We are circling over Buenos Aires. The airspace is crowded with other planes, all of them holding like ours. The pilot explains that it is the fault of the humo, or smoke, a word I will hear a great deal in the coming week.

An hour and a half later I am on the ground, head pounding, breathing in the humo. The cover of the Clarín newspaper shows someone gagging and declares: "The Worst Atmospheric Contamination in History."

Some things, such as slight overstatement, haven't changed in Buenos Aires. Still, it's hard not to think of the first time I came here. It was January 2002. The economy had just crashed, the banks had locked out their customers and Argentines had thrown out five presidents in three weeks. There was smoke in the air then, too, but it was from the bonfires in the streets.

Within an hour, I have heard three theories purporting to explain the humo. (1) It's a political protest by the farmers, who set their crops on fire to protest against a new tax on soy exports. (2) It's the government, which set the crops on fire in order to turn public opinion against the farmers after they went on strike against the export tax. (3) It may be the farmers who set the fires but it's the fault of the government, which is deliberately refusing to extinguish them.

The truth, I learn later, is that the fires are the result of a radical shift in Argentina's agricultural economy. This country used to be all about the grass-fed cows, raised by the famous cowboys of the Southern Cone, the "gauchos". But soy production is expanding so rapidly, thanks to high prices and huge demand in China, that ranchers are being forced on to ever-shrinking slices of land. They burn the brush to clear it quickly but this year, because of a drought, the fires spread out of control. Add to that high winds and you have humo in Buenos Aires.

It makes for a powerful symbol: the proud gauchos suffocated by soybeans. Argentina is certainly changing.

Soy isn't the only force displacing the cowboys this week; so is the annual Buenos Aires Book Fair, the reason for my trip. The fair is held in La Rural, the huge agricultural exposition grounds where Argentina's landowners have auctioned their prize cattle for more than a century. The book fair has transformed the space, covering the grubby floors with red carpet and sleek display booths. Occasionally, one catches a whiff of manure. We authors choose not to mention this in our presentations.

Besides the smoke, there are many other changes to note in this city. Last time I was here, the shops were empty, the streets were filled with protests, and the International Monetary Fund was calling the shots. This time, Argentina is no longer in debt to the IMF, the economy is booming and, in far-off Washington, the IMF is facing its own debt crisis, provoking self-imposed structural adjustment as the organization lays off hundreds of staff and dips into its gold reserves.

Today there is less "Yankee go home" graffiti and more... Yankees. Argentina's 2001 market crash was created, in large part, by the monetary policy that pegged the peso to the US dollar at a rate of one-to-one. The economy here was too weak to maintain the illusion and the currency crashed. This time, much of the boom comes from the fact that the US economy is in crisis and the dollar is weak. Buenos Aires, with its grand cafés and edgy designers, has gained a reputation with US holidaymakers as a discount Europe: Paris on the cheap.

At the book fair, an audience member asks me if I think he should sell his dollars. I accuse him of being a disaster capitalist, preying on the US economy in its time of crisis. In this country where so many disasters - coups, hyperinflations, debt - have been opportunities for foreigners to earn super-profits, it gets a good laugh. "To the Escuela de Mecánica de la Armada," we tell the taxi driver. "Why are you going to ESMA?" he demands. Because we are filming there." For a minute I think he is going to throw us out of the car. He opts to take the fare, but maintains a furious silence the entire journey.

In between festival events, I am beginning work on a documentary of my book The Shock Doctrine, directed by Michael Winterbottom and Mat Whitecross, the team that made The Road to Guantánamo. We are picking up that road a few decades earlier this time, in Argentina and Chile in the 1970s. The most notorious of the period's torture centres was ESMA, a Navy school converted into a clandestine prison. According to human rights groups, roughly 5,000 desaparecidos, or disappeared people, were tortured here, the vast majority of them murdered.

In 2002, the military still controlled ESMA, while the human-rights groups, such as the Mothers of the Plaza de Mayo, were outsiders to Argentina's official institutions. Men like my taxi driver, who denied most of the crimes, still held sway in public debate. Friends and family of the disappeared remembered their loved ones with protest signs, candlelight vigils, and spooky stencils spray-painted on sidewalks and walls.

Things have definitely changed. Now Buenos Aires has an official memorial wall, made up of 30,000 individual bricks - each representing one of the disappeared. The monument was unveiled by then President Néstor Kirchner less than six months ago. The version of history protected and nurtured by the mothers, grandmothers and children of the disappeared is at last becoming Argentina's accepted history.

We see the most dramatic change of all upon our arrival at ESMA, the human rights groups control it now, and they are turning the haunted houses into a new kind of school, one focused on the kind of country that the desaparecidos, most of them leftwing activists, were trying to build when they were erased.

There will always be those who deny the atrocities that happened here. But the past in Argentina is finally getting clearer, despite the smoke.

This article was first published in The Financial Times.