NARRATOR David Attenborrough: And that's not all. Climatologists like Peter Cox have begun to worry that Global Dimming has led them to underestimate the true power of global warming. They fear that the Earth could be far more vulnerable to greenhouse gases than they had previously thought.

DR PETER COX: We've got two competing effects really, that we've got the greenhouse effect, which has tended to warm up the climate. But then we've got this other effect that's much stronger than we thought, which is a cooling effect that comes from particles in the atmosphere. And they're competing with one another. And we know the climate's moved to a warmer state by about point six of a degree over the last hundred years. So the whole thing's moved this way. If it turns out that the cooling is stronger than we thought then the warming also is a lot stronger than we thought, and that means the climate's more sensitive to carbon dioxide than we originally thought, and it means our models may be under sensitive to carbon dioxide.

NARRATOR: The models that everyone has been using to forecast climate change predict a maximum warming of 5 degrees by the end of the century. But Cox and his colleagues now fear those models may be wrong. Temperatures could rise twice as fast as they previously thought with irreversible damage just twenty-five years away.

DR PETER COX: If we don't do anything by about twenty thirty we could have a global warming of exceeding two degrees, and at that point it's believed the Greenland ice sheet would start to melt in a way that you wouldn't be able to stop it once it started it, it would melt. Take a long time to melt but ultimately it would lead to a sea level rise of seven or eight metres.

NARRATOR: Once the Greenland ice cap begins to melt, nothing will stop it. Many of the world's major cities will be living on borrowed time. Decade by decade, the risk of catastrophic flooding would increase inexorably. But unless action is taken it won't stop there. Because after Greenland, the world's tropical rainforests will start to wither in the heat.

DR PETER COX: 2040 it could be four degrees warmer, the climate change could have led to big drying particularly in the Amazon Basin, that would make the forest unsustainable, we'd expect the forest to catch fire probably, turn into savannah and maybe ultimately even desert if it gets really really dry as our model suggests.

NARRATOR: And as the rainforest burnt away, it would release vast amounts of carbon dioxide into the atmosphere, driving global warming still further. Cox calculates that in just a century, the world could be 10 degrees hotter, a warming more rapid than any in Earth history. If this were to happen, the landscape of England would be utterly transformed.

DR PETER COX: We're talking about a change from er a lush, moist climate, environment like this, to a North African climate in just a few decades or a hundred years.

NARRATOR: Most British plant species could not survive a North African climate. With vegetation dying everywhere, soil erosion would become a severe problem. From a green and pleasant land, England would become a country of extremes, with winter flooding giving way to summer dust storms. And it will be far worse elsewhere.

DR PETER COX: You can imagine ten degree warming in the UK in a hundred years is catastrophic. Ten degree warming in a hot country already makes it essentially uninhabitable.

NARRATOR: And just when one might think things could get no worse in the far North a ten degree warming might be enough to release a vast natural store of greenhouse gas bigger than all the oil and coal reserves of the planet.

DR PETER COX: We will be in danger of destabilising these things called methane hydrates which store a lot of methane at the bottom of the ocean in a kind of frozen form, ten thousand billions tons of this stuff, and they're known to be destabilised by warming.

NARRATOR: At this point, whatever we did to curb our emissions, it would be too late. Ten thousand billion tons of methane, a greenhouse gas eight times stronger than carbon dioxide, would be released into the atmosphere. The Earth's climate would be spinning out of control, heading towards temperatures unseen in four billion years. But this is not a prediction - it is a warning. It is what will happen if we clean up pollution while doing nothing about greenhouse gases. However, the easy solution - just keep on polluting and hope that Global Dimming will protect us - would be suicidal.

DR PETER COX: If we carried on pumping out the particles it would have terrible impact on human health, I mean particles are involved in all sorts of respiratory diseases, that's why they're being brought under control, and of course they effect climate anyway. If you, if you fiddle with the, the balance of the planet, the radiative balance of the planet, you affect all sorts of circulation patterns like monsoons, which would have horrible effects on people. So it would be extremely difficult, in fact impossible, to cancel out the greenhouse effect just by carrying on pumping out particles, even if it wasn't for the fact that particles are damaging for human health.

NARRATOR: Instead we have to take urgent action to tackle the root cause of both global warming and Global Dimming - the burning of coal, oil and gas. We may have to make very difficult choices, about how we live and how we generate our electricity. We have been talking about such things for 20 years. But so far very little has been done in practical terms. The discovery of Global Dimming makes it clear that we are rapidly running out of time.

DR PETER COX: One of the real driving forces is that you leave an environment that is comfortable for your children. And we carry on going the way we're going, we're not going to do that, we're going to leave an environment that's much worse than the environment we lived in; and it will be down to what we did when we were using that environment, and that would be, um, tragic really, if that happened.