Robyn Williams: This is ABC Radio National and a Science Show all about Gaia, the idea that the Earth behaves like a complete integrated living thing, and where that leaves us in the face of global warming.

In recent years the arguments about global warming have shifted rapidly. At first the question was whether it was happening at all and if it is, are we guilty as charged? Then, as scientific evidence became indisputable, the debate centred on the urgency and the extent of the problem. But now as some of the impacts become more obvious, scientists have turned their attention to how we might cope. Preparations range from engineering drought-resistant crops to building a Noah's Ark for world seeds in the Arctic.

But James Lovelock, the father of Gaia theory, goes much further. He believes that the point of no return in climate change is beyond us and civilisation itself will suffer. In this Science Show we look at the climate from an Earth System point of view and investigate the arguments beyond Lovelock's apocalyptic vision. The program is written and produced by Annamaria Talas. It's presented by David Fisher.

David Fisher: The oceans are warming, ice is melting, the North Atlantic current is weakening. Soils and forests are releasing methane and carbon. Incredibly this is occurring following global warming of less than one degree.

Environmentalist David Suzuki, climatologist Vicky Pope, paleo-climatologist Richard Alley, and marine physicist Tim Barnett sum up the problem.

David Suzuki: We've added more than 30% more carbon dioxide into the atmosphere than existed 150 years ago. The really frightening aspect of that is that if we were to stop, it will be still over 1,000 years before what we have already added to the atmosphere will equilibrate in the air, the water and then the land.

Vicky Pope: We're already committed to a certain amount of temperature increase whatever we do, but obviously if we don't do anything now, then that increase will be worse.

Richard Alley: We are now changing the greenhouse gases in the atmosphere and we are changing them fast enough that nature cannot undo what we are doing.

David Suzuki: We've set in motion a massive experiment that won't be played out for literally hundreds and hundreds of years.

Vicky Pope: We are talking about the minimum sort of change in temperature over the next 100 years of 1.5 degrees which is double what the changes have been in the last 100 years. And the maximum change could be up to six or possibly even eight degrees.

Tim Barnet: All of the oceans in the upper layers are warming. The amount of energy that has gone to the oceans in the last 40 years to do this is enough, if we could harvest it, to run the state of California, the world's seventh biggest economy for over 200,000 years.

Richard Alley: There are points of no return; certain times in certain places, the Earth's climate system will slowly change, slowly change and then it will jump or flip to a new state.

Vicky Pope: Essentially the uncertainty is whether it's going to be bad or very bad, not whether it's going to be good or bad.

Tim Barnet: We are not talking about scare tactics here, we are not talking about doom and gloom, we're talking about a situation that we are creating for ourselves, and the best minds of the planet tell us we've got a problem.

David Fisher: One pre-eminent scientist, James Lovelock, who conceived the first entirely new way of looking at life on Earth since Darwin, believes we have already passed the point of no return.

James Lovelock: Its far too late, hopelessly too late. We are wasting our time and energies on all the wrong things. We have to face up for the fact that we are about to enter a major war with our own planet.

There is a huge challenge and problem before us of surviving ourselves and making sure that civilisation survives, and that's more than enough to exercise us and something that everybody will soon become quite desperately involved with and will want to do something about it.

David Fisher: Lovelock warns that like the rope of destiny in Wagner's The Ring Cycle, civilisation is at the end of its tether and could collapse. On The Science Show today we'll explore Lovelock's Gaia theory to see whether his vision of the state of the planet is just a bleak view or a wakeup call to stop us sleepwalking into disaster.

Excerpt from Notes of a Biology Watcher, by Lewis Thomas: Viewed from the distance of the moon, the astonishing thing about the Earth, catching the breath, is that it is alive.

If you look long enough you would see the swirling of the great drifts of white cloud covering and uncovering the half-hidden masses of land.

If you had been looking for a very long geologic time, you could have seen the continents themselves in motion, drifting apart on their crustal plates, held afloat by the fire beneath.

It has the organised, self-contained look of a live creature, full of information, marvellously skilled in handling the sun.

David Fisher: An excerpt from a book published 40 years ago, Notes of a Biology Watcher, by Lewis Thomas, the American scientist and poet.

It's almost like an image of Lovelock's idea; planet Earth with a special characteristic, deserving its own name, Gaia.

