I write this as a former left-winger. No, I haven't made the long, lifetime trek from Trot to Tory, like so many highly placed newspaper columnists (more of them later). My contention is, I hope, a little more rational: that the "left-wing" label no longer implies an acceptable position on the most crucial issue facing the world today. A new challenge has arisen which - by transcending the older and less significant divides - will eventually define the course of 21st-century politics. In comparison, today's "big" issues, on top-up fees, foundation hospitals and the Blair-Brown relationship, recede like bar-room chatter into the background hubbub.
This new challenge does not have an easily recognisable name. E O Wilson, one of the greatest contemporary biologists, calls it "the bottleneck". Most of its sub-issues are familiar, but the bigger picture is not. Piece together the disparate elements and the product is terrifying. The shadow thrown by this looming crisis is everywhere, its darkness growing slowly, almost imperceptibly, as it creeps over our planet.
The crisis is this: within the earth's biosphere, a single species has come to dominate virtually all living systems. For the past two centuries this species has been reproducing at bacterial levels, almost as an infectious plague envelops its host. Three hundred thousand new individuals are added to its numbers every single day. Its population of bodies now exceeds by a hundred times the biomass of any large animal species that has ever existed on land since the beginning of geological time.
The species is us. Now numbering more than six billion souls, the human population has doubled since 1950. Nothing like this has happened before in the earth's entire history. Even the dinosaurs, which dominated for tens of millions of years, were thinly spread compared to the hairless primate Homo sapiens.
Inevitably, our productive and consumptive activities displace other living species from the planetary food web. The result is mass extinction, which has historically accompanied human expansion everywhere, from North America to Easter Island. Wherever humans dwell, other species die out - displaced from land cleared for agriculture, killed for their flesh, or simply allowed to disappear as an unnoticed by-product of the thriving primate economy.
We are now in the early-to-middle stages of the sixth mass extinction to hit the planet since complex life began 2.1 billion years ago. Species are disappearing at between 1,000 and 10,000 times the natural background rate. A fifth of birds are threatened with extinction, as are 40 per cent of mammals and fish, a third of amphibians and up to half of all plant species. Humans now appropriate 40 per cent of the planet's organic matter produced by photosynthesis. The number of humans born in a single day almost equals the total global population of great apes.
Again, this situation is un- precedented: never before has an agent of mass extinction emerged from within the living systems of the planetary biosphere. Previous mass extinctions have been caused by external factors, such as the aster-oid that likely wiped out the dinosaurs 65 million years ago or the volcanism that seems to have caused the Permian mass extinction (when up to 95 per cent of species died out) 251 million years ago. In the lottery of evolution, as Wilson writes, Darwin's dice have rolled badly for Planet Earth. We are entering a new geological era: the Anthropocene.
All this would be bad enough. But there is more. Since the early 1800s, humans have been using buried carbon - first in coal, later in oil and gas - as a form of energy. When combusted in our oxygen-rich atmosphere, this carbon becomes the potent greenhouse gas carbon dioxide - the same gas that keeps Venus's surface temperature at a searing 460oC. Levels of carbon dioxide in the earth's atmosphere have risen by a third since the start of the industrial revolution, and global temperatures are rising in tandem. We are now on track to change the globe's average surface temperature by anything up to six degrees Celsius within a century, taking us into a climate regime last experienced between one million and 40 million years ago, well beyond the evolutionary experience of many creatures alive today - including humanity.
Add these trends together and Wilson's "bottleneck" sounds like an understatement. It is inconceivable that humanity and natural species can pass together through this bottleneck unscathed. And here lies the challenge. Will we emerge at the end of the century with a depleted, devastated planet, inhabited only by remnant super-adaptable species and artificial ecosystems created to support the remaining human population centres? Or will humanity take sufficient remedial measures to ensure that a reasonable proportion of the living biosphere survives?
This is the shadow under which the battle lines are forming in modern-day politics. On one side stands a loose and ragged coalition of those who want to see the survival of nature, not just for the sake of human survival, but because they believe in its intrinsic worth. On the other side stand those who don't know or don't care, or who actively oppose efforts to get us through the bottleneck unscathed. Let's meet these people.
First under the spotlight is the United States. Its current government is dedicated to environmental destruction on a scale hitherto unimaginable. President Bush and his vice-president, Dick Cheney, both deny the reality of global warming. They have approved legislation to speed up the logging of forests, and they are trying to gut the Endangered Species Act, among numerous other blindly destructive measures. Several of the most powerful US senators - including James Inhofe (Republican, Oklahoma, and chair of the Senate committee on environment and public works), Larry Craig (Republican, Idaho) and Craig Thomas (Republican, Wyoming) are dedicated anti-environmentalists. Inhofe calls global warming a "hoax", and alleges that the Kyoto treaty "is an economic weapon designed to undermine the global competitiveness and economic superiority of the United States". In evidence to support this contention, he cites numerous pseudo-scientific studies, many of them supported by fossil fuel interests and by far-right think-tanks such as the Competitive Enterprise Institute and the George C Marshall Institute.
Britain, too, has a powerful establishment of anti-environmentalists. The Adam Smith Institute is one of the most prominent, with strong connections to new Labour, despite its Thatcherite political creed. Of similar bent is the Institute of Economic Affairs, which has published several pamphlets denouncing "climate alarmism", opposing the Convention on International Trade in Endangered Species and accusing those who try to protect tropical forests of "eco-imperialism".
