Planet Earth is unwell, argues James Lovelock in The Green Room. Emissions of greenhouse gases and other environmental changes have, he says, brought humanity and the natural world to the edge of crisis.
The Intergovernmental Panel on Climate Change report, published in 2001, is one of the scariest documents you will ever read.
It talks about changes to the Earth by the end of this century which will be as great or greater than occurred between the end of the last Ice Age and the time when humans started changing the atmosphere; it is huge.
It alarms me, and it should alarm anyone.
Just imagine that you had lived 12,000 years ago, before the last Ice Age ended, in a tropical civilisation somewhere in South-East Asia.
What would have been your reaction if some scientist had told you that within not too many years the sea level would rise by 120m, by 400ft?
What the precise impacts of elevated greenhouse gas levels will be I cannot say; but we are on course for changes on that sort of scale.
In addition, as the world gets very hot, it will not be able to produce anything like as much food as it does now; so quite literally, billions of us are going to be faced with starvation.
These issues amount to a challenge far greater than anything humanity has faced since the shift out of the last Ice Age into the interglacial period.
As a Gaian scientist, a general practitioner of planetary medicine, I have spent decades trying to see the Earth and life on it as an integrated whole.
It is not only climate change and the emissions of gases which are causing it - carbon dioxide, methane, halocarbons, nitrous oxides - which concerns me.
At the same time we are taking for our own purposes more and more of the natural ecosystems that usually regulate conditions at the planet's surface. We are denuding forests, changing biodiverse lands into monoculture deserts, acidifying the oceans.
To put Earth's self-regulation into perspective, compare our planet with its neighbours, Venus and Mars.
If there were no life on Earth, it would be a giant arid desert, just like Mars and Venus
These I call "dead" planets - there is no life at all, and they show no sign of regulation. Their temperature follows what the Sun does; as it warms up, they grow hotter.
If there were no life on Earth, the temperature on our planet would be way up above 60C, possibly 100C; there would be no water, it would be a giant arid desert, just like Mars and Venus.
It is instead a cool, beautiful world, because of the life that is on it.
It has been present for three and a half billion years; and however the Sun's output of energy has changed, life has kept the planet comfortable for itself, for its continued survival.
The life out there is necessary for our welfare; we cannot just go taking it for our convenience, cutting down forests, turning the productive oceans into the marine equivalent of deserts, and expect Gaia not to take revenge.
In 100 years' time, I would expect life to be very grim.
I suspect that people will be migrating towards what will be more comfortable parts of the Earth like the Arctic basin. To an extent Siberia and northern Canada may flourish.
The British Isles, I have often felt, will be blessed, because our oceanic position means that the intolerable heat that will hit Europe even by mid-century will not affect us anything like as badly.
But social effects there will certainly be.
Many good scientists say that by 2050, almost every summer in Europe will be as hot as it was in 2003. In that case I can foresee a mass movement of people from mainland Europe to Britain, because they are free to come, it is their right to come.
We are overcrowded enough already; where are we going to put them?
Call to arms
During the last week I have had the benefit of wise comments from seven well-respected scientific peers who have examined the content and the message of my book. I have listened to what they have all said and taken it aboard.
Having done that, my general feeling is there is not a lot I would rewrite; I would be more careful in the way I phrased some passages, but the content would be pretty much the same.
And it is crucial to see what the book is not. It is often claimed to be a counsel of despair; critics say it will cause people to throw up their arms and say "what's the point of doing anything, let's just enjoy it while it lasts".
It isn't that at all.
I compare these times to the period just before World War Two; I remember it so vividly, because I was a young student in those days, and concerned about things.
People did not see the almost inevitable consequence of war coming as something to be frightened of; they saw it as an opportunity, strangely enough.
And once war did come, people were amazingly busy, finding jobs, doing all sorts of things; there was a sense of purpose around.
I hope that as climate change worsens that same sense of purpose, that almost tribal pulling together, will work again, to find such solutions as are still available in Gaia's damaged state.
Professor James Lovelock is an independent scientist and the originator of the Gaia hypothesis
His recent book The Revenge of Gaia formed the basis of a BBC panel discussion on Monday and Tuesday
The Green Room is a series of opinion articles on environmental issues running weekly on the BBC News website