James Lovelock: Gaia of course is the name of the ancient Greek Earth goddess, and it was suggested to me by the novelist William Golding for my theory that the Earth is a self-regulating system made up of the organisms, the rocks, the air and the ocean...always keeps the Earth a habitable place for life.

David Fisher: By using a mythological name for his theory, Lovelock caught the public imagination but alienated many of his colleagues. In the early 70s, a whole generation was in search of a new religion. Having just seen the first space images of the Earth, hanging in the vastness, in its shimmering, fragile beauty, they seized on the metaphor. Gaia was a reinvented Mother Earth, which needed to be worshipped and protected. Science, on the other hand, was preoccupied with reductionism and triumphant in investigating the world in its finest details. In this climate, a name with holistic connotations ignited fierce opposition and the scientific establishment was quick to reject it.

Tim Lenton is Lovelock's closest collaborator. Tim, what was at the root of the criticism, and did it influence Lovelock's thinking?

Tim Lenton: One of the big criticisms that came from biologists is that his idea implies some kind of teleology, some kind of conscious foresight or purpose on the part of unconscious organisms, and he answered that by developing his simple Daisyworld model which is a system that just demonstrates that life could automatically be involved in regulating the climate without any kind of conscious involvement. That, in turn, led him towards the view that we are talking about regulation of the planet as a whole system, both the living things and the non-living things intertwined, whereas originally he was talking in terms of life being the controller for the rest of the system.

David Fisher: Critics and supporters of Gaia have tended to react to the theory as a literal explanation of the world, but perhaps this overlooks its real value as a way of interpreting problems we face today. Should we see Gaia as reality or a metaphor? Andrew Watson, Lovelock's former colleague, and Fellow of the Royal Society.

Andrew Watson: That's a good question; it's both. If you concentrate on the idea of the Earth as being alive then I think that that's a metaphor. But, as with many good metaphors, it contains a lot of truth within it.

Peter Cox: Before Jim Lovelock came up with the concept of Gaia we really thought about the physical parts of the climate system, for example, being quite distinct from the biology, the life on Earth, and really the life on Earth was just a slave or responder to the physical environment. The physical world, the physical climate is actually just as much dependant on biology as the other way around, and that's been a very valuable thing. It's a very valuable metaphor. There are also some real aspects of that interaction that affect the way we think about the future of the planet.

David Fisher: Peter Cox, former climate modeller at the Hadley Centre in the UK and now director of the Centre for Ecology and Hydrology.

Here's Stephan Harding, Lovelock's collaborator on Gaian computer modelling.

Stephan Harding: For me, Gaia theory is so powerful because it has very important scientific implications, but perhaps more important than that it has incredibly serious and important philosophical implications for the way our culture lives in nature. Gaia gives us a chance to recover an ancient story about the Earth. The story is that the Earth is alive and that we are one of her many creatures. The story we must reject, I am suggesting, is that the Earth is nothing more than a dead machine that we can use and exploit as we wish. If we keep to the old story of the Earth as a dead machine, all our climate science, all our efforts towards sustainability will come to nothing because we will be acting from within the wrong story, whereas if we accept the Gaia story, which is now based on solid science, we can begin to relate to the Earth again. So it's a far more inspiring, a far more empowering story to be living our lives through than the old, dead, mechanistic story which just kills the Earth and makes it into nothing more than a whole set of mechanical cogwheels.

David Fisher: The idea of Gaia was born out of efforts to find life on another planet. It all started in the 60s when NASA invited Lovelock to a meeting.

James Lovelock: I remember vividly being invited to just come along and listen at a meeting of biologists who were making life detection experiments, and at the end of the day I was appalled. I'd been trained as a bacteriologist so I knew a bit about what they were up to. They were going to send culture media to Mars and just hope that a bit of soil that they put on it would reveal growing bacteria, and I said, 'How the hell do you know that they'd grow on your culture? It's pretty difficult here on Earth.' They naturally said, 'Well, what would you do?'

And without thinking I said, 'I'd look for an entropy reduction on the planet.' That would be a much more general life detection experiment, and it would work whatever form of life you had here. They got very annoyed. And they went to the boss at JPL, (Jet Propulsion Laboratory) my boss, and said, 'Look, this chap is interfering with the way we are trying to run our life detection experiments, he ought to be curbed.' And I was called to see him and he said, 'I hear you think an entropy reduction experiment would find life on Mars. Well, you have until Friday to come to me with a practical entropy reduction experiment.'