These viewpoints are popularised by media pundits, primarily but not exclusively attached to the right-wing press. Their empress dowager is the Daily Mail's Melanie Phillips, who claimed recently that "there's no correlation between rises in climate temperature and sea levels". This is untrue, even at the level of basic physics - heat makes water expand, raising sea levels at the same time that water from melting ice on land adds more mass to the global ocean. But the mistakes - and Phillips's article is full of them - are not the point: this is a statement of ideology, based not on scientific rationality or empirical evidence, but on a particular world-view. It is vital to understand this, because it reminds us that those with truth on their side will not necessarily emerge triumphant as this conflict deepens.
Joining Phillips on the far right is the self-parodying twitterer Peter Hitchens. He writes: "The Kyoto treaty is a silly waste of time. The greenhouse effect probably doesn't exist. There is as yet no evidence for it." Given that the greenhouse effect is basic atmospheric physics (and that, without it, the average global temperature would be well below freezing), this is a very stupid statement indeed. Again, truth is secondary.
But it is no use looking to the left for a more rational approach. Communists have always regarded nature as little more than raw material to be scraped up and melted down into pig-iron by an emancipated proletariat, marching in step to a glorious techno-industrial future. Variants of this view persist among the Socialist Workers Party and its various front groups in the Socialist Alliance and anti-war movements. Indeed, anti-environmentalism is such an article of faith among the extreme left that the cultish cabal which used to publish Living Marxism magazine, and which has now moved into the electronic media, called its Channel 4 series Against Nature.
More moderate leftists neglect ecological concerns in favour of their enduring obsession: human equality. Worthwhile as this objective may be, any consideration of how resources are to be shared with other species or with future human generations is excluded as irrelevant. Moreover, both left and right agree that economic growth can and should go on for ever.
For anyone with a basic understanding of mathematics, the impossibility of everlasting growth based on a finite resource base should be obvious. And this is the core of my argument: that it is time to de-prioritise the struggle over fair shares to the global economic pie, because the very existence of this pie is increasingly at stake. If global warming accelerates enough to turn the world's breadbaskets into dust bowls (as is already happening in northern China), then our squabbles over how to divide the spoils from the rape of Planet Earth will look very short-sighted.
Like cockroaches, human beings can scrape a living almost anywhere. The total extinction of our species is unlikely. But human society is complex and fragile, especially in an age where few people in rich countries have any experience of fending for themselves. Indeed, those who are most dismissive of environmental concerns are precisely those whose meal tables are likely to include green beans from Kenya, prawns from Bangladesh and beef from Argentina. The system of long-distance food transportation is far more vulnerable to ecological collapse than they realise.
Moreover, we are all deeply dependent on the "ecosystem services" provided free by the natural world. These include the purification and retention of fresh water (and flood control); the formation and enrichment of soil; the detoxification and recirculation of waste; the pollination of crops; the production of lumber, fodder and biomass fuel; and the regulation of the atmosphere and climate. The monetary value of these ecosystem services has been costed at $33 trillion a year, roughly the same as combined world GDP. If natural systems are mostly wiped out, we will need to replace these services artificially, which is a physical and economic impossibility.
So who is to blame for this blindness? It is tempting to follow the anti-globalisation movement in castigating multinational corporations, the World Trade Organisation and rampaging capital markets. I believe that something much baser is happening. The NS essayist John Gray gets it about right: "The destruction of the natural world is not the result of global capitalism, industrialisation, 'western civilisation' or any flaw in human institutions. It is a consequence of the evolutionary success of an exceptionally rapacious primate."
That is why we need to abandon the left-right battle lines. They offer us no guidance on how to survive the century ahead. Neither does Gray, for that matter. For him, all grand plans are by definition doomed to fail. Looking at history, one can see his point. But we have got to survive the bottleneck, and just muddling through won't do.
Thinking up solutions is not the problem. The "contraction and convergence" proposal for tackling climate change (global emissions contract to a sustainable level; per capita emissions converge between countries) knits both human equality and ecological survival into an elegant equation. Similarly, we can protect biodiversity by stopping habitat destruction and countering the spread of invasive alien species around the world, especially in highly biodiverse "hot-spot" areas. And increasing women's control over their fertility is a straightforward way to reduce population growth.
Yet these proposals are so vast and all-consuming as to require a strong and durable consensus before they can be agreed or implemented. Biodiversity protection cannot be bolted on to existing growth-oriented economics. Contraction and convergence would require enormous resource transfers from rich to poor countries, as the developed world pays the developing nations not to follow in its own dirty footsteps.
Hence the failure of the various UN environmental summits: they take place in a political vacuum, with little public knowledge or interest to support or enforce their decisions. It is the formation of any durable political consensus towards ecological survival that the anti-green movement is determined to prevent.
In the meantime, the rest of us get side-tracked. I still believe that Tony Blair, for all his faults, remains unusually committed - compared to other government leaders - to tackling global warming. But by joining Bush's war on Iraq, Blair helped deliver the world's second-largest reserves of oil into the hands of the only major country fully under the control of climate change deniers. Rather than chasing all over the desert in search of a few mouldering old canisters of mustard gas, those seeking weapons of mass destruction need only have drilled down a few hundred metres until they hit oil, the most potent and destructive WMD of all.
The government's chief scientist, Dr David King, recently found himself at the centre of controversy when he said global warming is a more serious threat than terrorism. Of course it is: just add up the numbers. Global warming: 150,000 deaths annually from the increased disease caused by higher temperatures, according to the World Health Organisation. Terrorism: 1,000 a year (at a guess). So why is terrorism the apocalyptic threat we all have to mobilise against? You're more likely, in a poor country at least, to die of flood-related diarrhoea. And the rich won't be safe for long. The much-vaunted "clash of civilisations" is at best a distraction, at worst a racist fiction. Preventing the clash between human civilisation and nature is the battle we ought to be fighting.
Mark Lynas is the author of High Tide: news from a warming world, published by Flamingo on 1 March (£16.99). www.marklynas.org