rivers would occur.

NARRATOR: Itís a horrifying prospect. A fifty percent increase in some of the worldís biggest rivers. If Petersonís projections are true a vast wall of fresh water will soon come flooding through northern Siberia. An extra thousand cubic kilometres a year more could flow in to the salty waters of the conveyor. The impact could be immense. And that was the warning that Bill Turrell had seen in the deep. Heís been monitoring the saltiness of the conveyor as it flows past the Faro Isles north of Scotland.

DR BILL TURRELL: This is the device we use to measure the salinity in the ocean. These bottles here collect the water samples that we bring back to the ship to analyse, to collaborate the electronics which are down here. This package measures temperature, salinity, about twenty five times a second as we lower it down from the surface down to the seabed.

NARRATOR: If the saltiness of the water is dropping itís a sign that the driving force of the conveyor is weakening.

DR BILL TURRELL: This graph shows the salinity or saltiness of the bottom water. Itís the saltiness from 1900 to the present day.

NARRATOR: Until the 1970s the salinity had been almost constant. But then it began to drop.

DR BILL TURRELL: After the late seventies we began to see a freshening of the bottom water. So much so that we, we began to doubt our own results. We took further samples, we checked with other countries who are sampling the same water, until eventually we became convinced that this change was actually happening.

NARRATOR: Turrell had measured the largest and most dramatic change recorded in the era of modern instruments. And there was worse. He took measurements from the very bottom of the ocean, from the return leg of the conveyor. Its flow had fallen by a massive twenty percent.

DR BILL TURRELL: Itís the first changes that weíd expect to see if we thought that global warming was beginning to effect the conveyor belt. Er a few years ago I probably wouldnít have said that because global warming was far more iffy, it wasnít so significant, it wasnít so certain. Now we really do know that fresh water input to the Arctic is increasing. The Siberian rivers are pumping out more fresh water. The Arctic ice sheets are melting and there is more release of fresh water. It, itís the most fundamental change Iíve observed in my career.

NARRATOR: The process that could cut off the conveyor has begun. We donít know where the cut off point is, we just know weíre getting closer to it.

Prof RICHARD ALLEY: I donít think that an abrupt sudden trip and fall down the stairs is the most likely outcome. But I think that the probability of that is high enough that we should really think about it.

Dr TERRY JOYCE : The likelihood of having an abrupt change is increasing because of global warming is moving us closer and closer to the brink. We donít know where that is but we know one thing, weíre moving towards the edge. And so I would say within the next hundred years itís very likely. In other words a fifty percent probability that this might happen.

NARRATOR: So there could be a fifty percent probability that the conveyor will stop flowing past the shores of Britain. Perhaps a one in two chance that one of the most important sources of heat in the world will just disappear. What would happen to us if the heat of a million power stations were to cut off? The answer depends crucially on how soon it happens. The Met Office has run a series of possible scenarios. In the first they examined what would happen if the conveyor cut off in fifty years. They have balanced the effects of fifty years of global warming against the regional cooling caused by the conveyor cutting off. In this scenario the area around Britain does indeed get colder.

Dr RICHARD WOOD: Cooling due to the ocean circulation collapsed is even stronger than the global warming. Weíre still looking at a climate which is a lot colder than todayís.

NARRATOR: But how much colder? This was the coldest winter in the past century. A freak event not easily forgotten.

PROF BILL MCGUIRE: The winter of 1962 to 1963 is something that people of my age particular remember because from Boxing day Ďtil March there was very deep snow on the ground across much of England.

NARRATOR: Blizzards lashed the country for day after day. In places snow lay eight meters deep. And temperatures fell to minus twenty two degrees.

Dr RICHARD WOOD: That would have had a huge impact on, on peopleís lives. Thereís a lot of disruption to all sorts of infrastructure, people were snowed in for long periods, over quite a wide part of the country.

NARRATOR: The electricity supply failed across the whole south east. Crops had to be drilled out of the ground. We asked the Met. Office to examine their scenario and calculate how often we could expect a winter like this. Their answer, once every seven years. But there is a second scenario the scientists have been examining.

PROF BILL MCGUIRE: Well I can only describe it as catastrophic.

BOB GASGOSIAN: The implications are huge.

PROF BILL MCGUIRE: Itís clearly going to influence every single one of us every day of our lives.

BOB GASGOSIAN: The economic implications, the political implications, er and the national security implications, for all countries.

NARRATOR: This scenario is that the gulf stream conveyor cuts off, not within fifty years but twenty. Itís described as a low probability, high impact event. The whole of north west Europe gets a lot colder.

Dr RICHARD WOOD: This is really a huge change in climate, a massive cooling. If this was to happen we would certainly have a future which was very cold, very different from what we have today.