So when you are challenged like that and you know your job is on the line you think fast, and I did. And after spending a somewhat miserable night in my hotel room, it suddenly dawned on me that all that one has to do is to measure the chemical composition of the planet's atmosphere. Quite simply, if there is life on it, it has to use the atmosphere as a source of raw materials and a place to put its waste products.

David Fisher: But you took it further when you realised that the composition of the air was constantly maintained.

James Lovelock: Yes, the moment I started thinking about the Earth's atmosphere...it's amazing! I mean you have got hydrocarbons like methane mixed with oxygen which is a reactive mixture, it's the sort of gas that goes into the intake manifold of your car, whereas Mars is exhaust gas; everything had been reacted as far as it will go. My next thought was, how extraordinary, here we have an unstable atmosphere yet it stays constant over periods of time vast compared with the reaction rates of the gases in it. Something must be regulating it. It would be utterly impossible for a mixture of methane and oxygen to exist in an atmosphere at the sort of levels we have unless something was both making it and regulating it. And then I made a big mistake. I thought it was life at the surface that was doing the regulation not the whole system. And it took a long time after that right until near the end of the 1970s and after fierce arguments with the biologists who kept on saying organisms can never regulate anything beyond their phenotype, that it dawned on me that it was the whole system and not just life that was doing the regulating.

David Fisher: Stephan, when and how did this planetary-scale regulation emerge?

Stephan Harding: Three and a half thousand million years ago when bacterial life emerged and then rapidly spread all over the planet. And as soon as that happened the bacteria began to influence the atmosphere very radically. They began to take carbon dioxide out of the atmosphere through photosynthesis, and they began to return methane to the atmosphere through decomposition, and they also began to produce oxygen, and that's when the coupling first began.

James Lovelock: The more it spreads and the more it changed the composition of the Earth, the more the two things were forced to evolve together and become a single system.

Stephan Harding: So these two components in the system interact very tightly together and what emerges out of that interaction is that the whole planet is able to regulate its own surface conditions.

James Lovelock: If that happens we have a Gaia and it's formed.

Stephan Harding: The self-regulation is emergent, it's unexpected, and it results in the whole surface of the planet remaining habitable for life over vast periods of time. The best form of evidence is actually found in the geological record and in ice bubbles in the Antarctic where, by sampling the air within them, we can see very regular rhythms of ice ages and interglacial warm periods with a 100,000-year interval. That's strongly suggestive of a self-regulating system. We know that almost certainly the temperature of the Earth has never been too hot or too cold for life over 3,500 million years, despite the fact that the Sun has been getting brighter and brighter.

David Fisher: Stephan, how can a planet regulate its temperature?

Stephan Harding: What happens is that...what Gaia has to do to regulate temperature is play with the amount of carbon dioxide in the atmosphere, because you know carbon dioxide is a greenhouse gas; too much and the Earth gets too hot, too little and the Earth could freeze over. There is a peculiar and very important chemical attraction between carbon in the air in carbon dioxide and calcium in certain kinds of rocks, like granite and basalt. Water actually puts the carbon and the calcium together through the process of rock weathering. Through this process, which is greatly aided by the forests that make the rock crush and pulverise much more quickly.

Now, what happens next is that the carbon and the calcium are washed down by the rivers to the sea and there microscopic marine algae absorb the calcium and the carbon together and make chalk, and then they die and the chalk sinks to the bottom of the ocean and we have wast deposits of limestone and chalk at the bottom of the ocean which are holding carbon that used to be in the atmosphere. Now, this could cool the Earth into a frozen ice ball because it has taken carbon out of the atmosphere.

So how the carbon gets back? It gets back through plate tectonics. Because the great movements of the plates subduct the limestone and chalk deep under the earth where the chalk melts and this recreates granite and releases carbon dioxide to the atmosphere. Any time there is too much carbon dioxide in the atmosphere we get more rock weathering, so we get more carbon taken out of the atmosphere and the Earth cools. But if the earth gets too cool, there is less carbon dioxide coming in from the atmosphere, there is less rock weathering and so gradually, gradually, gradually the carbon dioxide builds up again from the volcanoes and the Earth warms. So this is how Gaia has maintained a habitable temperature.

