Deep disagreements over political issues, and over the perceptions of reality behind them, have been the norm rather than the exception through American life. This is the nation that had to build its founders' conflicting views about slavery into its constitution. Through the following century, the main axis of legislative tension, economic rivalry, and ultimately war involved the inability of either side in the debate over slavery to persuade the other, peacefully, to change its beliefs.

Through the past century as well, American public life has struggled to contain deep and bitter disagreements, often based on misinformation, about the nation's character and goals, plus the main challenges to its well-being. Immigration has been a crucial long-term source of American revitalisation, but after World War I, it was brought almost to a halt largely because of bogus "scientific" theories about the inferiority of the newest arrivals from eastern and southern Europe.

Madison Grant's The Passing of the Great Race, first published in 1916, sold strongly for the next 20 years with its warning that the "Nordic" stock (from the British Isles and Germany) that had brought America to glory would be fatally undermined by the inferior intelligence and criminal traits of the Alpine and Mediterranean "races" (Greeks, Italians, Poles, and worse) who now wanted in. The seminal book Public Opinion, by the young Walter Lippmann, warned that the realities of the world had become too complex and fast-changing for average Americans to grasp or debate intelligently—and this was in the comparatively pastoral circumstances of 1922.

Three years later, at the "monkey trial" in Tennessee, later dramatised as Inherit the Wind (a 1960 film starring Spencer Tracy), the biology teacher John Scopes was found guilty of the crime of teaching evolution, an illustration of the long-standing gap between what we would now call faith-based and science-based views of the world.

During my own childhood in the Cold War/Baby Boomer era, some Americans thought their countrymen were dupes for failing to see that the addition of fluoride to drinking water, far from being a dental-health measure, was in fact the first step toward unaccountable world government.

Thus the aspects of today's American discourse that seem irrational—or that seem, at a minimum, to illustrate that Americans now occupy separate "fact universes", each with its own version of reality—may represent an unfortunate continuity with the nation's past rather than another indication of decline. Recently, I interviewed leading political scientists to ask whether they could show that public life in America had become in any measurable way "stupider" than before. What about the sizeable minority of Americans—and absolute majority of Republican primary voters—who believed their current president was born overseas and therefore held office illegally? Or the overlapping group who believe he is a Muslim? The clear majority with no idea that Medicare, the healthcare program for people over 65, is run by the government? I found many academics who lamented these findings, but none who said there had ever been a different golden age.

Some of today's illustrations of flawed or irrationally politicised versions of reality are self-limiting in their effects. For instance, no matter how many Americans believe that "cutting foreign aid" will eliminate deficit spending, the numbers will never add up that way. (On average, Americans think the federal deficit and the foreign aid budget are about the same size; in fact, the deficit is more than 100 times larger.) But there is one area of politicised knowledge with the potential to be more dangerous than most others. That is the battle within America over climate change.

Across the world, there are sharp differences over the proper response to climate change: who should pay, on what timetable, using which technology, with what possible risks. The acrimonious disagreements at the Copenhagen and Cancun climate meetings over the past two years, showed how far from consensus world opinion is, even at sessions where Americans played no part. And in nearly every country, including Australia, the range of opinion on climate change includes some deniers, those who dismiss the international scientific consensus that human activity since the Industrial Revolution has raised greenhouse gas concentrations, and that this change is altering the climate.

But in no other advanced country is the simple scientific argument about climate change regarded, by the deniers' camp, as just another partisan assertion, like opinions on the proper tax level or policy in the Middle East. America's status on this issue is comparable—in its isolation and in its consequences—to South Africa's in the years when then-president Thabo Mbeki dismissed the idea that the HIV virus had any connection with AIDS.

The effect of this internal US split on worldwide climate efforts is evident. With the world's largest economy and second-largest emitter of greenhouse gases divided on whether there is even a problem to be solved, it is even harder for anyone else to move. But what does it indicate about the situation of political consideration, debate, and action in the US? I think it shows something familiar but also something new and worrisome.

The familiar aspect is the increasing atomisation of the public into separate information spheres. The technological and business changes in journalism from the 1960s, when major newspapers reached most readers in each town and three broadcast TV networks brought most viewers their nightly news, to the vast flow of news today makes it harder for public officials or scientific authorities to reach people with information they have not chosen to hear.

The American media's habit of presenting most issues in the form of pro-con "debate" plays a part too. When covering Mbeki's disastrous delusion in South Africa, they felt free to say flatly, "his views are wrong". But when covering the climate "debate" they feel obliged to pair, say, a climate scientist with a Mbeki-style denier politician, as if their views deserve equal weight.

The additional element is a behaviour pattern known increasingly as "agnotology", or "epistemic closure". These are two elaborate ways of saying that certain parts of the American public have decided to exclude themselves from even the possibility of changing their minds. The pattern here is a variant of tribal politics, which is the bane of developing societies and which governing systems from the Renaissance onward have tried to offset. In a tribal model, positions are good or bad based on who espouses them, not on their merits.

Thus, if George W. Bush and Dick Cheney had endorsed a radical freedom-from-fossil-fuels program as part of their post-September 11 response, Fox News would have trumpeted it as a gesture of greatness; from John Kerry or Barack Obama, it would be (to Fox) another galling American retreat. "Epistemic closure" completes the model by persuading people that the more thoroughly their position is attacked, especially with contrary facts, the more fiercely they must cling to it. Thus, mainstream mockery of Sarah Palin only increased the loyalty of her supporters.

Stalwartness in the face of criticism has some admirable precedents. In 1936 Franklin Roosevelt said that the forces of plutocracy were "unanimous in their hate for me—and I welcome their hatred!" But it has been a while since Americans have seen such open hatred for facts. We don't know when this phase will end, but we can hope it will be soon.