NARRATOR: The coastline of Britain would become unrecognisable.

Dr RICHARD WOOD: You could expect to see er sea ice off the coast of, off the coast of south east England probably several miles off shore.

NARRATOR: We would struggle to keep our coastline open and our ports working. But the effects of a shutdown could be even greater on land. Winter blizzards would bring us entirely new hazards.

PROF BILL MCGUIRE: Weíd expect to see ice storms. Now these are severe winds but they bring with them either frozen rain or snow which clogs up and builds up on cables, power cables, telephone cables, brings them down very effectively. These conditions can persist for days and can really bring a country to its knees.

NARRATOR: Our infrastructure would be in danger of collapse.

PROF BILL MCGUIRE: The power lines would ice up, theyíd snap, theyíd collapse and this could happen virtually country wide during the worst of these storms.

NARRATOR: With enough warning we could make plans and adapt to our new landscape. The trouble is there wouldnít be any warning.

Dr TERRY JOYCE: Itíll be quick and suddenly one decade weíre warm, the next decade weíre in the coldest winter that weíve experienced in the last hundred years, but weíre in it for a hundred years.

NARRATOR: Winters like this would be commonplace.

PROF BILL MCGUIRE: I think weíd be extraordinarily poorly equipped to deal with a gulf stream shutdown. If weíre dealing with a situation where snow is on the ground for perhaps thirty days a year or maybe up to a hundred days a year, with temperatures regularly down in the minus twenties, I think weíll find it very, very difficult to cope with that.

NARRATOR: It could mark the end of the British way of life as weíve known it. But there is something even more disturbing about the conveyor cutting off. Something that suggests it could cause a catastrophe of truly global proportions if it were to happen. The Met Office have run another computer simulation. It shows a clear link between a conveyor cut off and patterns of rainfall across the world.

Dr RICHARD WOOD: One thing thatís really surprised us was the fact that, that the impact is not just confined to our part of the world.

NARRATOR: The dark red areas of his map suffer massive drops in rainfall.

Dr RICHARD WOOD: The main rain band in the tropics would move quite a bit south so all these countries in the red region here would lose a very large proportion of their rainfall and that could impact on a large number of people in those parts of the world.

NARRATOR: Central America would be one of the worst hit regions. It could lose up to forty percent of its rainfall.

Dr RICHARD WOOD: The model suggests that the present er vegetation in the rainforest wouldnít be sustainable in that situation, and the forest would die and be replaced by grassland.

NARRATOR: While we shivered through long and bitter winters huge tracks of forest would die away. Well thatís what the computer model said. And then came evidence that it had actually happened before. The clues came from within Richard Alleyís ice cores. They contained bubbles of ancient air, evidence of how the atmosphere has changed. Alleyís attention focussed on one gas that was present in the bubbles, methane. Itís produced naturally by bacteria in the tropics when they receive plenty of rainfall, and it disappears at times of drought.

Prof RICHARD ALLEY: And so you can ask of those bubbles how many swamps were there on earth. And this is wonderful because it tells you something thatís happening across a whole bunch of the land area. If the world dries up partially theyíll be much less methane in the air and it will show up, right there.

NARRATOR: He found that the methane levels plummeted at the same time as temperatures had dropped in the past. It seemed the conveyor cut off made the world drier.

Prof RICHARD ALLEY: And when the north Atlantic has been cold in the past the monsoon seems to have weakened or failed in places in Asia. Chill out the northern Europe and you dry a lot of places around a lot of millions of people with that.

NARRATOR: The monsoon is a lifeline to hundreds of millions of people living across the Asian subcontinent. Agriculture depends on its heavy rains. Should this scenario ever happen it would bring disaster on an unimaginable scale. There would be famine, economic collapse and refugees on the move.

DR BOB GASGOSIAN: We had only three billion people on this earth in 1962, we have over six billion now. Where are these people going to go if there are economic hard times? If we were to get on the same scale of the Irish famine today, that occurred then, I am very concerned at what would happen at that time.

NARRATOR: And there is a final twist to these scenarios. Switching the conveyor off can happen at speed. But switching it back on is altogether harder.

TERRY JOYCE: Weíre moving along and suddenly we drop down, how do we switch it back on? Well we would have to climb this cliff. Thereís an enormous amount of energy required to climb that cliff.

Prof WALLY BROECKER: And thereíd be nothing in the meantime we could do to change it.

TERRY JOYCE: We have to reveres this effect of global warming by hundreds of years.

NARRATOR: We are now preparing for more heat and more rain. But our new understanding of how climate change works challenges all that. It suggests our defences could be pointing the wrong way.


 
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