David Fisher: Lovelock's ingenious idea for recognising signs of life on a distant planet gave him a unique insight into the influence of life here on Earth. Always a practical man, he wanted to go beyond theory to find evidence of Gaia at work.

Jim, you've spent years travelling around looking for fingerprints of Gaia. Could you take us to some of the places where you've found those signs?

James Lovelock: I used to wander on the sea shore and pick up all different bits of kelp, you know, seaweed, and put them in jam jars, sealed, and see what gases they gave off because I thought that biology on the planet was producing the gases of the atmosphere. And I thought an easy one to look at was the sulphur cycle because I knew that sulphur was deficient on the land therefore there had to be things in the sea that were making up the loss and transferring it through the air back to the land. And sure enough I have found quite a few algae on the cost that were generating huge quantities of dimethyl sulfide gas. I managed to bum a ride on a ship, The Shackleton, from Britain down to Antarctica and back. I took along a home-made gas chromatograph and measured the gases on the way, including the CFCs incidentally which was the thing that really kicked off the CFC-ozone level because I was able to demonstrate that they were all the way in the Earth's atmosphere right the way from here down to Antarctica and that the amount that was in the air was just about the same as had been released, so they weren't vanishing very quickly, just adding up. But much more interesting to me was that I found dimethyl sulfide being given off from the oceans everywhere the ship sailed. So that was my first bit of evidence that the Gaia system perhaps had something in it.

David Fisher: In fact it's this gas, dimethyl sulfide, that gives the sea its distinct smell. It's produced all over the world by marine algae.

Tim Lenton, the DMS production; do you see that as a regulatory Gaian mechanism?

Tim Lenton: Very interesting one. So we are talking about, yes, the production of this dimethyl sulfide gas by marine algae and the fact that it contributes to making cloud droplets, and the more of those are the more reflective the clouds are. So I think we've established that this leads to a current cooling of the climate state.

Peter Cox: It is undoubtedly a feedback loop that connects biology to physics in the climate system. We are absolutely sure that DMS emissions from the ocean are very important in regulating cloud brightness and therefore climate, but I don't think we yet know how DMS emissions will change as climate changes. There are a couple of possibilities. One is that the global warming makes the surface ocean warmer and actually stabilises it, prevents mixing between the surface ocean and depth. And that means less nutrients available to phytoplankton, and so phytoplankton could be less productive and produce less DMS, which would mean less bright clouds and a warmer world. So that would be a positive feedback on the system.

Tim Lenton: We are still talking about a Gaian phenomena when we are talking about tiny organisms in the surface ocean acting to change the reflectivity of clouds and therefore the temperature of the surface of the ocean. In the broader picture of Gaia they are both positive and negative feedback mechanisms, both amplifiers and dampeners on any existing change. The concern is that the Gaia system currently has a number of its systems going into positive feedbacks. So going from negative feedback tending to damp change, into positive feedback where changes are amplified.

David Fisher: So the idea is that Gaia regulates the temperature of the Earth through negative feedbacks while positive feedbacks act as amplifiers, in a way adding fuel to the fire. Stephen Harding:

Stephan Harding: A negative feedback is a regulating feedback, it is a feedback which acts to keep conditions more or less constant, for a while at least. A positive feedback, as you say, is like adding fuel to the fire; it's a feedback that doesn't stay still. And we are now pushing the Earth into a positive feedback state.

David Fisher: In the 40 years as Gaia grew from intuition to theory, the signs of global warming have become increasingly apparent.

Peter Cox: The warming has been 0.7 of a degree over the last 100 years, roughly speaking, but most of that has actually been since the Gaia theory was conceived in the 70s actually. The big issue really is not the size of the change necessarily, it's the rate of the change, it's the fact that 0.7 degrees doesn't sound like very much but it's actually rather a big and rapid change in the context of what has been a very stable climate since the last glacial period.

David Fisher: Peter, has climate modelling have been influenced by Gaia over the last 40 years?

Peter Cox: There has certainly been an impact of Gaia theory on the climate modelling world, if you like. I mean, climate modelling, when I started out in it maybe 15 years ago now, was really about a sort of extension of weather forecasting; it was about winds, about circulation patterns. And now we actually talk not of climate models so much but of Earth System models.

