This is the moment at which we turn and face ourselves. Here, in the plastic corridors and crowded stalls, among impenetrable texts and withering procedures, humankind decides what it is and what it will become. It chooses whether to continue living as it has done, until it must make a wasteland of its home, or to stop and redefine itself. This is about much more than climate change. This is about us.

The meeting at Copenhagen confronts us with our primal tragedy. We are the universal ape, equipped with the ingenuity and aggression to bring down prey much larger than itself, break into new lands, roar its defiance of natural constraints. Now we find ourselves hedged in by the consequences of our nature, living meekly on this crowded planet for fear of provoking or damaging others. We have the hearts of lions and live the lives of clerks.

The summit's premise is that the age of heroism is over. We have entered the age of accommodation. No longer may we live without restraint. No longer may we swing our fists regardless of whose nose might be in the way. In everything we do we must now be mindful of the lives of others, cautious, constrained, meticulous. We may no longer live in the moment, as if there were no tomorrow.

This is a meeting about chemicals: the greenhouse gases insulating the atmosphere. But it is also a battle between two world views. The angry men who seek to derail this agreement, and all such limits on their self-fulfilment, have understood this better than we have. A new movement, most visible in North America and Australia, but now apparent everywhere, demands to trample on the lives of others as if this were a human right. It will not be constrained by taxes, gun laws, regulations, health and safety, especially by environmental restraints. It knows that fossil fuels have granted the universal ape amplification beyond its Palaeolithic dreams. For a moment, a marvellous, frontier moment, they allowed us to live in blissful mindlessness.

The angry men know that this golden age has gone; but they cannot find the words for the constraints they hate. Clutching their copies of Atlas Shrugged, they flail around, accusing those who would impede them of communism, fascism, religiosity, misanthropy, but knowing at heart that these restrictions are driven by something far more repulsive to the unrestrained man: the decencies we owe to other human beings.

I fear this chorus of bullies, but I also sympathise. I lead a mostly peaceful life, but my dreams are haunted by giant aurochs. All those of us whose blood still races are forced to sublimate, to fantasise. In daydreams and video games we find the lives that ecological limits and other people's interests forbid us to live.

Humanity is no longer split between conservatives and liberals, reactionaries and progressives, though both sides are informed by the older politics. Today the battle lines are drawn between expanders and restrainers; those who believe that there should be no impediments and those who believe that we must live within limits. The vicious battles we have seen so far between greens and climate change deniers, road safety campaigners and speed freaks, real grassroots groups and corporate-sponsored astroturfers are just the beginning. This war will become much uglier as people kick against the limits that decency demands.

So here we are, in the land of Beowulf's heroics, lost in a fog of acronyms and euphemisms, parentheses and exemptions, the deathly diplomacy required to accommodate everyone's demands. There is no space for heroism here; all passion and power breaks against the needs of others. This is how it should be, though every neurone revolts against it.

Although the delegates are waking up to the scale of their responsibility, I still believe they will sell us out. Everyone wants his last adventure. Hardly anyone among the official parties can accept the implications of living within our means, of living with tomorrow in mind. There will, they tell themselves, always be another frontier, another means to escape our constraints, to dump our dissatisfactions on other places and other people. Hanging over everything discussed here is the theme that dare not speak its name, always present but never mentioned. Economic growth is the magic formula which allows our conflicts to remain unresolved.

While economies grow, social justice is unnecessary, as lives can be improved without redistribution. While economies grow, people need not confront their elites. While economies grow, we can keep buying our way out of trouble. But, like the bankers, we stave off trouble today only by multiplying it tomorrow. Through economic growth we are borrowing time at punitive rates of interest. It ensures that any cuts agreed at Copenhagen will eventually be outstripped. Even if we manage to prevent climate breakdown, growth means that it's only a matter of time before we hit a new constraint, which demands a new global response: oil, water, phosphate, soil. We will lurch from crisis to existential crisis unless we address the underlying cause: perpetual growth cannot be accommodated on a finite planet.

For all their earnest self-restraint, the negotiators in the plastic city are still not serious, even about climate change. There's another great unmentionable here: supply. Most of the nation states tussling at Copenhagen have two fossil fuel policies. One is to minimise demand, by encouraging us to reduce our consumption. The other is to maximise supply, by encouraging companies to extract as much from the ground as they can.

We know, from the papers published in Nature in April, that we can use a maximum of 60% of current reserves of coal, oil and gas if the average global temperature is not to rise by more than two degrees. We can burn much less if, as many poorer countries now insist, we seek to prevent the temperature from rising by more than 1.5C. We know that capture and storage will dispose of just a small fraction of the carbon in these fuels. There are two obvious conclusions: governments must decide which existing reserves of fossil fuel are to be left in the ground, and they must introduce a global moratorium on prospecting for new reserves. Neither of these proposals has even been mooted for discussion.

But somehow this first great global battle between expanders and restrainers must be won and then the battles that lie beyond it – rising consumption, corporate power, economic growth – must begin. If governments don't show some resolve on climate change, the expanders will seize on the restrainers' weakness. They will attack – using the same tactics of denial, obfuscation and appeals to self-interest – the other measures that protect people from each other, or which prevent the world's ecosystems from being destroyed. There is no end to this fight, no line these people will not cross. They too are aware that this a battle to redefine humanity, and they wish to redefine it as a species even more rapacious than it is today.