In 2006 James Lovelock published a book that bluntly laid before us the consequences of the carbon imbalance. The Revenge of Gaia argues Gaia's climate system is far more sensitive to greenhouse gas pollution than we imagine, and the system is already trapped in a vicious circle of positive feedback.
Although there is still time to avert a catastrophe, Lovelock believes humans lack the foresight, wisdom and political energy required to do so. Instead, he predicts, before the 21st century is out our global civilisation will have collapsed and a new Dark Age will have dawned, wherein a few survivors will cling to the few remaining habitable regions, such as Greenland and the Antarctic Peninsula.
How probable is it that this bleak vision will come to pass? New scientific data means that in 2009 we are better placed than ever to determine the scale of the threat and its imminence.
The northern fridge
The sea ice that covers the Arctic Ocean is an ancient feature of our planet. It has glistened brightly into space for at least three million years.
The northern ice acts as a refrigerator that cools the entire planet. During the summer, the sun's rays beat down upon it 24 hours a day, but because the ice is bright, 90 per cent of that energy is deflected back into space.
By 2005 the Arctic ice cap had been melting at a rate of around eight per cent per decade for thirty years. At that rate, it would have taken until 2100 or thereabouts for the ice cap to disappear altogether.
But in the summer of 2005, a dramatic change occurred. The rate of melt accelerated, so that around four times as much ice melted as compared with previous summers.
These changes in the Arctic have left many scientists worried the region is already in the grip of an irreversible transition. During the winter months, the Arctic is now warming four times faster than the global average, while the existing temperature increase year-round already exceeds two degrees Celsius.
What will happen during that first iceless summer? Most likely, not much at all, for it will take several summers' worth of energy to warm the surface of the Arctic sea to a point where dangerous changes are generated further south. But each year thereafter, the ocean at the top of the world will warm inexorably, and the temperature gradient that controls climatic zones across the northern hemisphere will shift.
If we look back to the last time in Earth's history when such a great warming occurred — 55 million years ago — we see an ominously different world.
Back then, lemurs sported in the rainforests of Greenland, while the tropics were covered in a spiny, thin and alien-looking cover of vegetation, which is today entirely extinct. No one knows how quickly the world's climate altered back then, but one cannot help but fear what a similar scale of change might mean for humanity today.
A dying sea
New ramifications of rapid warming are continually being discovered. In 2006 scientists realised that the sea can die as a result of massive global warming. Indeed, it has done so several times during Earth's history, and when it does, it takes most life on land with it.
The most devastating example of oceanic death occurred around 250 million years ago, when 95 per cent of all life perished.
Geologists studying rocks in Western Australia discovered traces of the unique lipids (fatty molecules) made by strange kinds of bacteria known as purple bacteria and green sulphur bacteria.
These bacteria only thrive in waters that are well lit by the sun, yet are low in oxygen and high in hydrogen sulphide. Such conditions exist only in very restricted and unusual environments today, such as the 'jellyfish lakes' of Palau. Yet the story preserved in the rocks reveals that most, if not all, of Earth's oceans resembled this environment 250 million years ago.
Answers in the ice-core
How much time do we have to prove Lovelock wrong? On 31 March 2008, Dr James Hansen and eight of his colleagues provided a new, alarming, though still partial, answer to this question.
They looked back over the increasingly complete ice-core record, which documents the last three-quarters of a million years of Earth's climatic history, and tried to determine how much warming a given amount of atmospheric CO2 pollution would produce, and how long it would take to produce it.
Their most alarming discovery was that, when viewed over the long term, Earth's climate system is about twice as sensitive to CO2 pollution as is shown on the Intergovernmental Panel on Climate Change's century-long projections.
This implies that there is already enough greenhouse gas pollution in the atmosphere to cause two degrees Celsius of warming, bringing about conditions not seen on Earth for two to three million years and constituting, according to the authors, "a degree of warming that would surely yield 'dangerous' climate impacts".
Fortunately for us, some, perhaps half, of that warming is currently masked by other pollutants, known collectively as the agents of global dimming, which reflect sunlight into space, thus cooling Earth.
Today, China, India and other rapidly industrialising economies are releasing these pollutants in ever-increasing quantities. Yet because of their effect on visibility and their serious impact on human health, there's good reason to believe that in the near future such nations will move to curb their release.
Close to the point of no return
In their landmark paper, Hansen and his colleagues make a useful distinction between climatic "tipping points" and "the point of no return."
The climatic tipping point is the point at which the greenhouse-gas concentration reaches a level sufficient to cause catastrophic climate change. The point of no return is reached when that concentration of greenhouse gas has been in place sufficiently long to give rise to an irreversible process.
Humanity is now suspended between a tipping point and a point of no return. We still have a few years before we reach the point of no return, but there is not a second to waste. This is our greatest challenge: to draw the pollution out of the air and save ourselves from Lovelock's new Dark Age.
Professor Tim Flannery is an eminent scientist, writer and former Australian of the Year. He is also chair of the Copenhagen Climate Council. This is an edited extract from his book Now or never: a sustainable future for Australia? published in Australia by Black Inc.