Andrew Watson: If you go back 20 years, you know, people only modelled the atmosphere. Ten years ago they began to have coupled models which modelled the ocean and the atmosphere. Then five years ago they had carbon cycles and bringing life, if you like, into it for the first time.

David Fisher: So what is the benefit to climate change research in taking the Gaian perspective? How has it changed or benefited?

Andrew Watson: Well, the climate change research has to take on the Gaian perspective because that's the way the world is. The world is complex. And Gaia is basically saying that this is a complex system.

Peter Cox: So there is a whole series of feedbacks now that are being introduced into climate models that evolves them from climate system models to Earth System models, and that's a very kind of Gaian view point of the way the world operates.

David Fisher: So, Peter Cox, do you fully accept the idea of Gaia?

Peter Cox: I suspect in my case because I am thinking about climate change in the next 100 years I'm thinking less about the stabilising effects of the interacting components of Gaia and more about how life will affect climate change over the next 100 years. And that needn't be stabilising. In fact, my work suggests that it may well not be, that the existence of life on Earth could accelerate climate change to some extent over the next 100 years. That's an extension of the idea that the system is interacting but it doesn't sit with the original concept of Gaia that life would kind of retain conditions that would be suitable for it or that would stabilise us or get us out of the mess of climate change.

David Fisher: In January this year, the UK Meteorological Office published a report titled 'Avoiding Dangerous Climate Change'. It concludes that future emissions of greenhouse gases are likely to raise global temperatures by between 1.4 and 5.8 degrees this century. It suggests that we might be already on a path of crossing key thresholds where changes could be irreversible.

Peter Cox was one of the scientists who contributed to the report.

Peter Cox: We are going to see evidence of accelerating change over the next few decades. That isn't the same thing as irreversible change. There are some irreversible changes that we believe are close. For example, the one that's the most obvious is the Greenland icesheet melt at a warming of 2.5 to 2.7 degrees, which we are very likely to pass before the middle part of the century. What that means is that it would be difficult to restabilise the Greenland icesheet except by sucking CO 2 out of the atmosphere, which looks to be very unlikely. The change will be rather slow; it may take many hundreds of years for the Greenland icesheet to melt but it's almost inevitable that once you get beyond those temperatures it will ultimately melt and raise global sea levels by seven metres. So there are some thresholds we cross. They are not like falling off the cliff but they are like sliding down a slippery slope; you can't get back up very easily.

Stephan Harding: Once the world warms to four degrees centigrade, the Amazon can no longer keep itself cool and it gets taken over by savannah. There will be huge fires, lots of decomposition, a massive pulse of carbon to the atmosphere which will further warm the planet.

Tim Lenton: The concern is that we already seem to be on a path of more rapid change than are our baseline predictions. Certainly if there is a continuing link between growth of economies and growth of emissions then we could be losing the Artic sea ice in the next 50 years.

Stephan Harding: As the world warms, the ice melts, it's quite simple really. And as the ice melts, more dark ocean is exposed to the Sun and that warms the world even further which melts even more ice, which exposes even more dark ocean to the Sun, which warms the world even further...you get the idea, its another positive feedback.

Peter Cox: There is a whole issue of where the Earth system now sits. Now there is a very strong evidence that certainly over the last million years that the climate system has regulated itself between glacial and interglacial periods and these are quite tightly constrained, with carbon dioxide levels typically between 280 and 180 parts per million. We are now at 380 parts per million. So the issue is that we have pushed the system that does regulate itself beyond where it normally regulates. And that's why there is a possibility that what were previously stabilising effects become destabilising.

James Lovelock: We have chosen a very bad moment to put those gases into the atmosphere. When the Earth is in one of these interglacials it's in a very similar state to a person in a fever; all the systems that normally would regulate your body temperature when you have a fever go in reverse, they go into positive feedback, and you shiver...well, you should do that when you are cold, not when you are hot. You start sweating when you have a fever and sweating is a normal way you cool yourself, and all of the system go in positive feedback. The same is true for the Earth. Now, this means that small amounts of carbon dioxide that we put in, could have put into the air with impunity in the middle of the Ice Age. Their effect is amplified by the positive feedbacks and is much larger than would ordinarily occur. And not only that, but we always have to keep in mind that farming, taking away the natural ecosystems of the Earth that do the regulating, is just as harmful as putting gases in the air. So it's a kind of double whammy at the wrong moment for the Earth.

Stephan Harding: Everything we are doing seems to be pushing the Earth into an ever warmer state, and a lot of it is happening as our activities reduce the biodiversity of the planet. For example, the Amazon produces a huge amount of air conditioning service for us by keeping the Earth cool. If we cut down the Amazon, we couldn't possibly replace that service. So one key lesson from all of this way of seeing the world, from a Gaian way of seeing the world, is that biodiversity is critically important for maintaining a habitable Earth. We used to say that we must preserve biodiversity because of its intrinsic value. Well, that had no impact at all, no one cares about intrinsic value. Then there was the argument of we must preserve biodiversity because down in the tropics there's lots of plants with valuable medicines, that's why we've got to do it. But now we've got an even more powerful reason which is that we have to preserve biodiversity if we want to have a climate on this Earth which is going to be suitable for us.

David Fisher: The Earth has seen regular climate changes in its history, but Gaia, as theory suggests, has always kept conditions on the planet within a range that's favourable to life. Now Lovelock warns that Gaia's regulatory systems have changed direction and begun to amplify human-induced global warming. This phenomenon is the title of his latest book, The Revenge of Gaia.

James Lovelock: The Earth has now passed the point of no return and is rapidly moving towards a hot state that it has been in before, a state it was in 55 million years ago. There will be very little of the life we know left on the main continents which will have reverted to scrub or desert. There will be mass migration of life and of people towards the remaining habitable parts. What we have to aim at mainly is organising our survival and the survival of civilisation, and we shouldn't waste energy trying to do silly things like plant biofuels which is quite mad really when you think about it. The Earth has hardly got enough land to feed all the people, how on Earth can it feed all the cars which consume ten times as much as the people do? We're wasting our time and energies on all the wrong things. We have to face up to the fact we're about to enter a major war with our own planet. As in a war, we've got to think about our defences and we've got to think about security of supplies of food and energy and things like that.

David Fisher: It sounds pretty gloomy. Tim Lenton, how does your view of the future compare with that of James Lovelock? I mean, he is pretty dark on the future isn't he?

Tim Lenton: He is darker than dark, yes. I think he genuinely feels that there's a reasonably high probably that we've, in a sense, passed some kind of point of no return. I personally don't think that we have passed that point yet. There's an awful lot that we can do now in terms of reducing our greenhouse gas emissions and so forth that could help determine how severe our future climate change is, and indeed we could avoid a number of these critical thresholds or tipping points in Gaia.

David Fisher: I suppose it could be also argued that the tipping point is actually not a point but a range, over a range of time.

Tim Lenton: Exactly. In fact, we have different components of the system that we mentioned; the Arctic sea ice, the Greenland icesheet, the Amazon rain forest, maybe the El Nino southern oscillation might switch to a different state. But we would expect that individual tipping points for each component to be set at, say, a different global warming or a different particular level of carbon dioxide in the atmosphere. So it's not clear that there would be what some of us would describe as domino dynamics, which is you pass one tipping point and that changes the climate enough to trigger the next one and so forth like the toppling of a line of dominos. There is a danger that things may be set up that way but personally it's not clear to me that they are.

Peter Cox: At the moment what is happening is that the natural systems (Gaia, if you like) are absorbing about 50% of our emissions of carbon dioxide and really slowing down climate change, otherwise carbon dioxide would be going up twice as fast as is now and the climate would be approximately changing twice as fast. It's been kind of assumed that it would continue into the future and there would always be this buffering mechanism and that Gaia would slow down climate change indefinitely. What our modelling results, that are partly motivated by discussions with Jim Lovelock, suggest is that that may not happen, that in fact climate change itself could suppress the ability, particularly of vegetation to remove carbon dioxide from the atmosphere. Carbon dioxide could therefore could go up faster and global warming could accelerate because of interactions between biology and the physical climate system.

David Fisher: Andrew Watson, what are the Gaian feedback systems that you see are changing as the Earth gets hotter?

Andrew Watson: The uptake of carbon dioxide by the land biota, we expect to change. It's very variable, even on a year-to-year basis. We know that it is climatically sensitive therefore. And we are also actually are beginning to think that the uptake of carbon dioxide by the ocean will change quite substantially. As the oceans warm, they become less able to take up carbon dioxide. There's just a simple chemical reason for that. There is also the fact that the circulation, the vertical overturning of the oceans may tend to slow down, and this means that carbon dioxide can't get into the deep waters as fast as it previously could, and that would tend to slow down the rate of uptake of carbon dioxide by the oceans.

Peter Cox: We should start seeing evidence that, for example, the carbon dioxide concentration in the atmosphere starts to accelerate upwards, even if emissions aren't going up. So I am looking out year-on-year at the carbon dioxide records, comparing them to the models and see if we can detect anything.

Stephan Harding: The soil holds lots and lots of carbon, much more than there is in the atmosphere, but, again, if the Earth warms too much, then the microbes that live in the soil like the high temperatures and they begin to decompose the organic carbon in the soil and again we get a massive pulse of carbon into the atmosphere.

Tim Lenton: So our predictions are that it maybe only another 40 or 50 years before the land biosphere becomes...instead of a sink for carbon becomes a source of carbon, and tends to be a net addition of carbon dioxide to the atmosphere.

Peter Cox: In the UK there was a study suggesting that the soil carbon dropped very alarmingly. And I think people are still trying to confirm that. It seems such a large change that it was larger even than the most pessimistic models. So we have to wait and see about that, but it is the basic direction we expect things to go.

David Fisher: Last month, a panel of leading scientists convened by the BBC spent two days debating Lovelock's dire view on the state of the planet. Although they didn't quite share Lovelock's apocalyptic vision, there was a consensus that in our lifetime we'll face serious climate change. Andrew Watson was one of the panellists.

Andrew Watson: I think there will be substantial change whatever we do. If we do nothing over the next 20 years it will be catastrophic. If we do nothing over the next 50 to 100 years it might even be terminal.

David Fisher: Andrew, Lovelock suggests that we are moving into a much warmer but stable situation for Gaia. Do you accept that idea?

Andrew Watson: I don't know.

David Fisher: And that it is something that, you know, hasn't existed for millions, perhaps 50 million years.

Andrew Watson: Well, that I think is certainly right. The Earth has been, broadly speaking, cooling over the last 50 million years and we are going to push it back into a very much warmer state. Is it going to be stable? That's a very good question. I don't know the answer to that. The fact is that the last time we had high levels of carbon dioxide in the atmosphere was 100 million years ago and the Sun was a little bit cooler at that time. Now if we push it up...this is not something that most climatologists will talk about but I think that there is a small chance, maybe a 1% chance, that if we really hit the planet too hard we may push it into a runaway system in which the temperature simply goes up and up until the oceans boil into the atmosphere, and that would extinguish all life on Earth.

Peter Cox: One of the things that is quite difficult to get across but is very important is the idea that we need to understand what might be called high impact low probability events. In a sense, in the last 20 years or so the climate modelling community has been kind of obsessed by working out what the most likely outcome is of us doing nothing. The issue now is that that's probably not the most important thing to know about. The most likely thing is not the thing that you should necessarily be most concerned with. What you should be concerned with is a bit like the insurance problem, is the highest risk, and the highest risk can be actually a very high impact, like a very large climate change that is unlikely but could still the highest risk of all to the system you are in and human society.

David Fisher: Andrew let's get out the crystal ball and just notch it forward to say 100 years time; what changes do you see?

Andrew Watson: If one takes the business as usual scenario in which we continue to emit a lot of carbon dioxide, then the planet will be probably four or five degrees warmer, the north polar cap will have been completely disappeared and much of the planet will be difficult to live on.

David Fisher: Why is that?

Andrew Watson: It will be just too damn hot. Many of the tropical regions are already actually quite hot to live in, but there will be large regions where people will not want to be because it is too hot. I would imagine therefore that there will be human conflicts on a grand scale. There are many, many people who live in those regions now. They will presumably be blaming the industrialised nations for their poor living conditions. I would also think that all of the natural ecosystems on the planet will be in crisis. There will be regions which would be further to the north presumably, such as in Southern Europe, where it would be possible to grow tropical forest but there won't be any because it won't have had time to grow. So I don't see a very happy situation. We will have to be using all of the land area that still can support crops to grow it. So that will mean there will be no space for the natural ecosystems that are important for the carbon cycle, for the other cycles of the planet.

David Fisher: Stephan Harding, what do you think conditions might be like in 100 years time?

Stephan Harding: Business as usual scenario...

David Fisher: Hundred years time, you're at the window looking out...

Stephan Harding: Britain will probably be reasonably okay. It will probably be sunnier here. The Arctic will be quite nice. Norway will be a terrifically nice place to live. Britain will be a bit marginal. We'll have terrific energy shortages here. We'll have much more severe storms in the winter, probably hurricane strength. There will be severe and sudden flooding events which will leave many people homeless, and then in the summertime there will be droughts. In 100 years time I suppose it is conceivable that sea level rise will have begun to bite away at London, so it's possible that we start getting the first 100,000 or 200,000 or so ecological refugees from the flooding of London, and this will create huge problems with law and order. You know, where will these people go? So, I mean, this is the worst-case scenario but it's certainly not off the cards.

David Fisher: Tim, can you tell us what you see if we go forward in time, say 100 years down the track, in Australia, assuming that we've changed our ways just a little, just a little tinkering here and there.

Tim Lenton: I would be worried about the fate of the Great Barrier Reef, for example. I suspect much of the reef would have disappeared.

David Fisher: What about where we live, Tim?

Tim Lenton: On the continent the challenge is really of food production in a land which is...I mean, Australia is remarkable for the antiquity of its rocks, it's underlying geological base. You have really some of the oldest continental rock in the world, but unfortunately this means that the weathering and extraction of nutrients from that rock has reached a point where soils are relatively less fertile in Australia than in many other regions where we have agriculture. So you've actually got a much more sensitive baseline environment than, say, Western Europe or much of North America. So I'd be very concerned that Australia would become more and more marginal for a lot of types of agriculture.

David Fisher: What do we have to avoid? What are the key things to avoid, Tim?

Tim Lenton: We need to avoid provoking climate changes that are severe enough to threaten our own civilisation. We should be clear that past civilisations have degraded their environments to the degree that it played a part in their downfall. Now, what would really threaten civilisation? It is hard to prescriptive about that. We see in certain case studies that it's an environmental trigger that then provokes conflict over dwindling resources, perhaps of water or of food. I would be concerned then particularly about changes in the atmospheric regime or the atmosphere ocean modes like the El Nino southern oscillation, if they change in a way that really affects our ability to produce food crops in certain key regions then that will have large ramifications, I think. Then perhaps less urgent but a concern in the longer term is any significant rise in sea level displacing people.

Andrew Watson: We can be mesmerised by the difficulty of this problem. If you look at it in another perspective, it's not that difficult. If we really think that carbon dioxide emission is endangering the planet then there are lots of things that we can do about it. We can use nuclear energy, we can increase our efficiency of use of energy, we can capture the carbon dioxide from fossil fuel burning.

David Fisher: Peter Cox, do you think we can make it in time?

Peter Cox: There are elements of ingenuity that humans show that will help us to do that if we get the right incentives in place. So I don't think it's too late but I do believe that we can't mess about for two more decades without doing anything, we have to get on with it because a lot of the systems that we're dealing with take a long while to turn around, and human systems amongst them. It's got a lot of inertia in it, we have to slow down the tanker and start turning it way before the threshold is obvious to you.

David Fisher: The future is not inevitable, but we'll have to work hard and fast if nature can continue to provide the necessities for life, services we take for granted today. Some people believe the means already exist for us to move from a carbon-dependant lifestyle and economy but we need political will to make it happen. This leads to a crucial question.

Andrew Watson, can democracy as we know it be effective in tackling this global problem?

Andrew Watson: Democracy is not particularly effective at these kinds of problems. But like Winston Churchill said, it's the worst solution but the only solution, the only type of government that we have. This is why it's so important to pressurise governments. Individuals have to be concerned about this so that they are not only doing their own individual things to lower carbon emissions but they are making it so difficult for governments to govern unless they do something sensible about this.

Peter Cox: One of the things we have not got to yet, certainly not in the Western democracies, people are not yet voting on environmental issues, me included. And until we do that we won't be using our ultimate leverage as individuals in democracies, which is to actually choose governments based on what they do and how they act with regard to the environment, and that to me is the key. We have to vote with our feet in that sense.

Andrew Watson: If governments do not take this on, there will not be governance in 100 years time.

Robyn Williams: That Science Show special on Gaia and the warnings of Jim Lovelock was written by Annamaria Talas and presented by David Fisher.